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Warm deck vapour barrier - what's the difference?

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26 Sep 2025
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We're having a warm deck flat roof on a new & existing extension and the builder has turned up with what looks like any other DPM. He said it's what he uses on the roof as the vapour barrier.

A Google search does give this an option when searching for "roof vapour barrier" but also various foil based vapour barriers, both loosefit and self-adhesive ones.

So what's the difference and does it really matter? I'm inclined to buy the "better" version (if there is such a thing) for him to fit but I'm not sure if there is any benefit? The overall space is almost 70 sq meters so there is a notable price difference (and amount of work too).

What the builder has on sight:
1760120220541.png


Others available (Screwfix) - I'm thinking this would need to be cut to size, laid and foil tapped down - and is loose-fit?

1760120165011.png


What I see in lots of YouTube videos recently - self adhesive version which requires a premier I understand?

1760120192532.png
 
Last edited:
Moisture still has to escape through a warm deck, the vcl afaik prevents build up of condensation while allowing the roof to breath. Your builder presumably thinks his spec is correct to achieve this balance and meet building regs.
 
We're having a warm deck flat roof on a new & existing extension and the builder has turned up with what looks like any other DPM. He said it's what he uses on the roof as the vapour barrier.

A Google search does give this an option when searching for "roof vapour barrier" but also various foil based vapour barriers, both loosefit and self-adhesive ones.

So what's the difference and does it really matter? I'm inclined to buy the "better" version (if there is such a thing) for him to fit but I'm not sure if there is any benefit? The overall space is almost 70 sq meters so there is a notable price difference (and amount of work too).

What the builder has on sight:
View attachment 395303

Others available (Screwfix) - I'm thinking this would need to be cut to size, laid and foil tapped down - and is loose-fit?

View attachment 395301

What I see in lots of YouTube videos recently - self adhesive version which requires a premier I understand?

View attachment 395302
It's fine.

Also use Duplex plasterboards for belt and braces approach.
 

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