Component materials with timber frame
In a timber frame wall construction, the windtight layer (also known as the breather membrane) can be a rigid board or membrane, which will need taping to eliminate thermal bypass. Modern ‘vapour open’ but windtight and water-resistant materials allow any moisture within the fabric to evaporate harmlessly to the outside.
Insulation in timber frame walls can be of any type, either flexible insulation can be used between studs to cope with the thermal movement of the timber, minimising the possibility of air pockets which can lead to thermal bypass, or if rigid board can be used between timber studs, in which case careful installation is essential to ensure all gaps are fully filled.
The airtightness, or vapour control, layer is often achieved by using OSB board, well taped at the joints. However OSB is not supplied as an airtightness product and can vary in its effectiveness; it is therefore recommended to use at least an 18mm board. Polythene sheeting is a costeffective barrier, but may be difficult to install into the difficult corners to achieve a good seal. The main problem with polythene is its vulnerability to damage during the build process and also during any subsequent alteration works. Modern reinforced purpose-made membranes, with the appropriate tapes, are the safest route in achieving the desired airtightness.