Assessment of long-term and mid-term building airtightness durability: field study of 61 French low energy single-family dwellings

The French ongoing research project “Durabilit'air” (2016-2019) aims at improving our knowledge on the variation of buildings airtightness through onsite measurement and accelerated ageing in laboratory controlled conditions. This paper presents the final results of the second task of the project. This task deals with the quantification and qualification of the durability of building airtightness of single detached houses. It is done through field measurement at mid-term (MT) and long-term (LT) scales.  
We first present the field measurement protocol. For the MT campaign, a sample of 30 new single-detached houses has been selected nationwide. During the study, the airtightness of each building was measured once per year over a 3-year period. A part of this sample (5 houses) was also measured twice per year during two different seasons in order to investigate the impact of seasonal variation. In addition, the air permeability of a window was measured once per year over the 3-year period in 5 houses. The LT campaign was carried out with a second sample of 31 existing single-detached houses constructed during the last 10 years. The airtightness of each house was measured once. 
A specific measurement protocol was defined after a detailed literature review. The main challenge is to understand the variations of the airtightness and to identify whether it is related to the products/assembly ageing, the maintenance conditions or other factors such as the occupants’ behaviour. The protocol is based on the standard ISO 9972 for the measurement method of building air permeability with additional requirements for the measurement conditions. It also includes a detailed qualitative leakage detection and questionnaire for occupants. Secondly, this paper presents the construction characteristics of both samples. All houses were tested upon completion. The air changes per hour at 50 Pa pressure difference (n50) of both samples show the same mean value of 1.4 h-1, with larger variations among the LT sample. 
Finally, we discuss measurement results. Regarding MT sample, the air permeability slightly increases during the first year (mean increase by 18%), and then stabilizes during the second and third year. However, for some houses with exposed timber framing, n50 has increased by more than 100%. Regarding LT campaign, the air permeability (n50) show a similar increase after 3-10 years with a mean value of 20%. Measurements performed during two different seasons did not show a significant impact of seasonal variation. The results show globally an increase in the number of detected leakages for all houses, but this increase is not always correlated with the change in air permeability. For 10 houses of both samples, the building airtightness has improved. For 6 houses, this improvement is maybe due to the building material (wood), the maintenance of windows, or the sealing of leaks by occupants but for 4 houses, we have not been able to explain this improvement.