Consequences of Varying Airtightness in Wooden Buildings

Previous research has shown that the airtightness in wooden buildings can vary over the year and that buildings can be less airtight during wintertime, likely caused by variations in humidity. In order to investigate possible causes and consequences, such as increased energy use and moisture damage, a number of numerical simulations and laboratory measurements have been performed. In a wooden guesthouse, without a moisture barrier, the indoor relative humidity is kept at 90 % during 8 days and then decreased to 25 % during 7 days. The airtightness is measured frequently during both periods and shows a change in airtightness from 0.74 l/sm2 to 1.21 l/sm2 at 50 Pa pressure difference. The moisture scenario can be related to common levels of inbuilt moisture as well as moisture loads from indoor activities. In order to investigate the consequences of varying airtightness, a numerical model is set-up to resemble a typical wooden detached house. Airflows and pressure profiles are then calculated for different values of airtightness. Simulations show that the total increase in exfiltration is dependent not only on airtightness value but also on leakage distribution. For example, exfiltration will increase with 1.9 % per percentage point of decrease in airtightness, if the distribution of leakages are concentrated to the lower parts of the building, compared to 1.6 % if leakages are more evenly distributed. Interestingly, results also show that if the cold attic is accessible through an attic hatch that is not airtight, air is likely to leak from the indoor environment to the attic during some periods of the year and downward to the indoor environment during other periods. Results show that variations in airtightness in wooden houses is affected by surrounding humidity and that the consequences of varying airtightness will affect both moisture and energy performance