Warm Flat Roof Construction Vapour Barrier

A vapour control layer or vcl on a warm flat roof sits between the roof decking and the insulation.

Warm flat roof construction vapour barrier.

Grp insulated decking board ecotherm insulation 120mm with 6mm ply attached firrings joists plasterboard i have read that i need a vapour barrier. 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. For all warm flat roofs a fully intact vapour barrier would be required on the room side under the insulation. Provided your vapour control layer is intact then there should not be any risk of condensation.

A vapour barrier will then need to be installed to reduce condensation in the roof structure. Condensation within a flat roof mainly occurs during cold weather when moisture vapour in the air which has been generated within the heated building rises from the room below into the cold roof void above the ceiling. When the temperature of the vapour falls to or below its dew point the water vapour condenses on cold surfaces. Ideally you need to ventilate the roof and have a 50mm air gap above your insulation.

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. 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. If this is a habitable room beneath the roof then you should be constructing a warm roof as per building regulations c l.

Where in the roof construction should this be installed. 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. 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.

A vapour control layer helps you shield your building from the consequences of condensation. 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.