Vapour control layer

Buildings with a level of internal humidity which is higher than normal may require a vapour control layer. The purpose of this is to prevent moisture passing from the inside of the building into the roof construction. Unless this can be achieved, interstitial condensation will form in adverse climatic conditions. The vapour control layer must be positioned close to the warm internal face of the insulation. This can be achieved by using an effectively sealed liner sheet, or by including a separate membrane with a high resistance to vapour. In some cases, both methods may be adopted.

It is worth remembering that site workmanship and conditions can seriously affect the performance of a vapour control layer. The performance of a vapour control layer can also be adversely affected by the poor fixing of throughpenetrations and roof lights and by poor detailing around them. The correct use of seals and filler blocks, together with appropriate details, will have a marked effect on the overall performance of the vapour control layer.