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.

Localised paths of increased heat flow through an insulated construction where a more conductive element, such as a metal fixing, rafter or parapet junction, bypasses the insulation. Thermal bridges increase heat loss and can produce cold internal surfaces that lead to condensation and mould growth.

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.

A roof construction in which the insulation is placed at ceiling level below the structural deck, leaving the deck and the void above it at close to external temperature. The void normally requires ventilation to remove moisture vapour, and this arrangement is discouraged for flat roofs in current UK guidance because of condensation risk.