Airtightness of cross-laminated timber envelopes: Influence of moisture content, indoor humidity, orientation, and assembly

Abstract

Decrease in the relative humidity (RH) of the indoor air causes cracks in the cross-laminated timber (CLT) surface resulting in air leakages in the CLT panel. The effect of construction moisture on CLT air tightness properties is not clearly defined, which is important for moisture safe construction. The aim of this study was to investigate the effect of high MC on CLT air tightness properties compared to conventional factors such as indoor RH change, insulation type and façade orientation. An air permeability test was carried out on 12 different test walls with CLT as the airtight layer exposed to indoor environmental conditions. Test walls were constructed from CLT panels with two different initial moisture content (MC) values, ≈13%, and ≈26%, and in addition, different insulation materials were used in the wall assemblies: mineral wool, cellulose wool and polyisocyanurate (PIR) plates. Air leakage measurements were carried out at two different times, first in mid-autumn and second at the end of winter. Based on the results of this research it was concluded that high initial moisture content in the CLT panels significantly weakens the airtightness of the external wall. CLT exposure to water and wetting the CLT panels during construction carries more than a risk of moisture damage, and the planning and implementation of moisture safety in the construction of CLT buildings is therefore essential if the CLT is to be considered as an airtight layer. The effect of a change in indoor RH is small on the air permeability properties of the CLT wall. Therefore the 5-layer CLT panel can be considered and used as an air-tight layer in external wall construction as long as its initial low moisture content (about 13%) is maintained during both construction and service life.