Insulating an interior wall of a cavity wall

22 Nov 2010
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United Kingdom
Hello all,

Looking for some guidance here. We are having an extension built onto our 1930's house (semi). The new walls are built from block with a 100mm cavity. The cavity is filled with 50mm celotex. Once finished, the exterior walls will be rendered.

My question, is it worth insulating the inside of the wall too, prior to any plaster/plasterboard finish?

I understand that there is a danger of condensation if the interior insulation is greater than the cavity (50mm) so a vapour barrier is required. But is it even worth considering or is 50mm celotex in the cavity sufficient?

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Not really, the easiest/cheapest way to reduce the heat loss in that wall construction would be to increase the cavity insulation thickness to say 70mm, that way the only additional cost is the thicker insulation and slightly longer wall ties but at least there's no additional labour fixing another board inside.
Thanks for that. The walls are already up and 50mm celotex already in place. Roof is almost complete and I'm turning my head to the interior.

It's a big enough space to not worry about insulating the interior - I just wanted to know if it was worth it.

From what you say, it doesn't sound like it is.
I wouldn’t bother tbh, the heat will only escape via the existing poorly insulated walls/roof etc anyway.
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The roof has 140mm of celotex in it.

The floor 100mm celotex.

The doors which are the folding sliding type have a value of 1.2 - it seems that anything I do to the walls is negated by one side of the extension being glass.

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