NCC Compliance: Roofspace Ventilation and Timber Sheathing Boards
Tech Note 08
It is not technically incorrect to describe any timber sheathing board as being vapour permeable, but the question remains just how vapour permeable the different boards are. Many factors such as thickness, board type, number of layers of adhesive, timber type, adhesive used, moisture content and manufacturing homogeneity, all have an impact on the vapour permeability of a given product or manufacturing batch. It is important to have this tested value as an input for the hygrothermal modeling that is needed to prepare a Performance Solution.