Yearly Variations in Airtightness of Detached Wooden Houses: Simulations and Laboratory Measurements to Investigate Causes and Consequences

Abstract

It is well known that leaky buildings have higher risks of acquiring moisture problems, are more draughty and are harder to make energy efficient compared to more airtight buildings. Previous research has shown that the airtightness varies over the year and that buildings can be less airtight during winter months. The aim of this thesis is therefore to investigate possible consequences and causes of variations in airtightness of detached wooden houses. Investigations are performed both with laboratory measurements as well as with computer calculations. The thesis starts with describing the theory relevant for the calculations and analysis performed. Following chapters describe methods and results from performed investigations. For a chosen number of common construction details the change of leakage through the detail as a function of relative humidity is calculated. Results show that for the ideal cases, the leakage changes drastically when relativity humidity changes. One such example is the floor-wall connection where the airflow increases from 2.4 l/sm to 6.7 l/sm when the relative humidity inside the building drops from 55 % to 23 %. Laboratory measurements are conducted on a wooden guest house. The relative humidity inside the guest house is increased to 90 % during an 8-day period and then decreased to 25 % during a 7-day period. The airtightness is measured several times during these two periods with the use of blower door equipment. Results show that the airtightness changed from 0.74 l/sm^2 when the indoor air was more humid to 1.21 l/sm^2 when the indoor air was more dry. A computer model is set-up to resemble a typical wooden detached house. Airflows and pressure profiles are then calculated for different values of airtightness. Results show, for instance, that for every percentage point of decrease in airtightness the total exfiltration may increase with 1.9 %. Results from the simulations also show that a decrease in airtightness increases the flow of indoor air up to the attic if an attic hatch is present. Conclusions should not be interpreted as generic but rather as indications of likely outcomes. In order to more fully understand the impact that varying airtightness has on building performance more in-depth research is needed.