celotex and condensation

Hollie6464 · 14 Jul 2020

On a dormer bungalow conversation the builder has put 90mm celotex on a 90mm stud wall. All joins are taped.

To the outside is ply sheeting, a breathable vapour barrier and then hanging tiles.

To the inside is std 12.5mm plasterboard and plaster.

It has been drawn to our attention that there should also be a vapour barer to the inside as well to prevent condensation and damp which is now causing some concern.

How much or a risk is this?

Appreciate any help/advice anyone can offer.

Many thanks,
Hollie

Notch7 · 14 Jul 2020

Did the builder tape across each stud to continue the foil across the whole wall?

If so that will act as a vapour barrier.

Was the insulation cut for any services like cables or pipes?

Did the builder use foil backed plasterboard?

Hollie6464 · 14 Jul 2020

Thanks so much for your reply.

Yes, there was tape across all joins and studwork. It was also taped where it was cut for rafters, pipes and sockets. So as you looked in the rooms it was all silver!

It was not however taped where it was cut out from the cold side for the cables to sit in from behind.

We asked the plaster about silver backed plasterborard, but he said it was not needed because the celotex is foil backed, so only std plasterboard went on.

Notch7 · 14 Jul 2020

Hollie6464 said:
Thanks so much for your reply.

Yes, there was tape across all joins and studwork. It was also taped where it was cut for rafters, pipes and sockets. So as you looked in the rooms it was all silver!

It was not however taped where it was cut out from the cold side for the cables to sit in from behind.

We asked the plaster about silver backed plasterborard, but he said it was not needed because the celotex is foil backed, so only std plasterboard went on.

The foil on celetex is a vapour barrier and since all joints have been taped, you have a continuous vapour barrier. I personally think it's a more robust detail than using a vapour barrier sheet because it's difficult to seal where it gets cut around Back boxes. I suppose doing both would be considered belt and braces, but I don't see any risk of interstitial condensation in your project. It seems to me the builder has been thorough.

^woody^ · 14 Jul 2020

Risk is 0-1%

Hollie6464 · 14 Jul 2020

Thank you so much for your thoughts and providing the peace of mind we needed. Thank you!

JP_ · 15 Jul 2020

Make sure the windows have trickle vents open - i had a loft conversion like this, window always got condensation if vent want open. Especially if you have en suite, even with extractor. Nowhere else for moisture to escape.

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celotex and condensation

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