Air leakage variations due to changes in moisture content in wooden construction - magnitudes and consequences

The airtightness of buildings is important for several reasons, such as being a prerequisite for low-energy buildings and for a healthy indoor air quality (without i.e. mould or radon). The airtightness of buildings can vary over time and investigations are made on these variations due to moisture induced movements in wooden constructions, and subsequent consequences, using both measurements and numerical simulations.
Measurements were performed in a wooden guest house that was built in a laboratory hall with an approximate relative humidity of 30 %. The guest house was designed with a wind barrier but without a moisture barrier, and with a majority of the leakages situated in the wall/floor connection. The relative humidity in the guest house was varied so that the indoor relative humidity was kept at 90 % during 8 days and then decreased to 25 % during 7 days. This variation in moisture content in the wooden part can also illustrate the built in moisture in construction timber (starting at a moisture content of 16%). The air permeability was measured frequently during both periods and showed a change in air permeability from 0.74 l/sm2 to 1.21 l/sm2 at 50 Pa pressure difference. Consequently, for a wooden construction with a moisture dependent air permeability, it is easier to fulfill airtightness demands (checked by measurements), when the building is just erected, compared to a couple of months later.
Numerical simulations on the moisture induced leakage variations and the impact of the resulting variation in air permeability are performed in Simulink and MATLAB. Air leakage is calculated using a set of object oriented functions within the MATLAB environment. These functions follow the same mathematical principle as presented in the airflow simulation software CONTAM. The simulations are made for the climate of Gothenburg in south-west part of Sweden. Results show, among other things, that the orientation of the building is decisive for the magnitude of the total exfiltration rate and that air leakage from the indoor environment up to the cold attic is higher during winter months compared to summer months.