Vapour barriers / vapour checks

Vapour barriers and checks prevent the ingress of moisture into the thermal insulation and, as an airtight layer, prevent air from flowing through the thermally-insulated roof structure. They have different sd values (diffusion-equivalent air layer thickness). Vapour barriers have an sd value of ≤ 1,500 m; vapour checks have an sd value of between 0.5 and < 1,500 m. Whether to choose a vapour barrier or a vapour check is a decision that depends on the materials used.