Bassam Moujalled
email: bassam.moujalled@cerema.fr

Valerie Leprince

Adeline Bailly Mélois

Keywords: Building airtightness, measurements, database, field data

The French database of building airtightness has been fed by measurement performed by qualified testers since 2006. In 2015 and 2016, the database was enriched by 63,409 and 65,958 measurements respectively, which is 74% more than in 2014, making the total number of measurements about 215,000. However, residential buildings (multi-family and single dwellings) account for almost all of measurements, only 4% of tests are performed in non-residential buildings. Indeed, since 2013 the French EP-regulation requires a limit for airtightness level for all new dwellings. The justification of the building airtightness level shall be done either by an airtightness test or by the application of a certified quality management approach. In the first part, this paper summarizes the recent results of the database regarding buildings' characteristics (building area, main material, ventilation system, insulation...). The second part proposes first results regarding the evolution of air permeability. In single dwellings, the air permeability at 4 Pa (per unit of envelope area) slightly decreases from year to year with a mean value around 0.41 m 3 .h -1 .m -2 in 2015. In multi-family buildings, the yearly mean air permeability fluctuates between 0.60 and 0.65 m 3 .h -1 .m -2 . In non-residential buildings, it fluctuates around 1 m 3 .h -1 .m -2 . However non-residential buildings cover a wide variety of buildings. A special focus is made on non-residential buildings depending on the use of the building and its size. The last part of this paper deals with the impact of the seasonal variations on the measured air permeability in single dwellings depending on climatic zones and buildings construction materials (wood, concrete and brick constructions). An impact of seasonal variations on air permeability is only observed in the case of wood constructions, with slightly higher values during summer in the south of France in particular.

French database of building airtightness, statistical analyses of about 215,000 measurements: impacts of buildings characteristics and seasonal variations 1 INTRODUCTION

Building airtightness is a key issue to achieve low-and very low-energy targets. Therefore, an increasing number of tests are performed in European countries. To improve the reliability of those tests, qualification schemes for airtightness testers are developing all over Europe [START_REF] Leprince | Building and Ductwork Airtightness Requirements in Europe -Comparison of 10 European Countries[END_REF]. France, UK and Belgium have taken advantage of those schemes to impose systematic record of airtightness test and to develop databases [START_REF] Leprince | Building and Ductwork Airtightness Requirements in Europe -Comparison of 10 European Countries[END_REF]) [START_REF] Cope | Statistics, Analysis and Conclusions from 250,000 Blower Door Tests, Including Ventilation Types[END_REF]. The French database of building airtightness was created in 2007 following the implementation of a national qualification scheme for building airtightness measurement.

Thus each qualified tester is required to register all test results in a formatted table and send this register table to the certification body every year. The database is fed annually by these tables. The structure of the table is presented by [START_REF] Bailly | 6 years of envelope airtightness measurements performed by French certified operators: analyses of about 65,000 tests. 36th[END_REF].

The number of measurements in the database has strongly increased since 2013 thanks to the mandatory requirement of the French Energy Performance (EP) regulation which requires a limit airtightness level for new residential buildings. The justification of the building airtightness level shall be done either by an airtightness test performed by a qualified tester, or by the application of a certified quality management approach on building airtightness. Also low-energy labels have helped to strengthen airtightness requirements. Currently, about 215,000 measurements have been recorded in the database. It includes all the measurements that were performed by certified testers till the end of 2016. In June 2018, they are 871 certified testers. This paper summarizes the recent results of the database regarding buildings' characteristics, the evolution of air permeability, and the impact of seasonal variations.

DATABASE OVERVIEW

Figure 1 shows the evolution of the number of building airtightness measurements and the percentage of measurements depending on the use of the building.

In 2015 and 2016, the database was enriched by 63,409 and 65,958 measurements respectively, which is 74% more than in 2014, making the total number of measurements about 215,000. The number of measurements has been boosted by the mandatory requirement for new residential buildings which has been introduced since January 1 st 2013 based on the date of building permit. However, it is only since 2015 that every new residential building is being measured given the delay between the dates of building permits and the achievement. We can expect that the number of tests will be stable around 65,000 tests per year. Residential buildings account for almost all of measurements (64% for single-family dwellings with 140,542 measurements, and 32% for multi-family buildings with 70,632 measurements), only 4% of tests are performed in non-residential buildings (7,997 buildings). This is due to the fact that the mandatory requirement applies only for residential buildings: for non-residential buildings, it is still possible to use default values in the EP-Calculation. However, since 2013 new "Effinergie" labels require an airtightness measurement for nonresidential building with an area below 3,000m². Thus, more data should be collected for these buildings in the next years. Figure 2 presents the distribution of the measurements number depending on the measurement time and the measured extent of building. It shows that only 10% to 15% of residential buildings are tested during construction, against more than 30% in non-residential buildings.

Commissioning tests in residential buildings are always reported in the database. However, it is possible that some tests during construction are not filled in the database by the testers.

Regarding the measured extent of the building, almost all measurements in single-family dwellings are performed on the whole building. Conversely, more than 85% of the measurements in multi-family buildings are carried out on a part of the building. In nonresidential buildings, almost 70% of measurements are performed on the whole buildings. This results are in accordance with the compulsory measurement protocol which allows measurements based on a sampling method for multi-family buildings over 10m in height (NF EN ISO 9972, 2015) (FD P50-784, 2016).

RESULTS

Main characteristics of buildings

Figure 3 presents the share of buildings with EP-labels depending on the building use and the year of construction. During the first years, measurements in residential buildings were carried out mainly in the buildings with EP-labels. Since 2014, the share of residential buildings with label have dropped due to the mandatory requirement. Thus, measurements from 2015 can be considered as representative of new French residential buildings.

For non-residential buildings, almost 70% of the tested buildings have no label, which means that for non-residential building the main reason for testing is probably to improve the EPcalculation and not to comply with a label. However, to increase the number of test performed either a mandatory requirements or more penalizing default values are needed. Figure 4 presents the characteristics of buildings regarding main construction material and ventilation system for single-family, multi-family and non-residential buildings. The majority of single-family houses are built of masonry, mainly with brick (47%) followed by concrete (28%). Wooden houses account for 8%. For Multi-family buildings, concrete is the main material (44%), followed by brick (28%). Wood accounts only for 2%. For non-residential building, concrete is also the main material (36%), but the wood seems to be more widely used (17%) as it more than brick (15%).

It is important to note that the airtightness of concrete and brick constructions is treated in the same way in France, in most cases it is made through plasterboards and mastics at the inside facing of the walls. The wooden constructions are mainly wood frame structure, and the airtightness is ensured by the vapour barrier.

Regarding ventilation systems, we first observe that all residential buildings have been equipped with mechanical ventilation, mostly single exhaust systems (94% for single-family houses and 98% for multi-family buildings buildings). In fact, the French regulation requires general and permanent airing of residential buildings. Single exhaust systems include humidity sensitive ventilation system which is the most commonly used system in French residential buildings. Non-residential buildings are also almost all equipped with mechanical ventilation system, with mainly balanced systems (52%).

Evolution of measured building airtightness

The results presented here are expressed according to the air permeability French indicator Q4Pa-surf, which is the airflow rate at 4 Pa per unit of envelope surface area excluding lowest floor (m 3 .h -1 .m -2 ). Only measurements performed upon completion are presented and analysed hereafter in order to perform relevant comparisons.

Figure 5 presents the annual evolution of the number of tests and the average value of the air permeability for residential and non-residential buildings.

For residential buildings, results show a fast increase of the number of airtightness measurements since 2007. This dynamic was first triggered in 2007 by the French EP-label "BBC-Effinergie" and then accentuated with the mandatory requirement in 2013.

The annual increase in the number of measurements comes together with a significant drop in the average value of the air permeability (and thus an improvement in airtightness) during the first years. Since 2013, it has stabilized around 0.4 m 3 .h -1 .m -2 in single-family houses and 0.65 m 3 .h -1 .m -2 in multi-family buildings. The slight increase in multi-family since 2015 can be explained by the fact that every new building is now tested and not only exemplary ones that were applying for a label. The EP-regulation requires that Q4Pa_surf is below 0.6 m 3 /h/m² for single-family houses and 1 m 3 /h/m² for multi-family buildings. For single-family dwellings, we observe the threshold effect of the mandatory requirement of the EP-regulation at 0.6 m 3 .h -1 .m -2 which creates a discontinuity in the distribution of the measured values of the air permeability. This is due to last-minute corrections during the commissioning test to force the measured air permeability below the regulatory threshold. Thus 93% of the measurements are below 0.6 m 3 .h -1 .m -2 . Also more than half (53%) are below the threshold of EP-label "Effinergie+" 0.4 m 3 .h -1 .m -2 . For multi-family building, the distribution is more regular and is close to a skew-normal repartition. The threshold of 1 m 3 .h -1 .m -2 seems to be easier to reach in multi-family buildings with 86% of measurements below this threshold, and 73% below the threshold of EP-label "Effinergie+" 0.8 m 3 .h -1 .m -2 . Note that most tests are performed by sampling, so in most cases only dwellings are tested and not common parts.

For non-residential buildings, as seen above, the number of measurements is much lower. However, results show an annual increase in the number of measurements since 2011. Also the mean value of air permeability has decreased annually to reach 0.82 m 3 .h -1 .m -2 in 2016. In addition, this value is similar for buildings with or without an EP-label.

As for multi-family buildings, Figure 7 shows that the distribution is close to a skew-normal repartition as there is no mandatory requirement; 87% of measurements are below the default value of the EP regulation (1.7 m 3 .h -1 .m -2 for all non-residential buildings except commercial, gym, industrial and transportation buildings for which the default value is 3 m 3 .h -1 .m -2 ), and 67% below the threshold of multifamily buildings (1 m 3 .h -1 .m -2 ). The mean value of air permeability of non-residential in 2016 (with or without EP-label) is approximately half of the default value of the EP-regulation. According to [START_REF] Leprince | State of the Art of Non-Residential Buildings Air-tightness and Impact on the Energy Consumption[END_REF], we can expect an energy saving from 10% to 30% when using the average value (0.8 m 3 .h -1 .m -2 ) rather than the default value.

Despite the absence of a mandatory requirement, airtightness of non-residential buildings continue to improve, but only for those who have decided to care about it. Non-residential buildings cover a wide variety of buildings. The average value can mask different situations depending on the type of building and the volume. Therefore, a special focus is made on non-residential buildings depending on the use of the building and its size. We present in Figure 8 main summary of the volume of non-residential buildings depending on their use, and in Figure 9 the variations of air permeability depending on the building use and the range of volume for office buildings. In order to perform relevant comparisons, this analysis is based only on non-residential buildings constructed since 2013 as they present comparable values of the annual average air permeability.

The main parts of non-residential buildings are offices (25%), schools (20%) followed by hospitals (10%), gym, restaurants and other non-residential buildings (commercial, hotel, nursery, industrial…). All non-residential buildings present similar volumes with average values below 5,000 m 3 , with the exception of Gyms with higher value which is logical for this kind of buildings. The median values are globally less than 1000 m 3 except for schools and gyms with 1,311 m 3 and 1,294 m 3 respectively. The statistical values of the air permeability of the different uses are globally equivalent, with the exception of restaurants with slightly higher values. This is probably due to the hoods, and to the complexity of the technical equipment which multiplies the number of penetrations through the building envelope. The distributions of air permeability by building use are globally heterogeneous with mean values closer to the 3 rd quartile than the median. We also observe the same heterogeneity as a function of volume classes for office buildings.

Impact of the seasonal variations

The impact of seasonal variations on airtightness was already considered by [START_REF] Kim | Seasonal Variation in Air Tightness of Two Detached Houses[END_REF]) [START_REF] Wahlgren | Seasonal Variation in Airtightness[END_REF]. A variation of the airtightness depending on the season has been observed in particular for the wood structure houses which can be air tighter in summer than in winter. Another study has also shown that the airtightness of a timber building is affected by the indoor relative humidity [START_REF] Domhagen | Air leakage variations due to changes in moisture content in wooden construction-magnitudes and consequences[END_REF].

In order to investigate the seasonal impact on airtightness, we have analysed the seasonal variation of air permeability of single-family houses according to the method of construction and local climate. We have classified houses into 3 categories depending on their sensitivity to moisture and the treatment of airtightness:  wood structure houses where the airtightness is ensured by the vapour barrier;  heavy structure with interior insulation where the air barrier is ensured by plasterboards and mastics at the inside facing of the walls;  heavy structure with exterior insulation where the air barrier can be ensured either by plasterboards and mastics (like interior insulation) or by coating on the masonry. As the relative humidity varies according to the season and the climatic zone, we considered 3 different climates based on the climatic zones of the French EP-regulation: continental climate (H1), oceanic climate (H2), and Mediterranean climate (H3).

Figure 10 to Figure 12 show the seasonal variations of air permeability of single-family dwellings depending on the construction method respectively. For wooden houses, we do not observe seasonal variations for continental and oceanic climates. On the other hand, for the Mediterranean climate, we observe globally higher values for all seasons. In addition, the summer period shows higher values than in winter for the Mediterranean climate. This result is different from what was observed before for wood houses with higher airtightness during summer period. However, in France, wood constructions are almost always done with a vapour barrier, so the impact of wood distention with humidity is assumed to be low which may explain the difference in conclusion. In addition, the number of measurements in the Mediterranean climate is relatively low compared to other climates, which limits the statistical interpretation of these results.

For heavy structure houses with interior insulation, the results do not show seasonal variations for any climate. As for wooden houses, we observe slightly higher values of air permeability for the Mediterranean climate during all seasons. Finally, for heavy structure houses with exterior insulation, the number of measurements is not sufficient to make statistical analyses, in particular for the Mediterranean climate. It seems that houses are less air tighter during mid-season. This results needs to be further investigated.

CONCLUSIONS

Since its creation in 2007, the French database of building airtightness has been annually fed by measurements performed by qualified testers. The total number of measurements is now about 215,000 mostly residential buildings (multi-family and single dwellings). This is due to the fact that the French EP-regulation requires since 2013 a limit for airtightness level for all new dwellings. Therefore, the share of residential buildings with EP-label has dropped since 2014 due to the mandatory requirement. Measurements from 2015 can thus be considered as representative of new French residential buildings.

In single dwellings, the air permeability at 4 Pa (per unit of envelope area) slightly decreases from year to year with a mean value around 0.4 m 3 .h -1 .m -2 in 2016. In multi-family buildings, the yearly mean air permeability fluctuates between 0.60 and 0.65 m 3 .h -1 .m -2 . In nonresidential buildings, it is around 1 m 3 .h -1 .m -2 . However non-residential buildings cover a wide variety of buildings. Offices and schools are the main part of tested non-residential buildings in the database. The air permeability of the different uses is globally equivalent, with the exception of restaurants with slightly higher air permeability. Also air permeability of office buildings is globally equivalent for different volume ranges. Finally, the impact of the seasonal variations on the measured air permeability in single dwellings was analysed depending on climatic zones and buildings construction materials (wood, concrete and brick constructions). An impact of seasonal variations on air permeability is only observed in the case of wood constructions, with slightly higher values during summer in the south of France in particular. This result needs to be deepened with more measurement results especially in the Mediterranean climate.

Figure 1 :

 1 Figure 1:Evolution of the number of building airtightness measurement in France (left) and percentage of measurements depending on the use of the building

Figure 2 :

 2 Figure 2: Number of building airtightness measurements depending on the measurement time (left) and the measured extent of building (right)

Figure 3 :

 3 Figure 3: Percentage of buildings with EP-label

Figure 4 :

 4 Figure 4: Number of building airtightness measurements depending on the buildings main material (left) and ventilation system (right)

Figure 5 :Figure 6 :

 56 Figure 5: Evolution per year of construction of the number of measurements and the mean air permeability in residential buildings (left) and none-residential buildings (right)

Figure 7 :

 7 Figure 7: Distribution of building air permeability measured in non-residential buildings

Figure 8 :Figure 9 :

 89 Figure 8: Descriptive statistics of the volume of non-residential buildings since 2013

Figure 10 :

 10 Figure 10: Variation of air permeability for wood structure houses depending on climate and season (mean value is indicated by the red dashed line)

ACKNOWLEDGEMENTS

The database is the property of the French Ministry in charge of the Construction. The analysis was performed by Cerema. The sole responsibility for the content of this publication lies with the authors.


Bassam Moujalled
email: bassam.moujalled@cerema.fr

Valerie Leprince

Adeline Mélois

Statistical analysis of about 1,300 ductwork airtightness measurements in new French buildings: impacts of the type of ducts and ventilation systems

Keywords: Ductwork airtightness, building, measurements, database, field data

This paper summarizes the most recent results of the French database of ductwork airtightness. This database was created in 2016. It is fed through measurements performed by qualified testers according to a national scheme regarding ductwork. Measurements are mainly performed in building applying for the Effinergie + label which requires class A for ductwork airtightness. Therefore, results discuss in this paper only apply to the buildings of the database and cannot be generalized to all new buildings in France. The database was enriched in 2017 by new measurements, making the total number of measurements around 1300. This enables new analyses regarding the ductwork characteristics and its impact on ductwork airtightness. First, this paper summarizes the recent results regarding main characteristics of the buildings and ventilation systems. Then, the paper analyses the measurement results regarding ductwork airtightness classes depending on several factors, in particular building's use, type of ventilation system, targeted class and the type of ducts. The results show that the number of measurements increases annually in both residential buildings and nonresidential buildings. In particular, measurements in residential buildings are growing fast. In 2016, they represent 73% of the total measurements. Regarding ventilation system, residential buildings, are mainly equipped with single-exhaust ventilation systems (80%), whereas a large part of non-residential buildings (84%) are equipped with balanced ventilation systems. Regarding the type of ducts, flexible ducts are widely used in single dwellings in association with single-exhaust ventilation system (89%). Rigid metallic ducts are mostly used in multi-family and non-residential buildings in association with both single-exhaust and balanced ventilation systems respectively (around 90% for both). Regarding ductwork airtightness performance, it seems to be related to the ventilation system and especially to the type of ducts. Class A is the most frequent result for residential buildings, which are mainly equipped with single-exhaust ventilation system. However, in single dwellings where flexible ducts are mostly used, 55% of measurements achieved Class A or better, against 77% in multi-family buildings with a large part of rigid metallic ducts. In non-residential buildings mainly equipped with a balanced ventilation system and rigid metallic ducts, class B is the most frequent result, and 90% of measurements achieved Class A or better.

INTRODUCTION

Recent studies on ductwork airtightness [START_REF] Berthault | Ductwork airtightness: reliability of measurements and impact on ventilation flowrate and fan energy consumption[END_REF] (Leprince & Carrié, 2017) have shown that a leaky ductwork can multiply by 2 the fan energy use. It also has an impact on indoor air quality if the system no longer provides required air flowrates at air terminal devices. And finally, when the air is pre-conditioned a leaky ductwork also have an impact on heating and cooling loads. Therefore, ductwork airtightness is becoming a key issue to build low energy and healthy buildings. In the context of the French Energy Performance (EP) regulation, the test is only required if a better value than the default value is used in the EP calculation. However, since 2013 to get the French EP-labels "Effinergie+" and "BEPOS Effinergie 2013" (and related subside) it is required to justify by testing that the ductwork airtightness is at least Class A. In both cases the test has to be performed by a qualified tester. According to a survey performed in the context of the Tightvent Airtightness Association Committee (TAAC) (Leprince et al., 2017), France is one of the only European country that has developed a qualification for ductwork airtightness testers (Qualibat 8721) with 82 qualified testers in 2018. Qualified ductwork airtightness testers must register every measurement data and send it to the French qualification body (Qualibat). Field data are gathered by Cerema in a common database since 2017. Currently, 1,306 measurements have been recorded in the database. It includes all the measurements that were performed by the certified testers till the end of 2017. The structure of the database is presented by [START_REF] Bailly Mélois | About 1,000 ductwork airtightness measurements performed in new French buildings: database creation and first analyzes[END_REF]. This article presents a statistical analysis of those field data regarding main characteristics of buildings, ventilation systems and ductworks.

DATABASE OVERVIEW

Figure 1 shows the evolution of the number of ductwork airtightness measurements and the percentage of measurements depending on the use of the building. In 2016, the database was enriched by 696 measurements, which is 89% more than in 2015, making the total number of measurements about 1,306. For 2017, only measurements of January are yet included. The measurements are almost all performed in new buildings (98%). Residential buildings account for 67% of measurements (29% for single-family dwellings with 383 measurements, and 38% for multi-family buildings with 501 measurements), 32% of tests are performed in non-residential buildings (422 measurements). The part of measurements performed in residential buildings is significantly increasing, especially in multi-family buildings. Figure 2 presents the distribution of the measurements number depending on the measurement time and the measured extent of ductworks. It shows that 52% of measurements are performed upon building completion, 29% upon ductworks completion (i.e. the rest of the building is still in progress but the ductwork is completed), and 14% before ductworks completion. Contrary to building airtightness, almost half of the tests are performed before building completion. Therefore, in this database most probably a building appears twice (once before completion and once upon completion) which means that almost all building are "pre-tested" which is wise. Concerning the measured extent of the ductworks, almost all measurements in single-family dwellings are performed on the whole ductworks (61% with extractor, and 29% without extractor). Conversely, more than half of the measurements in multi-family (52%) and nonresidential buildings (64%) are carried out on a part of the ductworks. This results are in accordance with the compulsory measurement protocol which allows measurements based on a sampling method in case of ventilation system serving several dwellings or non-residential buildings (FD E51-767, 2017).

RESULTS

Main characteristics of buildings, ventilation systems and ducts

Figure 3 shows the distributions of measurements according to buildings 'use and EP-label.

As mentioned before, residential buildings are the main part with almost 40% for multi-family buildings. For non-residential buildings, office buildings, schools and hospitals are the main parts with 18% (342 measurements), 8% (143 measurements) and 3% (52 measurements) respectively. Few measurements are performed in hotels, commercial or other non-residential buildings.

Almost half of residential buildings are applying to EP-label, against 25% for non-residential buildings. So the test is either performed to improve the default value in the EP calculation or for quality reasons. French EP-labels "Effinergie+" and "BEPOS Effinergie 2013" represent the main parts of EP-label with 76% and 8% respectively. As mentioned before, they require to achieve at least class A. Figure 4 shows the distributions of measurements according to the type of ventilation system and building's use. Residential buildings are mainly equipped with single-exhaust ventilation system (78% for single-family houses and 85% multi-family buildings). Conversely, nonresidential buildings are mainly equipped with balanced ventilation system (85%).

The proportion of balanced ventilation system in this database is globally higher than its proportion compared to all new buildings in France. According to the building airtightness database (which includes more than 200,000 buildings), only 50 % of non-residential buildings have balanced ventilation, and less than 5% in residential buildings. It makes sense to test more balanced ventilation systems because of the higher impact of ductworks leaks on heat/cooling loads.

Figure 4: Type of ventilation system depending on buildings' use Figure 5 presents the distributions of measurements regarding the type of ducts. Table 1 presents the distribution of the type of ductwork depending on the building's use and the type of ventilation system. Three different types of ducts are mainly used: rigid metallic ducts, semi-rigid ducts, and flexible ducts with either cylindrical or rectangular section. Rigid metallic ducts represent the main part with 61% followed by flexible ducts with 27%. The ducts are mostly circular (79%). The 8% of rectangular ducts are sheet metal ducts. For balanced ventilation systems, metal ducts are mainly used especially in non-residential buildings. For single-exhaust ventilation system, metal ducts are widely used both in nonresidential buildings and multi-family buildings, and flexible ducts in single-family houses.

According to (Bailly Mélois & Moujalled, 2017), this practice is consistent with the type of ducts generally implemented in all buildings in France, as it corresponds to the French standards and professional recommendations.

Results of measured ductwork airtightness

Figure 6 presents the distribution of ductwork airtightness measured classes depending on target classes. The class "2.5*A" is the default value of the French EP-regulation. It can be used in the EP calculation without any justification. More than half of the measurements (56%) are performed without any target class (">2.5*A"). When a target class is selected, class A is the most frequent (19%). This is logical since the class A (or higher) is the class to reach for the EP-labels. It is followed by the class "2.5*A" (16%). Target Classes B and C are less frequent with 6% and 2% respectively. Target classes "A" and "2.5*A" are mostly chosen for residential buildings (almost 90%). Whereas most target classes B and C (90%) are chosen for non-residential buildings.

As shown in Figure 6, the distribution of the specific ductwork airtightness measured class depends on the chosen target class:

-For the target class C, only 55% of measured ductworks achieve class C (mainly offices and hospitals). 23% of measurements achieve class B, and 19% are two classes lower at class A. -For the target class B, the result is slighter better with 63% of measured ductworks achieving class B or higher. 28% of measurements achieve class A, and 8% are at least two classes lower than class B.

-For the target class A, most measured ductworks achieve class A (90%), and 25% achieve a better class. Only 10% do not achieve class A. -For measured ductworks with "no target class" (>2.5*A), the results are quite good as 69% of the measured ductwork reach class A or better. It means that, even with no target class, good results can be achieved thanks to the commitment to a measurement procedure encouraging a careful implementation of ductworks. However, this result only applies to the database and cannot be generalized to all new buildings in France. Figure 7 presents the results of ductwork airtightness measured class in residential and nonresidential buildings. For residential buildings, most measured ductworks achieve class A (46% for single-family houses and 48% for multi-family buildings). In multi-family buildings, 23% of measured ductworks achieved a better class (mainly B), against 11% in single-family houses.

For non-residential buildings, most measured ductworks achieve class B in particular office buildings (55%) and hospitals (44%). Also 15% of measured ductworks in non-residential buildings achieve class C (against 5% in residential buildings). Ductworks in non-residential buildings are overall tighter than in residential buildings. This result is consistent with the target classes of non-residential-buildings where classes B and C are the most targeted. Finally, Figure 8 shows the distributions of ductwork airtightness measured classes depending on type of ventilation systems and implemented ducts. The distribution of the specific ductwork airtightness measured class is clearly depending on the type of ventilation system and especially on the type of ducts:

-For single-exhaust ventilation systems that are widely used in residential buildings, 55% of measured flexible ducts achieve class A or better, against 80% for spiral metal ducts and 70% for sheet metal ducts. Flexible ducts are widely used in single-family houses with single-exhaust ventilation, whereas metal ducts are the most used in multi-family buildings. The wide use of flexible ducts in single-family houses could explain the difference in results between single-family houses and multi-family buildings in Figure 7. -For balanced ventilation system, ductworks are clearly tighter than for single-exhaust ventilation system, especially metal ducts: 68% of flexible ducts achieve class A or better, 87% for spiral metal ducts, and 81% for sheet metal ducts. Balanced ventilation systems are mainly used in non-residential buildings where classes B and C are the most targeted as mentioned before. This is can help to explain the better results in nonresidential buildings.

CONCLUSIONS

The French database of ductwork airtightness has been created nearly two years ago as part of the qualification scheme for ductwork airtightness testers. It now includes almost 1,300 measurements. Although the number of measurements is still low compared to the building airtightness database, the number of ductwork airtightness measurements that are performed by qualified testers continues to increase annually in both residential buildings and nonresidential buildings. In particular, measurements in residential buildings are growing fast. In 2016, they represent 73% of the total measurements. Regarding ventilation system, residential buildings, are mainly equipped with single-exhaust ventilation systems (80%), whereas a large part of non-residential buildings (85%) are equipped with balanced ventilation systems.

Regarding the type of ducts, flexible ducts are widely used in single dwellings in association with single-exhaust ventilation system (89%). Rigid metallic ducts are mostly used in multi-family and non-residential buildings in association with both single-exhaust and balanced ventilation systems respectively. Regarding ductwork airtightness performance, it seems to be related to the ventilation system and especially to the type of ducts. Class A is the most frequent result for residential buildings, which are mainly equipped with single-exhaust ventilation system. However, in single dwellings where flexible ducts are mostly used, 57% of measurements achieved Class A or better, against 76% in multi-family buildings with a large part of rigid metallic ducts. In non-residential buildings mainly equipped with a balanced ventilation system and rigid metallic ducts, class B is the most frequent result, and 86% of measurements achieved Class A or better. All measurements in the database were performed according to specific and not common demands. Thus, all results presented in this paper only apply to the buildings of the database and cannot be generalized to all new buildings in France.

Figure 1 :

 1 Figure 1: Number of ductworks airtightness measurements depending on the construction year and the use of the building

Figure 2 :

 2 Figure 2: Number of ductworks airtightness measurements depending on the measurement time (left) and the measured extent of ductworks (right)

Figure 3 :

 3 Figure 3:Distribution of buildings 'use (left) and EP-label (right)

Figure 5 :

 5 Figure 5:Distribution of type of ducts

Figure 6 :

 6 Figure 6: Distribution of ductwork airtightness measured classes depending on target classes

Figure 7 :

 7 Figure 7: Distribution of ductwork airtightness measured classes in residential buildings (left) and nonresidential buildings (right)

Figure 8 :

 8 Figure 8: Distribution of ductwork airtightness measured classes depending on type of ventilation systems and implemented ducts

Table 1 :

 1 Type of implemented ducts depending on building's use and type of ventilation system

	Type of ventilation	Type of ducts	Office buildings	Schools	Hospitals	Multi-family buildings	Single-family houses
	Balanced ventilation	Metal ducts Semi-rigid ducts	79% (146) 5% (10)	83% (77) 2% (2)	83% (26) 6% (2)	4% (18) 1% (6)	8% (32) 4% (17)
		Flexible ducts	3% (6)	6% (6)	0% (0)	2% (13)	5% (19)
	Single-exhaust ventilation	Metal ducts Semi-rigid ducts Flexible ducts	11% (21) 0% (1) 0% (0)	7% (7) 0% (0) 0% (0)	6% (2) 0% (0) 3% (1)	80% (360) 1% (6) 10% (46)	5% (20) 3% (11) 72% (265)

ACKNOWLEDGEMENTS

The database is the property of the French Ministry in charge of the Construction. The analysis was performed by Cerema. The sole responsibility for the content of this publication lies with the authors.


Gabriel Remion
email: gabriel.remion@cerema.fr

Bassam Mouhalled

Mohamed El Mankibi

Romuald Jobert

Laurent Deleersnyder

Assessing the performance of natural ventilation systems: a review of existing methods

Keywords: Natural airflows, Modelling, Tracer gas methods, Occupant-generated CO2

Natural and Hybrid ventilation systems, by using exclusively or partially natural driving forces, help to reconcile building energy sobriety and good Indoor Air Quality (IAQ). However, in France, Building Regulations restrict the use of natural ventilation by imposing minimum airflows in buildings. Natural ventilation, whose driving forces are atmospheric conditions, has an efficiency depending on the climatic region. Concerning climatic regions where natural ventilation is not likely to provide sufficient airflow, the ventilation system may be featured with a mechanical assistance, which is called Hybrid ventilation systems. Hybrid ventilation systems, taking advantage from natural driving forces when the induced airflow is strong enough, and assisting it otherwise, allow using natural ventilation while verifying airflow requirements. The assessment of mechanical systems performance has already been questioned by the French protocol PROMEVENT. The absence of a similar protocol restricts the development of natural and hybrid ventilation systems. Such a protocol would widen the use of natural ventilation in favourable climatic regions and enable the improvement of the control strategy of hybrid ventilation systems. However, the plurality of openings, variable airflows, and unstable flow patterns make the measurement of the performance of natural and hybrid ventilation systems a challenging task. It represents the objective of the French research project VNAT. This paper is issued from the first task of the VNAT project: the state of the art on the assessment of natural and hybrid ventilation systems. It presents a comprehensive review of existing studies and protocols regarding the measurement of airflow and Air Change Rate (ACR). It appears that direct measurement method of airflow may interfere with the flow pattern, which is troublesome for natural ventilation. Thus, the direct measurement is not likely to be representative. Indirect methods allow measuring Air Change Rate (ACR). They may be conventional tracer gas methods or occupant-generated CO2 methods. Tracer gas methods are numerous and they are based on assumptions which can differ from a method to another. Two of them, called the constant injection and the concentration decay methods, are widely used to characterize the performance of ventilation systems thanks to their ease of implementation. The violation of assumptions leads to important measurement uncertainties. Occupant-generated CO2 methods depend on the occupation rate and on the CO2 emission rate, which induces uncertainties too. These measurement methods as well as tracer gas methods are compared regarding their accuracy and their limits. Modelling methods are also discussed. Results from this paper will help to build a new protocol more suited to the assessment of the performance of hybrid and natural ventilation systems under real conditions.

INTRODUCTION

The building sector is the most consuming sector of European countries, with a share of 42% of the final energy consumption. Natural and Hybrid ventilation systems, taking advantage of natural driving forces, allow to reconcile energy efficient buildings with an acceptable Interior Air Quality. In France, building regulations impose general and permanent airflow rates in buildings. It promotes the use of mechanical ventilation systems in new buildings. Moreover, the protocol PROMEVENT has improved the reliability of their performance evaluation's procedure, which eases the verification of the required hygienic airflows [START_REF] Mouradian | Projet PROMEVENT -Rapport bibliographique : Etat des lieux des protocles et du matériel utilisés pour caractériser les débits de ventilation[END_REF]. Direct airflows or pressure measurements allow to assess the ability of a mechanical system to deliver standardrequired fresh air. Natural and Hybrid ventilation's implementation in new buildings is challenging, as specific procedures are required intending to prove that the system actually leads to the hygienic airflow rate prescribed by regulations. The problem is that it exists no protocol assessing accurately the performance of these kind of systems, such as the protocol PROMEVENT for mechanical ventilation. Under these conditions, the airflow meter could interfere with the unstable flow pattern, which would result in a non-reliable measurement. The other challenge regarding natural systems is that airflows are dependant from several parameters including the building geometry, openings configuration, and climatic conditions. Hybrid ventilation systems add the issue of the plurality of operating modes: pure natural, mixed, or pure mechanical. As a consequence, a measurement alone would only refer to a performance at a given state, which is non-representative of the global performance. Modelling methods seems to be required to allow the extrapolation of the global performance from measurements. Thus, the comprehensive review presented in this paper has two objectives: comparing the accuracy and applications' conditions of measurement methods used to characterize natural airflows, and comparing the suitability of modelling methods to account for natural and hybrid ventilation systems. Concerning measurement methods, tracer gas methods are often used to characterize natural ventilation systems. They are indirect methods based on the concentration of a tracer gas, so they do not interfere with the flow pattern. However, they rely on assumptions whose violation results in an increase of uncertainty. Most critical assumptions are the homogeneity of the tracer gas in the enclosure and the invariant nature of airflow rates during the measurement. The latter restricts the number of suited tracer gas methods. Other measurement methods derived from tracer gas methods are occupant generated CO2 methods. They often require to have access to the occupancy rate, as well as the emission rate. The emission rate, depending on people and activities, introduces another uncertainty source. Also, in most of the methods, the occupancy should not vary during a long time. Regarding modelling methods, they are essentials to assess the performance of natural and hybrid ventilation systems to account for their numerous dependencies. Ventilation modelling methods may come from empirical models, network models or Computational Fluid Dynamics (CFD) models. However, models have to be experimentally validated. They are often validated thanks to measurement methods mentioned above, which may be unreliable regarding the studied case. Their suitability for natural and hybrid ventilation systems will be discussed in the present paper.

We made a comprehensive review of articles focused on the evaluation of methods intending to assess natural ventilation's performance. It includes articles that analyse the accuracy of tracer gas methods, of occupant-generated CO2 methods, and of modelling methods. This state of the art, realised in the context of the VNAT project's first task, aims at identifying methods assessing natural ventilation's performance, as well as their limits. A lot of articles have been tackling the issue of the accuracy of tracer gas methods since 1990. These methods are very interesting for the measurement of natural ventilation airflows, but they are characterized by important uncertainties. Occupant-generated CO2 methods have been first considered in the 80s [START_REF] Turiel | Occupant-generated CO2 as an indicator of ventilation rate[END_REF]. A recent review paper on these methods has been found, especially used in school buildings applications [START_REF] Batterman | Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms[END_REF]. Their accuracy has also been questioned in several papers since the beginning of the 21 st century. Finally, modelling methods also show a great interest, as the performance of natural and hybrid systems depend on several fluctuating parameters. Models only are able to consider this. Network models have been used for a long time, but CFD models are quite recent thanks to the increasing computational resources implemented on computers. Articles treating CFD models are mostly found from 2010.

DESCRIPTION OF MEASUREMENT METHODS

Tracer gas measurement methods

Tracer gas methods are widely used to characterize natural airflows. They have the double advantage not to interfere with the flow pattern and to take into account infiltration and exfiltration flows. Also, they account for the effective ventilation rather than the expected one (Νikolopoulos, 2012). They are based on the concentration of a tracer gas. Three main ways of injecting the gas within the zone are available: delivering it before the measurement, during the measurement at a constant flow rate, or during the measurement at a variable flow rate allowing to keep its concentration at a fixed target value. The first one, called tracer decay method, uses the decrease of the gas to obtain the mean Air Change Rate (ACR). The second one is called the constant injection method and get the airflow from an equilibrium of the concentration occurring after, at least, 3 air changes. The last one is called the constant concentration method and requires sophisticated servo-controlled air mass flow controller looped to a target concentration. Tracer gas methods are based on zonal assumptions including three requirements [START_REF] Sherman | Tracer-Gas Technique For Measuring Ventilation in a Single Zone[END_REF]:

-The tracer gas should be homogeneous in the entire zone; -The zone should be isolated, with an exclusive exchange with the outdoor air; -The outside air should be perfectly mixed within the zone, meaning that no shortcomings should occur.

As a consequence, performing tracer gas measurement in a building would assume the whole building to be a well-mixed single zone, with a homogeneous tracer gas. However, this is possible to perform tracer gas measurements in a multi-zone buildings by two means: either by using multiple gases which result in a matrix illustrating inter-zonal flows and outdoor air flow [START_REF] Roulet | Air Flow Patterns within Buildings Measurement Techniques[END_REF], or by applying the constant concentration method while keeping each neighbouring room at the same concentration level [START_REF] Walker | Field Measurements of Ventilation Rates in Attics[END_REF]. The last technique does not provide information about inter-zonal flows but inhibits its influence [START_REF] Bekö | Diurnal and Seasonal Variation in Air Exchange Rates and Interzonal Airflows Measured by Active and Passive Tracer Gas in Homes[END_REF].

Tracer gas methods ensue from the mass balance equation [START_REF] Sherman | Tracer-Gas Technique For Measuring Ventilation in a Single Zone[END_REF]:

𝑉 𝑑(𝐶(𝑡)) 𝑑𝑡 + 𝑄 * 𝐶(𝑡) = 𝐸(𝑡) (1)
With V the volume, C(t) the concentration, Q the airflow and E the emission rate.

Then, Sherman proposed three techniques for resolving equation ( 1), with their specific assumptions [START_REF] Sherman | Tracer-Gas Technique For Measuring Ventilation in a Single Zone[END_REF]: the regression technique, the integral technique and the averaging technique. The regression and the integral techniques assume steady airflows whereas the averaging technique tolerates variable airflows. We saw above a first source of distinction between methods by ways of dosing the gas. The three resolution techniques result in another distinction inducing sub-methods. The standard ISO 12569 (AFNOR 2017b) describe the application of each method and sub-method to measure airflow rates in a single zone.

Constant concentration method

The constant concentration method may be considered as the reference tracer gas method. Its servo-controlled mass flow controller allows to describe the evolution of the ACH during the measurement thanks to the evolution of the emission rate of the tracer gas. This is the only tracer gas method able to account for a dynamic airflow rate. As the concentration is assumed to remain constant, the resolution of the mass balance equation is immediate, and the airflow is obtained by the ratio of the emission rate by the target concentration value. It can theoretically be computed instantaneously but according to the response time of the system, it is recommended to average the equation on successive periods [START_REF] Chao | Ventilation performance measurement using constant concentration dosing strategy[END_REF]. Chao et al. prescribe to start the injection hours before the beginning of the test. This method requires sophisticated materials and trained staff. Its application is restrained to research projects [START_REF] Bekö | Diurnal and Seasonal Variation in Air Exchange Rates and Interzonal Airflows Measured by Active and Passive Tracer Gas in Homes[END_REF][START_REF] Blomsterberg | Air flows in dwellings-simulations and measurements[END_REF][START_REF] Chao | Ventilation performance measurement using constant concentration dosing strategy[END_REF]. It is often used to assess the accuracy of other tracer gas methods, considering the constant concentration method as the reference value.

Tracer decay method

As aforementioned, for the tracer decay method, the gas is released before the measurement.

When the concentration reaches its expected value, the injection is stopped. It is common to use a mixing fan during the injection and a few minutes later to ensure the homogeneity of the tracer gas before the measurement, which is a requirement of the standard ISO 12569 (AFNOR 2017b). Then, the exponential decrease of the gas provides the mean ACH between the first and the last measurement points. The tracer decay method is the most used tracer gas methods thanks to its ease of implementation and to the small amount of tracer gas required. Two results analyses are then available, based on different resolutions of equation ( 1). The two-points decay method is based on only two measurement points. It ensues from the averaging techniques. On the contrary, the multi-points method represents a least-squares regression, allowing to significantly smooth measurement errors.

Sherman gave the formula for the standard deviation of both methods, ensuing from the error propagation law (M. Sherman 1990). Cui et al. computed it with experimental data and found a difference in the standard deviation of both methods of about 6% (Cui, 2015). However, the least squares regression implies steady airflows, which is hardly suited to natural ventilation whereas the two-points method tolerates variable airflows. Two other sub-methods are possible: the pulse technique and the step-down exhaust concentration method. Because methods require either an identified inlet or outlet, these methods are less suited to natural ventilation systems.

Constant injection method

The constant injection method is also widely used but it requires an important amount of tracer gas. The gas is dosed at a constant flow rate. The constant injection method requires the tracer gas' emission to be homogeneous within the enclosure. A mixing fan should be avoided during the measurement, as it would affect the flow pattern. An important number of dosing points should be used. Just like the concentration decay method, different analysis resulting from different resolutions of equation 1 are possible. The linear regression technique implies to wait for the equilibrium. The accumulation term of the mass balance equation tends to zero after 3 air changes, and the airflow becomes the ratio of the emission rate with the concentration. The averaging resolution technique induces the inverse concentration method and is suited to variable airflows (AFNOR 2017b). It does not require to wait for the equilibrium of the concentration.

A passive method is derived from the constant injection method. It is called the Constant Injection Long-Term Sampling (CILTS) method. A liquid is evaporated at a relatively constant rate through a controlling membrane. They are commonly called "PFT" methods as gases used are PerFreons Tracer gases. The sampling technique may also be passive with absorption tubes, or with sampling bags. This method allows to measure the mean airflow rate during months in occupied houses, and showed a great interest in the literature [START_REF] Bekö | Diurnal and Seasonal Variation in Air Exchange Rates and Interzonal Airflows Measured by Active and Passive Tracer Gas in Homes[END_REF][START_REF] Blomsterberg | Air flows in dwellings-simulations and measurements[END_REF]Okuyam, 2009).

Occupant-generated CO2 methods

Occupant-generated CO2 methods are very interesting because they do not require the injection of any tracer gases, taking advantage of a gas directly produced by people. Turiel et Rudy were of the firsts to identify the potential of occupant-generated CO2 [START_REF] Turiel | Occupant-generated CO2 as an indicator of ventilation rate[END_REF]. Batterman recently presented a review of existing occupant-generated CO2 methods [START_REF] Batterman | Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms[END_REF]. He identified 4 main methods relying on tracer gas methods' principles: the concentration decay, the built-up method, the steady-state method, and the transient mass balance equation method. The main problem arising from these methods is the uncertainty in the estimation of the emission rates which depends on each individuals, occupancy, activity, gender. The approximation, which is often made concerning emission rates, introduces significant errors. Persily et de Jonge proposed an update of the emission rate formula, which had not been revised since 1980 [START_REF] Persily | Carbon Dioxide Generation Rates for Building Occupants[END_REF]. They included, among others, the influence of the gender and of the age. In this respect, it has recently been proven that the old formula overestimated males' emission rate by 15% and females' emission rate by 25%.

Concentration decay

The concentration decay is based on the same principle than the tracer decay method. The measurement begins when occupants leave the studied zone. It has the advantage to be independent from the emission rate. However, it requires a significant rise in CO2 levels in the enclosure. Thus, this method is often used in classrooms or offices [START_REF] Claude-Alain | Simple and Cheap Air Change Rate Measurement Using CO 2 Concentration Decays[END_REF]. The occupancy in those buildings are convenient with period hosting a huge and constant number of occupants, and period where everybody leaves the building like during breaks or at the end of the day. The increase of CO2 must be in the order of several hundred in order to be far enough from typical CO2 measurement errors [START_REF] Batterman | Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms[END_REF].

Steady-state method

The steady-state method is the regression constant injection method. The occupancy should not change during the measurement which has to last at least 3 air changes. This represents a significant obstacle to the method as the equilibrium is very hard to achieve [START_REF] Batterman | Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms[END_REF].

The airflow is often over-estimated with this method.

Build-up method

The build-up method requires a step-up in the occupancy. The occupancy should not vary after the beginning of the measurement. These two requirements are similar to the steady-state method. However, it uses the transient increase of the CO2 concentration and not the steadystate. As a consequence, the measurement requires less time than the steady-state method. A non-linear regression is required, often performed thanks to softwares. Bekö et al. used principally this method to assess the airflow of 500 children bedrooms [START_REF] Bekö | Ventilation Rates in the Bedrooms of 500 Danish Children[END_REF]. [START_REF] Coley | Carbon Dioxide Levels and Ventilation Rates in Schools[END_REF]. The main advantage of this method is that it does not require a fixed occupancy, once the evolution of the number of occupant is recorded [START_REF] Batterman | Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms[END_REF]. However, the airflow rate should be constant during the fitting. If not, the model has to be computed on successive periods, in which the airflow rate is constant.

DESCRIPTION OF MODELLING METHODS

Empirical and Analytical modelling methods

Empirical and analytical models are based on the assumption that the whole building is homogeneous and surrounded by an envelope, on which balance equations may be realized. They aim at characterizing the openings behaviour. The envelope is decomposed of elements that account for energetic transfers, such as walls and roofs, and other elements that allow the air transfer, such as air inlets or windows. Then, the flow crossing openings is characterized by laws connecting pressure and airflows such as 𝑄 = 𝐶 * (∆𝑃) 𝑛 . The most used empirical model is the following [START_REF] Wullens | Étude numérique de la ventilation naturelle, mise en oeuvre d'un modèle fin dans une simulation de thermique du bâtiment[END_REF]:

𝑄 = 𝐴. 𝐶 𝑑 . √ 2.∆𝑃 𝜌 (2)
With A the effective area, Cd the discharge coefficient, ∆𝑃 the pressure difference across the opening, and 𝜌 the density.

These models are used on standards to compute airflows within buildings (ISO 16798-7 2017) (ISO 16798-7, 2017). They are convenient because they can quickly account for the order of magnitude of airflows. It is suited to energetic considerations to assess the heat losses by the air renewing. They require to measure wind speed and direction, outdoor and indoor temperatures, and ideally pressure coefficients. However, the amount of assumptions required to close the system impacts their accuracy [START_REF] Omrani | Natural Ventilation in Multi-Storey Buildings: Design Process and Review of Evaluation Tools[END_REF]. The assumed homogeneity of the whole building introduces other sources of uncertainty. Nevertheless, authors keep trying to improve models, especially for single-sided ventilation and cross-ventilation, in which accurate wind effects are difficult to assess [START_REF] Larsen | Calculation methods for single-sided and natural ventilationsimplified or detailed?[END_REF]. Caciolo et al. developed a new empirical correlation to characterize the flow crossing a window located leeward [START_REF] Caciolo | Development of a New Correlation for Single-Sided Natural Ventilation Adapted to Leeward Conditions[END_REF].

Network modelling methods

Network models consists in dividing the building into zones where each state variable is homogeneous. Zones may be rooms or floors. Mass balance equations are resolved within each zones and they are connected to each other thanks to pressure/airflow laws, or by different models such as wide openings models [START_REF] Wullens | Étude numérique de la ventilation naturelle, mise en oeuvre d'un modèle fin dans une simulation de thermique du bâtiment[END_REF]. Several softwares include multizone network models, such as MATHYS (Demouge, 2017), developed by the french Building's Scientific and Technical Center (CSTB), COMIS, or CONTAM. These models are very convenient as their lightweight allows to simulate during 1 year, describing the evolution of each state values. However, homogeneous zonal assumptions may introduce uncertainties and they cannot give any information about the flow pattern. The assumed pressure field homogeneity is often withdrawn on last versions on softwares aforementioned [START_REF] Wullens | Étude numérique de la ventilation naturelle, mise en oeuvre d'un modèle fin dans une simulation de thermique du bâtiment[END_REF].

Other network models with finer meshes may be realised. They may be called "zonal models". They rely on the resolution of mass and energy balance equations on each zones.

Computational Fluid Dynamics methods

CFD modelling is the most used modelling method to characterize natural airflows. It consists in discretizing the building volume and resolving Navier-Stokes equations on each sub-volume. Thus, they account for the flow pattern within the building, which makes a huge difference with other models. The other main difference is that wind effects are better treated. Two CFD approaches are in use: the Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulations (LES). RANS methods average wind pressure and temperatures to reduce the computational cost. An approximation is then realised by adding a turbulence models intending to keep the dynamic nature of physical mechanisms. Several models are used in the literature and the choice of one of them is often complicated. LES methods reduce the computational cost by ignoring small length scales thanks to low pass filters. The meshing is coarser than RANS models.

CFD models rely on several assumptions about the domain, the mesh, boundaries, solver parameters and the mesh sensitivity ( [START_REF] Hooff | On the Accuracy of CFD Simulations of Cross-Ventilation Flows for a Generic Isolated Building: Comparison of RANS, LES and Experiments[END_REF]. Thus, they need to be experimentally validated. According to Omrani et al., CFD models can be used through two main approaches [START_REF] Omrani | Natural Ventilation in Multi-Storey Buildings: Design Process and Review of Evaluation Tools[END_REF]. The first one is the association of CFD models with measurements. Measurements would allow to validate the model, and to visualize the data computed by the simulation. Once validated, CFD models are very useful to test different parameters, assessing for example the influence of openings types, locations and sizes. The second approach would be to couple CFD models with network models. First, CFD models would accurately compute boundary conditions by modelling the surrounding of the building of interest. Computed boundary conditions can be implemented in the multi-zone models thanks to adequate interface. Then, multi-zone models compute airflows of each room and it saves a huge computational cost [START_REF] Wullens | Étude numérique de la ventilation naturelle, mise en oeuvre d'un modèle fin dans une simulation de thermique du bâtiment[END_REF]. It remains a lot of work to do for those coupled models and we will not further analyse them in the present paper.

Tracer gas measurement uncertainties

Tracer gas measurement methods are subject to several studies, which tackle the issue of their accuracy. These methods are very promising for the characterization of natural airflows as they do not interfere with the flow pattern. However, they rely on assumptions which induce several sources of uncertainties. Table 1 compiles the synthesis of tracer gas methods with their estimated accuracy and their limits.

Concerning the decay method, the homogeneity assumption is assumed to be the most critical as spatial differences in the concentration may be as large as 10% (Cui, 2015). Van Buggenhout et al. found a deviation of the ACH about 86% for one of the sensors, situated near the inlet [START_REF] Van Buggenhout | Influence of sampling positions on accuracy of tracer gas measurements in ventilated spaces[END_REF]. Several studies indicate that the best sampling locations are near the outlet (Cui, 2015;[START_REF] Van Buggenhout | Influence of sampling positions on accuracy of tracer gas measurements in ventilated spaces[END_REF], ensuring a deviation below 10%. However, the outlet is not systematically clearly identified in natural ventilation. Okuyama et Onishi developed a method intending to reduce the influence of inhomogeneity [START_REF] Okuyama | Uncertainty analysis and optimum concentration decay term for air exchange rate measurements: Estimation methods for effective volume and infiltration rate[END_REF]. They performed a sinusoidal injection at the inlet. Another issue in the decay method is the choice between the 2 points and the multi-points decay method. As mentioned above, with steady airflows the multi-points method should be around 6% more accurate (Cui et al. 2015). However, unlike the 2 points method, it does not tolerate variable airflow. Remion et al. showed that a linear variation of 10% of the airflow during the multi-points decay measurement increases the deviation by 3% in controlled conditions (Remion et al. 2018). Anyway, due to its greater accuracy, the multi-points method is often preferred, even to characterize fluctuation airflows [START_REF] Cui | Performance evaluation of natural ventilation through windows with horizontal blade shutters[END_REF][START_REF] Caciolo | Analyse expérimentale et simulation de la ventilation naturelle mono-façade pour le rafraîchissement des immeubles de bureaux[END_REF][START_REF] Gough | Field Measurement of Natural Ventilation Rate in an Idealised Full-Scale Building Located in a Staggered Urban Array: Comparison between Tracer Gas and Pressure-Based Methods[END_REF]. Chao et al. found an overestimation of the multi-points decay method with variable airflows, against the constant concentration method, of 10 to 15% [START_REF] Chao | Ventilation performance measurement using constant concentration dosing strategy[END_REF]. Under steady airflows, Sandberd et Blomqvist found a difference between both methods of 6% [START_REF] Sandberg | A quantitative estimate of the accuracy of tracer gas methods for the determination of the ventilation flow rate in buildings[END_REF].

The Constant Injection Long Term Sampling technique was investigated by Sherman et al. [START_REF] Sherman | Uncertainties in air exchange using continuous-injection, long-term sampling tracer-gas methods[END_REF]. Under ideal conditions, an accuracy of 15% could be expected. On in-situ buildings, with a trained staff, the accuracy decreases to 20 -25%, mainly because of the variation in parameters, such as the airflow rate, the emission rate or the sampling rate.

Carrihlo et al. developed a method based on the daily CO2 cycle [START_REF] Dias Carrilho | Air Exchange Rates from Atmospheric CO2 Daily Cycle[END_REF]. It provided values within 10% of the reference value measured by the occupant-generated CO2 decay.

Occupant-generated CO2 measurement uncertainties

Table 1 also includes the analysis of occupant-CO2 generated tracer gas methods. Batterman et al. realised a comprehensive review of occupant-generated CO2 methods applied in school buildings [START_REF] Batterman | Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms[END_REF]. His conclusion is that the transient mass balance method is very convenient as it is flexible with respect to occupancy. Coley et al. compared the accuracy of this method with the tracer decay measurement performed with SF6. They found for closed windows configuration a deviation of 15% between both methods whereas the deviation increases to 40% with opened windows. Differences are justified by interzonal flows which influence the CO2 measurement as its concentration is higher in buildings than outdoor. Santamouris et al. also reported an accuracy of 15% for the transient mass balance analysis [START_REF] Santamouris | Experimental Investigation of the Air Flow and Indoor Carbon Dioxide Concentration in Classrooms with Intermittent Natural Ventilation[END_REF]. Another advantage of the transient mass balance equation method is that this is the only occupant-generated CO2 method that can account for a fluctuating airflow rate.

When the occupancy does not change, authors often prefer the build-up method rather than the steady-state method because of the significantly reduced required time [START_REF] Hou | a. « Air Change Rates at Night in Northeast Chinese Homes[END_REF][START_REF] Bekö | Ventilation Rates in the Bedrooms of 500 Danish Children[END_REF][START_REF] Bekö | Diurnal and Seasonal Variation in Air Exchange Rates and Interzonal Airflows Measured by Active and Passive Tracer Gas in Homes[END_REF]. Bekö et al. tried each method depending on the occupancy in 500 Danish rooms but often retained only the build-up method. The decay method would require a significant charge in CO2 concentration, which is difficult to obtain with a small occupancy. They estimated the inaccuracy due to interzonal flows thanks to CONTAM, a building simulation software, around 30%. In a later paper, Bekö et al. compared the build-up method with constant concentration method and found that the occupant-generated CO2 method resulted in airflow rates twice as large as those measured by the tracer gas method. Hou et al. investigated Chinese rooms and reported that the build-up method's accuracy has been estimated by Stavova, in a thesis project, around 15% in controlled conditions (Hou et al. 2018b). Lu et al. proposed an improvement of the occupant-generated CO2 decay method by applying the Maximum Likelihood Estimation (MLE) to the decay. It is assumed to be more powerful and to give results faster, which would allow to apply the method without huge rise in CO2 concentration levels [START_REF] Lu | A Novel Methodology for Estimating Space Air Change Rates and Occupant CO2 Generation Rates from Measurements in Mechanically-Ventilated Buildings[END_REF].

Concentratio n decay

Multi-points

No 10 -25% ---Volume < 500 m 3 (Caciolo, 2011); [START_REF] Sandberg | A quantitative estimate of the accuracy of tracer gas methods for the determination of the ventilation flow rate in buildings[END_REF]; (Chao, 2014) 2 points

Yes 15 -30% - (Cui, 2015); [START_REF] Sherman | Tracer-Gas Technique For Measuring Ventilation in a Single Zone[END_REF] Constant injection

Steady

Constant concentration

Yes * 3 -9% 1 ++ -Sophisticated equipment [START_REF] Bekö | Diurnal and Seasonal Variation in Air Exchange Rates and Interzonal Airflows Measured by Active and Passive Tracer Gas in Homes[END_REF]; (Chao, 2014); [START_REF] Blomsterberg | Air flows in dwellings-simulations and measurements[END_REF]; [START_REF] Sandberg | A quantitative estimate of the accuracy of tracer gas methods for the determination of the ventilation flow rate in buildings[END_REF] Occupant-generate CO2 tracer gas methods

Build-up

No 15 -100% ----No change in occupancy -Need emission rate [START_REF] Hou | a. « Air Change Rates at Night in Northeast Chinese Homes[END_REF]; [START_REF] Bekö | Ventilation Rates in the Bedrooms of 500 Danish Children[END_REF]; [START_REF] Bekö | Diurnal and Seasonal Variation in Air Exchange Rates and Interzonal Airflows Measured by Active and Passive Tracer Gas in Homes[END_REF]

Comparison of modelling methods

Omrani et al. performed a comparison of each modelling methods [START_REF] Omrani | Natural Ventilation in Multi-Storey Buildings: Design Process and Review of Evaluation Tools[END_REF]. As many authors agree on, empirical formulas are very convenient as they provide a fast estimation of general airflows in buildings. However, the whole building's homogeneity assumption, and the averaging of the wind introduced in empirical models may cause huge uncertainties. Several studies compared empirical models with tracer gas measurements, especially for single-sided ventilation and cross-ventilation (Gough,2018;[START_REF] Caciolo | Development of a New Correlation for Single-Sided Natural Ventilation Adapted to Leeward Conditions[END_REF][START_REF] Cui | Performance evaluation of natural ventilation through windows with horizontal blade shutters[END_REF].Cui et al. found a difference for cross-ventilation between empirical models and tracer decay method between 45% and 60% depending on correlations [START_REF] Cui | Performance evaluation of natural ventilation through windows with horizontal blade shutters[END_REF]. Gough et al. deduced directly airflows thanks to pressure taps sensors installed across openings [START_REF] Gough | Field Measurement of Natural Ventilation Rate in an Idealised Full-Scale Building Located in a Staggered Urban Array: Comparison between Tracer Gas and Pressure-Based Methods[END_REF]. Comparison with tracer decays showed a weak correlation with a coefficient R² <0,17. Caciolo et al. found differences between empirical models and tracer decay measurement for single-sided ventilation above 25% for every models they tried [START_REF] Caciolo | Analyse expérimentale et simulation de la ventilation naturelle mono-façade pour le rafraîchissement des immeubles de bureaux[END_REF].

Network models are also often validated by means of tracer gases. Belleri et al. used PFT methods to validate a multi-zone model [START_REF] Belleri | Natural Ventilation Design: An Analysis of Predicted and Measured Performance[END_REF]. When windows are closed, the model shows a good agreement with the measurement (5%). However, open windows lead to large deviations between both methods. As for empirical formulas, network models cannot accurately predict the effect of wind. Blomsterberg compared a COMIS network model with constant concentration tracer gas measurement [START_REF] Blomsterberg | Air flows in dwellings-simulations and measurements[END_REF]. He found a difference of 1% for a single-family house and 20% for an apartment. Results from buildings with several floors may be less realistic because standard pressure coefficients are involved in the simulation. However, there is a significant spatial discrepancy of pressure coefficients on façades, especially for highrise buildings. Accurate pressure coefficients can only be obtained in wind tunnels. Zhai et al. compared different network models and found no significant differences between models as they often rely on same physical principles [START_REF] Zhai | Review of Natural Ventilation Models[END_REF]. They seem to work efficiently for simple geometries without open windows. However, they cannot accurately account for wind effects.

CFD models are supposed to give best results as they theoretically resolve every issues mentioned in this paragraph: they do not assume any homogeneity of building zones, they take into account the fluctuating wind, they compute pressure coefficients and they determine the flow pattern inside the building. However, they require such an amount of assumptions, that the validation is even more required than for other models. These assumptions are different than for previous models as they are made by the modeler and not by the modelling software. Validation methods are more diversified than for other models as CFD outputs give a lot more

Concentration decay

No 10 -25% ----Require a huge CO2 charge [START_REF] Turiel | Occupant-generated CO2 as an indicator of ventilation rate[END_REF]; [START_REF] Claude-Alain | Simple and Cheap Air Change Rate Measurement Using CO 2 Concentration Decays[END_REF]

Steady-state

No 10 -30% ++ ---No change in occupancy -Need emission rate [START_REF] Batterman | Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms[END_REF]; [START_REF] Santamouris | Experimental Investigation of the Air Flow and Indoor Carbon Dioxide Concentration in Classrooms with Intermittent Natural Ventilation[END_REF]

Transient mass balance equation

Yes 15 -40% * ---Need the occupancy -Need the emission rate [START_REF] Batterman | Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms[END_REF]; [START_REF] Santamouris | Experimental Investigation of the Air Flow and Indoor Carbon Dioxide Concentration in Classrooms with Intermittent Natural Ventilation[END_REF] information. Validation may be done by means of tracer gas (Νikolopoulos, 2012), by measurement of state parameters [START_REF] Elshafei | Numerical and Experimental Investigations of the Impacts of Window Parameters on Indoor Natural Ventilation in a Residential Building[END_REF], by direct measurement of airflow in an opening ( [START_REF] Hooff | On the Accuracy of CFD Simulations of Cross-Ventilation Flows for a Generic Isolated Building: Comparison of RANS, LES and Experiments[END_REF], and finally by Particle Image Velocimetry (PIV) measurement in an opening [START_REF] Bangalee | Flow Visualization, PIV Measurement and CFD Calculation for Fluid-Driven Natural Cross-Ventilation in a Scale Model[END_REF]. The higher accuracy of CFD models is induced by way more sophisticated models requiring a huge computational time, even more for LES models which last between 80 and 100 times longer than RANS models ( [START_REF] Hooff | On the Accuracy of CFD Simulations of Cross-Ventilation Flows for a Generic Isolated Building: Comparison of RANS, LES and Experiments[END_REF]. Van Hooff et al. also compared the accuracy of RANS and LES models and found that all results where within 8,6% of the reference value.

CFD models are the most accurate and give the more information which is very useful to treat the air distribution's function of ventilation systems but they require significant computational resources. Also, caution have to be given to CFD results as modelling choices may introduce significant impacts. The validation is challenging but crucial. Network models may be convenient compromises between computational cost and accuracy. However, they hardly account for wind effects. Empirical and analytical methods are convenient for quick singlezones airflow's estimation. Finally, models' validation is very challenging for natural ventilation systems for the moment because of the lack of an accurate natural airflow's measurement method, as seen in 5.1 and 5.2.

CONCLUSION

A comprehensive review has been done on articles treating the assessment of natural and hybrid ventilation's performance. Two main methods were identified to characterize natural airflows, including tracer gas methods and occupant-generated CO2 methods. Modelling methods characterizing natural ventilation systems, as a whole, were also analysed.

Regarding the in-situ measurement of natural airflows, tracer gas methods seem to be promising but only a few are suited to variable airflows. Moreover, they are characterized by important uncertainties because of the violation of assumptions, the required homogeneity of the tracer gas, and measurement uncertainties. The constant concentration method would be ideal as it provides the evolution of the ACH, but its implementation on in-situ buildings in a reasonable cost is nearly impossible. Occupant-generated CO2 methods take the advantage of tracer gas principles, but have the great benefit not to require any dosing instruments or gases. However, neither the emission rate has to be accurately determined, which is challenging, nor the rise in CO2 concentration levels has to be significant, meaning a long or important occupancy preceding the measurement. Moreover, 3 out of 4 occupant-generated CO2 methods require strict conditions concerning the occupancy.

To account for the dependency of natural and hybrid ventilation systems, modelling methods are essential but their validation is very complicated as no measurement methods provide accurate ACH results. Network models are convenient for simple geometries but are hardly suited to account for wind effects. CFD models may be more accurate, but rely on several modeller-made assumptions, which increases the importance of the validation. They may be validated by other means than tracer gases, but often in wind-tunnel with reduced-scales buildings. Also, they require a huge computational cost, which restrains their use for research projects.

Finally, the development of a protocol assessing the global performance of natural and hybrid ventilation systems could come from the coupled measurement/modelling approach, which is the aim of the french VNAT project. It intends to overcome the difficulty to measure natural airflow while allowing to extrapolate the performance's assessment from measurements of indicators chosen thanks to a sensitivity analysis.

  Hou et al. also used this technique to investigate almost 400 child bedrooms in northern China(Hou et al. 2018a).Transient mass balance equation methodThe transient mass balance equation uses computer programs performing a non-linear fit. The occupancy can change but must be recorded as it represents input of the model. The emission rate as well as the airflow rate is deduced from the model. Coley et Besteiner performed it in classrooms

Table 1 :

 1 Limits of main natural ventilation's performance measurement methods

			Tracer gas methods
					Measure
	Method	Sub-method	Fluctuatin g ACH	Accurac y	-ment	Cos t	Other limits References
					time

ACKNOWLEDGEMENTS

This work is supported by the French ministry of building construction. Being part of the VNAT project, it is also supported by ADEME, the French environment and energy management agency.