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

Klaus Viljanen*, Jari Puttonen, Xiaoshu Lu

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

1 Citation (Scopus)
136 Downloads (Pure)


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.

Original languageEnglish
Pages (from-to)967-1021
Number of pages55
JournalJournal of Building Physics
Issue number2
Early online date10 Aug 2022
Publication statusPublished - Sept 2022
MoE publication typeA1 Journal article-refereed


  • Exfiltration
  • air leakage
  • moisture convection
  • highly insulated
  • dew point
  • mold growth risk
  • WOOD


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