Hygrothermal Behaviour of Ventilation Cavities in Highly Insulated Envelopes

¹ Department of Civil Engineering, Aalto University, Finland
² Ramboll Finland Oy, Finland
³ Department of Electrical Engineering and Energy Technology, University of Vaasa, Finland
⁴ Department College of Construction Engineering, Jilin University, China
⁵ Key Laboratory of Ministry of Land and Resources on Complicated Conditions Drilling Technology, China

^* Corresponding author: klaus.viljanen@aalto.fi

Abstract

This article presents long-term experimental studies on the moisture safety in the ventilation cavities of highly insulated (HI) structures. The tested HI-walls had thermal transmittances of 0.11-0.13 W/m²K. A wall with a thermal transmittance of 0.23 W/m²K represented the baseline wall in the test. In addition to walls, an HI-roof of a newly built house with a U-value of 0.08 W/m²K was measured. The results indicate that, in the ventilation cavity, the relative humidity of an HI-wall exceeds 1-7% of the humidity measured from the baseline wall during winter, which coincides with the 0.4-1.5ºC lower temperatures observed in the HI-walls. The mold risk in the ventilation cavities of the walls is low, as the value of the mold index (MI) remains below one, which indicates small amounts of microscopic mold only on surfaces. However, at the bottom of the cavity, the MI value reaches 1.4 due to lower temperatures. In the HI-roof, the MI values are between 1.0 and 2.0 in the middle of the cavity in winter. The reasons for the higher mold risk of the roof are the humid weather, the built-in moisture of the roof and the low heat flux from inside. The study confirms that, in the future, warmer weather and increased humidity can increase moisture risks in the ventilation cavities. The results support the use of materials that are more resistant to mold in the outer parts of structures.