In some wall and roof build-ups a carrier board is used to take the internal plaster or external render finish. In a natural building, a vapour-open carrier board is needed to maintain a vapour-permeable build-up, with a lime plaster applied as the final finish. These boards can be constructed from a range of different materials, depending on the specific use, but all have a relatively open structure that ensures vapour can easily pass through. When selecting a carrier board, it is important to consider several factors. For example; Is the application internal or external? Will the board also be giving racking resistance to a timber frame? Will it also be providing insulation? Does it need to form a curve? Will it be used as a permanent shuttering for hempcrete? In the case of permanent shuttering for hempcrete the board must be suitable for large volumes of water and vapour to dry through it quickly.
Installation of vapour-open carrier boards is similar to that of gypsum plasterboard, although some of them look very different to plasterboard, and certain types may require special fixings because of the structure of the board fabric. In general, it’s recommended to use stainless steel / other non-ferrous fixings for carrier boards where a lime plaster is to be applied. Otherwise, the highly alkaline lime will quickly rust standard screws and the expanding corrosion will damage the plaster. Good quality galvanised fixings may suffice inside, but externally stainless steel is a must.
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