Sarking is used to prevent wind driven rain blowing under the weatherboard lap. It is essential that sarking be used where this is likely to be a problem, for example, in upper storey extensions. Sarking is recommended under Weathertex cladding, but it must be a vapour permeable (breather) grade. Vapour permeable sarking such as Bradford Enviroseal Wall Breather Foil may be used directly under Weathertex weatherboards. Foil laminated sarking will also act as a reflective insulation. For further information please refer to the Weathertex Installation Guide. In cold climates, a vapour barrier should be used on the inside of the frame or on the internal wall to prevent transmission of water vapour to the wall cavity, for example, from clothes and dish washing, unflued gas heaters etc. Subsequent condensation can cause decay of the timber frame. Expert advice should be sought on this aspect of building design. Typical "R" values of wall systems are given in the Weathertex Installation Guide.