Warm Flat Roof Construction Vapour Barrier

It s the first line of defence against high vapour pressure air entering the roof structure and causing interstitial condensation.

Warm flat roof construction vapour barrier.

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. I am having an extension built and my builder is constructing the flat warm deck roof in the following way. There is no requirement for any air gaps. A vapour control layer or vcl on a warm flat roof sits between the roof decking and the insulation.

Provided your vapour control layer is intact then there should not be any risk of condensation. When the temperature of the vapour falls to or below its dew point the water vapour condenses on cold surfaces. 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. If this is a habitable room beneath the roof then you should be constructing a warm roof as per building regulations c l.

A vapour barrier will then need to be installed to reduce condensation in the roof structure. Where in the roof construction should this be installed. To use epdm for a warm deck flat roof construction solid closed cell roof insulation such as kingspan or celotex should be placed on top of the roof deck rather than in between the rafters. Ideally you need to ventilate the roof and have a 50mm air gap above your insulation.

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. Condensation is generated when warm moist air rises and condenses into a liquid on contact with the cooler areas above the insulation. Because of this the vcl always sits at the warm side of the insulation. I was looking to use this vapour barrier from screwfix but not sure if it s suitable for the job it only mentions underfloor heating and laminate flooring in the description.

Grp insulated decking board ecotherm insulation 120mm with 6mm ply attached firrings joists plasterboard i have read that i need a vapour barrier. A vapour control layer helps you shield your building from the consequences of condensation. I m in the process of building a warm flat roof for a garden room and need to put down a vapour barrier between the sub deck and insulation.