Hygrothermal Behaviour of Ventilation Cavities in Highly Insulated Envelopes

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.

Factors affecting the performance of ventilation cavities in highly insulated assemblies

“…It has been experimentally found that the temperature in the cavities of HI walls is close to, but slightly above, the outdoor temperature (Viljanen et al, 2020). The difference in the thermal behavior leads to a slightly higher relative humidity (RH) in the cavities of HI assemblies compared to a baseline (BL) wall, which may increase the risk of moisture damage (Viljanen et al, 2020). However, mold growth was observed at the bottom of the cavity battens of both HI and BL walls, for which the fundamental causes remained unresolved.…”

“…The recession classes are 1 (strong), 0.5 (significant), 0.25 (relatively low), and 0.1 (almost no decline). The values for the sensitivity and recession classes represented in Figures 5(d), 6, and 8(d), were selected based on the materials in the assemblies (Viljanen et al, 2020). A laboratory test was assembled to define the pressure losses of some common cavity and gap types of walls and roofs ( Figure 3).…”

“…Nevertheless, the restriction of the cavity ventilation should be carefully considered, because it requires very low moisture transfer to the cavity, and thus, for example, the sensitivity of the structure to rain leakages increases. The drying stage of construction moisture may also weaken the cavity performance of the HI roofs (Harderup and Arfvidsson, 2013;Viljanen et al, 2020); therefore, the restriction of the ventilation should be implemented only after the initial drying period. The use of a roof underlay, which in the test roof was partially represented by the coated wind barrier wool, equally prevents wind washing of the thermal insulation.…”

“…However, as a low U value decreases the temperature on the other side of an HI assembly, the thermal conditions in the cavity are close to those of outdoor air (Technical Research Centre of Finland, 2008). It has been experimentally found that the temperature in the cavities of HI walls is close to, but slightly above, the outdoor temperature (Viljanen et al, 2020). The difference in the thermal behavior leads to a slightly higher relative humidity (RH) in the cavities of HI assemblies compared to a baseline (BL) wall, which may increase the risk of moisture damage (Viljanen et al, 2020).…”

“…The risk of moisture damage is increased in HI roof structures, where the cavity temperature and RH are even closer to the outdoor level than it is in walls (Viljanen et al, 2020). Thermal conditions may be deteriorated further by thermal radiation toward the sky at night.…”