Vapour barriers for cold roofs is one of the top questions I'm asked by email and on the phone.
What is the correct vapour barrier for a cold flat roof? The answer is the one that's on the specifications for the project you are building. If you are building a domestic building in the UK you need Building Control and possibly planning permission. To get through Building Control you should have some sort of drawings and some specifications (these specifications should have all the details you need to construct the building, however normally they have been cut back to keep costs down). The drawings will help to get you through Building Control but what you really need are construction drawings with specifications containing details on them. A lot of the time the details are missing. This omission makes up a large part of the problem we have in construction today; without the details and specifications you don't know what air Vapour Control Layer (AVCL) or vapour barrier to use. Depending on the buildup of the roof or the wall, the designers should have done dew point calculations, and worked out the resistance needed to stop condensation forming on the inside of the building. This is called interstitial condensation when it forms in a wall or roof structure. There are many vapour barriers on the market, some have very high resistances to moisture and some low, all are vapour control layers and nearly all of them also control the movement of air. This is what we call the Air Vapour Control Layers.
If you use a vapour barrier that's very strong, i.e. it stops a lot of vapour passing through a wall or a ceiling into a roof, then you need to deal with the moisture left in the room with ventilation of some kind. If however, the vapour barrier you use is too weak and moisture gets through into the buildup of the wall or roof, then it may get stuck there and cause problems like rot in the internal timbers or the back of the roof. With a cold roof it's essential to make sure that above the insulation and below the roof there is a good vented void to remove any moisture that makes it up into the construction. The vented void can then allow the wind to blow the moisture out, basically it's a second line of defence.

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Steven Dickinson
London flat roofing
07802300099
info@londonflatroofing.co.uk

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