Warm Flat Roof Construction Vapour Barrier

Condensation is generated when warm moist air rises and condenses into a liquid on contact with the cooler areas above the insulation.

Warm flat roof construction vapour barrier.

A vapour barrier will then need to be installed to reduce condensation in the roof structure. 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. A vapour control layer or vcl on a warm flat roof sits between the roof decking and the insulation.

Where in the roof construction should this be installed. 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. 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. Ideally you need to ventilate the roof and have a 50mm air gap above your insulation.

When the temperature of the vapour falls to or below its dew point the water vapour condenses on cold surfaces. Because of this the vcl always sits at the warm side of the insulation. 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. 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. Provided your vapour control layer is intact then there should not be any risk of condensation. 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. For all warm flat roofs a fully intact vapour barrier would be required on the room side under the 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. There is no requirement for any air gaps. 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.