Water vapour permeability of Cross-Laminated Timber: preliminary results on the impact of adhesive layers

Abstract

The hygrothermal performance of Cross-Laminated Timber (CLT) envelopes differ fundamentally from timber-frame constructions due to their composite structure. Despite this, CLT is often modelled as solid timber, presuming minor impact of the adhesive layers. This study investigates the water vapour permeability of CLT and its dependence on wood moisture content (MC), with particular focus on adhesive type and amount. Cup tests were conducted on solid spruce and glued timber specimens representing CLT using polyurethane (PUR), melamine–urea–formaldehyde (MUF), and emulsion polymer isocyanate (EPI) adhesives. Results show that adhesive layers increase water vapour resistance primarily at low relative humidity, and thus at low wood MC, with EPI exhibiting the highest resistance, while differences largely decrease at high RH. The amount of adhesive had no systematic effect, whereas application method can influence the vapour permeability. The derived material data and analytical functions provide improved input for future hygrothermal simulations of CLT envelopes.