Why a Vapour Control Layer is Essential in a Flat Roof

Vapour Control Layers For Flat Roofs

Why a Vapour Control Layer is Essential in a Flat Roof

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

A vapour control layer helps you shield your building from the consequences of condensation. Condensation is generated when warm moist air rises and condenses into a liquid on contact with the cooler areas above the insulation. 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.

Why is a VCL so Important in a Flat Roof

A VCL is a very important component in your build up acting as one of the protective layers that minimises the amount of warm moist air that comes into a construction aspect. A VCL in conjunction with the correct use of venting and membranes will essentially eradicate the risk of interstitial condensation. It’s the two design fundamentals working collectively that greatly reduce the destructive effects of condensation on the structure.

Consider it like this; a jumper will keep you lovely and warm but a jumper AND a water-resistant coat will give you a better possibility of keeping warm and dry in a storm.

A VCL prevents damp and mould which generally, helps to prevent major respiratory problems, according to the NHS. As well as major health ramifications, condensation can also cause structural problems, weakening wooden framed structures and corroding other materials. An additional major benefit of the VCL is that it will prevent installed insulation becoming damp and losing it’s thermal properties.

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  • David Anderson Reply

    Hello Steve,
    Thanks for all your great video’s for a warm flat roof, however what I am not sure about is my builder has only bent up the vapour barrier which sites on the SOB layer to level with the top of the insulation and it is not taped to the built up timber kerb it is just loose will this be ok or should it be taped up against the kerb, or go right over the wooden kerb, I will have a layer of OSB on top of the insulation and will put a layer of EPDM to finish the roof.
    Many Thanks
    David

    11th May 2019 at 3:06 pm

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