Hygrothermal performance of wood-framed, mineral-wool-insulated walls and roofs with low thermal transmittance

Low thermal transmittance of external walls and roofs supports sustainability and energy savings, but the improved insulation efficiency may increase moisture problems in these building components. As the performance of the highly insulated (HI) building envelope subjected solely to water vapour diffusion has been found acceptable, this study analyses the consequences of built-in moisture, rain leakage and indoor air exfiltration for HI building envelopes and assesses the hygrothermal conditions and moisture safety of the ventilation cavities of HI walls and roofs. The extensive experiments of the thesis focused on mineral-wool-insulated wood-framed external walls and roofs with thermal transmittances of 0.12–0.13 W/(m2K) and 0.08 W/(m2K), respectively, and with wind barriers that had water vapour diffusion equivalent air layer thickness (sd) of 0.1–0.2 m. The experimental results received indicated that exfiltration is the most potential source of moisture to deteriorate the hygric performance of the wall studied. However, no connection was observed between the thermal transmittance and hygric performance of a wall that was subjected to air leakage. This also applied to the performance of the walls during the drying in the built-in moisture tests: all the walls dried to the state of equilibrium without significant risk of moisture problems. The HI walls had a slightly better tolerance to rain leakage than the baseline wall. The results of the wall experiments support the use of an insulating exterior sheathing in HI walls. Such a sheathing protects the wall against air exfiltration and decreases the level of relative humidity during the drying of the wall. The results indicated that a ratio of the thermal resistance of the exterior sheathing to that of the whole HI wall should be at least 10%. The measurements revealed that in the ventilation cavities of HI walls the hygrothermal conditions were the weakest at the bottom of the cavity. The performance of the cavities of the HI roofs studied was satisfactory at most, but in the future, the performance will deteriorate if climate changes as is predicted. Based on these results, it is recommended to use mould resistant materials in the cavity of HI walls and roofs, to use effective vapour barriers in HI roofs (sd at least 50 m), and to monitor the performance of the cavity of HI structures regularly. Based on the numerical analysis conducted, if necessary, the hygrothermal performance of HI roofs can be improved by using quintuple thermal resistance compared to the conventional level of the structural members above the ventilation cavity such as 0.13 (m2K)/W and by restricting the ventilation openings in windy regions so that the air change rate is about 20 1/h.

Department dc:contributor.department

Rakennustekniikan laitos

Grantor dc:publisher

Aalto University

Year dc:date.issued

2023

Author dc:creator

• Viljanen, Klaus

Advisors dc:contributor.advisor

• Lü, Xiaoshu, Prof., Vaasa University, Finland
• Puttonen, Jari, Prof., Aalto University, Department of Civil Engineering, Finland

Contributors dc:contributor

• Aalto-yliopisto
• Aalto University

Language dc:language.iso

en

Repository record dc:identifier.uri

https://aaltodoc.aalto.fi/handle/123456789/121180

OAI record oai:identifier

https://aaltodoc.aalto.fi/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Aaaltodoc.aalto.fi%3A123456789%2F121180