Vapour Control Layer Cold Flat Roof

An effective vapour control layer at ceiling level an unobstructed 50mm ventilation space above the insulation.

Vapour control layer cold flat roof.

Be it a cold or warm roof construction the primary function of a vcl is to keep warm moist air from inside the building inside the building. Cold flat roofs are difficult to detail correctly but where used they should be in accordance with bs 5250 and have. The roof is an unventilated warm roof design finished with a single ply membrane. Breather membranes are positioned to the outside of the insulation acting as a weather barrier while still allowing moisture particles to escape from the inside.

The architect plans stated requirement for a vapour control layer below the insulation or a 3 in 1 insulation board including vapour control layer and an additional foil backed plasterboard. It s the first line of defence against high vapour pressure air entering the roof structure and causing interstitial condensation. The notion behind a vapour control layer is to install it on the room side of the insulation so it stops the passage of warm moist air entering the structure. It has transpired that there is no vapour control layer fitted.

Condensation is generated when warm moist air rises and condenses into a liquid on contact with the cooler areas above the insulation. A vapour control layer or vcl on a warm flat roof sits between the roof decking and the insulation.