Studies on Building Air Leakage -a transient pressurisation method, measurements and modelling
Doctoral thesis, 2007
Airtightness of the building envelope is a key factor for good performance of a building. The main part of this thesis deals with the exploration of a transient pressurisation method for air leakage measurements.
In the transient method, an enclosure is pressurised to a desired pressure. The air flow into the enclosure is then stopped. The pressure starts to decline due to air leakage through the walls of the enclosure. The pressure decline is measured. The air leakage rate, as a function of the excess pressure, is obtained from a formula that involves the time derivative of the pressure. The formula also involves air heat capacity, potential volume change due to excess pressure, and heat exchange between air and enclosure. The main benefit of the method, compared to the static method, is that it eliminates the need of airflow meters. It also gives a continuous relation between pressure and air leakage rate. The method is validated against the common static method, and against empirical values of air leakage through sharp-edged openings. The agreement is very good. The development of the method involves studies and analysis of curve fitting, volume change, heat flux, and oscillations due to inertia of the airflow. The particular complication of an unknown volume change is overcome by performing two measurements, one without and one with an added opening with known leakage.
The method is applied to cases of air leakage through a wood frame wall, an attic floor and a knee wall construction. The measurements are performed in a laboratory environment. The results from these measurements show that the method can be used also in cases of large and non-linear volume change of the enclosure. Transient opening and closing of leakage paths may be detected. The testing of the building components also show that it is quite easy to make these components sufficiently airtight, with moderate efforts only.
The method is also applied to air leakage through well-defined leakage paths. Air leakage through these paths is also simulated with a CFD-tool. Simulations and measurements show in most cases very good agreement. In cases involving porous materials, even a small void in the insulation near an inlet or outlet opening may have a large impact on the air leakage rate.
A second part of the thesis deals with modelling of air leakage in a single-family house. The modelling shows that the ventilation system has a large impact on the result. The models also show that wind is a mitigating factor, in most cases, regarding transport of indoor air to the attic. In cases of indoor air transport to the attic it is necessary to ventilate the attic. In a climate similar to the one in Gothenburg, ventilation of an attic can, however, cause moisture problems. There is a delicate balance between providing sufficient ventilation of the attic and too much ventilation.
air change rate
VK, Sven Hultins gata 6, Chalmers tekniska högskola
Opponent: Johnny Kronvall, Professor, Teknik och samhälle, Byggteknik och arkitektur, Malmö tekniska högskola, Sverige