Hygrothermal performance of highly insulated external walls subjected to indoor air exfiltration

Abstract

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°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 (Γ) were 20%–22% and 1%–10% for the HI and baseline (BL) walls. The HI walls that presented Γ 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°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 Γ values of 1% were exposed to a high relative humidity and even interstitial condensation because the DPD values were often below −2°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°C last for a maximum of about 1 month every year.

Study of ventilated low-slope and large span wooden element roofs in the current and future climate

“…The review states that several different factors have a great influence on ventilation ranging from the details of a structure to climatic factors such as temperature and wind speed. The studies presented suggest recommended theoretical values for air change rate (ACH [1/h]) for the considered situations such as 20 1/h [3] and values applied in simulation for similar structures such as 30 1/h [4]. However, values presented in these studies cannot be directly applied for the evaluation of the ACH of the ventilation gap in studied large span roofs.…”

“…In terms of the hygrothermal performance of ventilated structures, the airtightness of the vapor barrier has been identified as a significant factor, as Viljanen has summarized [3]. This is taken into account in the Finnish guidelines [5], stating that maximum air infiltration must be less than 4 (q50 [m 3 /(m 2 h)]) and the recommendation is 1.…”