Component materials with timber frame
This section looks at the types of materials used with timber frame construction, to make up the rest of the walls.
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.
Advantages and disadvantages of timber frame construction
The key advantages of timber framing are the speed of erection and the ability to easily monitor the quality of insulation and membranes. Airtightness testing can be carried out at an early stage of the build process, when any faults can be rectified relatively easily.
Disadvantages of timber frame walls are the need for additional studs to perform a structural role (e.g. around window openings), which create extra thermal bridging. When timber frame is used with a brick or stone rainscreen, there is a danger of moisture build up in the timber frame particularly during the summer months when strong sunshine can drive moisture from a rain soaked masonry façade into the cavity (known as ‘solar driven moisture’). This situation can be prevented and controlled by ensuring that there is an adequately ventilated cavity, but research in this area is still in its infancy.