Journal
JOURNAL OF BUILDING PHYSICS
Volume 46, Issue 2, Pages 967-1021Publisher
SAGE PUBLICATIONS LTD
DOI: 10.1177/17442591221109956
Keywords
Exfiltration; air leakage; moisture convection; highly insulated; dew point; mold growth risk
Categories
Funding
- Ramboll Foundation/Ramboll Finland Oy
- Aalto University School of Engineering
- Fabian and Jaakko Ahvenainen Fund
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This study shows that highly insulated walls with a thermal resistance ratio of at least 20% are resistant to air exfiltration. The moisture accumulation rate in wood-based material has a negative linear correlation with the dew point depression value. Even for low dew point depression values, significant moisture accumulation does not occur if the exterior sheathing is vapor permeable. However, walls with lower thermal resistance ratios and dew point depression values may pose a risk for mold growth.
The study comprises three laboratory tests in which typical Finnish highly insulated (HI) walls were exposed to concentrated leakages of indoor air under steady outdoor temperatures of 1-5 degrees C. Airflows with a relative humidity of 50% and at rates of 1-3 L/min were directed close to the wooden frames inside the walls. The thermal resistance ratios between the exterior sheathing(s) and the whole wall (Gamma) were 20%-22% and 1%-10% for the HI and baseline (BL) walls. The HI walls that presented Gamma values of at least 20% were observed to be resistant to air exfiltration, and their durability was not affected by the addition of a gypsum sheathing outside the wooden frame or a more permeable vapor retarder. This is related to the negative linear correlation that exists between the moisture accumulation rate in wood-based material and the dew point depression (DPD) value. The developed approach, called the DPD method, shows that a significant degree of moisture accumulation does not occur even for DPD values of as low as -2 degrees C if the exterior sheathing is vapor permeable. The airflow does not penetrate into the rigid mineral wool sheathing, which helps to avoid interstitial condensation. Regardless of thermal transmittance, the HI and BL walls with maximum Gamma values of 1% were exposed to a high relative humidity and even interstitial condensation because the DPD values were often below -2 degrees C. For these walls, the mold index analysis and visual observations confirmed the local risk for mold growth on the opposite side of the leakage point. In practice, long-term mold growth may be limited if the seasonal periods during which the outdoor temperature is 1-5 degrees C last for a maximum of about 1 month every year.
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