Timber frame wall: Hygrothermal properties and vapour barrier damage

Abstract

A study of moisture distribution in plywood sheathing, and the heat transfer processes, in an experimental timber frame wall (unvented), has been carried out under steady state simulated winter conditions of 20°C indoors, and 0°C outdoors. The experiments were performed with intact and perforated vapour check/plasterboard inner linings, under 'dry' (25% RH), and 'wet' indoor conditions (60% RH). In the absence of a vapour check, 'wet' conditions would cause condensation on the plywood sheathing. Under these conditions, sheathing moisture content is increased when the inner lining is perforated, the strongest concentrations occuring opposite the holes. With two 20 mm perforations, inducing moist air circulation effects, the sheathing moisture opposite the holes was found to be significantly more than that for one 20 mm perforation, approaching levels necessary to sustain decay. Small changes of 7% in the thermal transmittance of the wall, were measured under 'wet' conditions and with the lining perforated. An apparent reduction in the transmittance was observed when the wall contained two perforations, due to vertical transfer of heat resulting from air movement through the insulation.