Cold roof

A roof construction in which the thermal insulation is placed above the structural deck, keeping the deck and supporting structure at close to internal temperature. A vapour control layer is normally installed on the warm side of the insulation to limit the risk of interstitial condensation.

A membrane or sheet installed on the warm side of insulation to restrict the passage of water vapour from inside the building into the roof or wall construction, reducing the risk of interstitial condensation. Materials range from polyethylene sheet to bitumen membranes and foil composites, and effectiveness depends on sealed laps, edges and penetrations.

Condensation that occurs within the layers of a roof, wall or floor construction rather than on its internal surface, when water vapour moving through the build-up reaches a plane at or below its dew point temperature. It can degrade insulation, corrode fixings and decay timber, and is controlled through vapour control layers, ventilation and correct layer sequencing.

A vapour-permeable but water-resistant membrane used on the cold side of insulation, typically as a pitched roof underlay or behind cladding, allowing water vapour to escape from the construction while resisting liquid water from outside.