Vapour Barrier For Warm Flat Roof

Ideally you need to ventilate the roof and have a 50mm air gap above your insulation.

Vapour barrier for warm flat roof.

A roof design that includes an adhered roof membrane with multiple layers of insulation with board joints offset and staggered over a vapor retarder air barrier helps lower the risk that air and the moisture it carries will infiltrate the roof system. A vapour barrier sometimes referred to as vapour retarder is typically a plastic or foil sheet used for damp proofing to prevent interstitial condensation from forming in various building assemblies such as walls roofs foundations and floors. That reduction of air and moisture infiltration can then help improve roof longevity. If it is not a habitable space or as per the old design for flat cold roofs then you would use foil backed plasterboard which acts as a vb.

It is frightening indeed that construction practices can be so dramatically. Condensation is generated when warm moist air rises and condenses into a liquid on contact with the cooler areas above the insulation. Often the vcl in a warm roof is bonded to a timber substrate which helps the integrity however in a warm roof the vcl has two important functions. The buildup of a warm flat roof starts with a good vapour barrier followed by insulation then more decking or just the roofing materials depends on if you want the roof soft or solid.

A vapour control layer helps you shield your building from the consequences of condensation. The history of cold climate vapor barriers itself is a story based more on personalities than physics. The vcl is easier to maintain in a warm roof construction as it s generally out the way of the dangers of plasterboard fixings and downlighters. Much like the cold roof it stops the transfer of warm moist air from inside the building into the warm roof layers.

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. Rose 1997 regales readers of this history. A vapour control layer or vcl on a warm flat roof sits between the roof decking and the insulation.