Insulating external stud wall

3 Dec 2015
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United Kingdom
We have a large timber framed dormer. It is made of standard 4"x2" studwork and rendered onto lath (which is a bit the worse for wear - but that's another job).

Anyway, I'd like to get some insulation in as the internal plaster is off at the moment. So what is the best thing to use. I was planning to stuff it with rock wool and put some 50mm insulation backed plasterboard on ths inside.
But then I started reading about VCLs and aslso started wondering whether it wanted to be filled with 100mm celotex, not rock wool.

Any advice would be much appreciated,
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VCLs get important in modern houses with draught-sealed spaces- any condensation will stick around. Older houses with draughty roofs don't have the same issues.

Strictly speaking, since you're upgrading/renewing a significant percentage of a thermal element in a room your job is notifiable and building regs apply, especially required insulation values. The target for walls can usually be met with 75mm Celotex/equivalent or 150mm polystyrene or rockwool or earthwool. If you have the depth available, rockwool is a lot easier and cheaper than the solid stuff. Have a read here for what's meant to happen page 25 onwards is the fun stuff.
100mm Celotex in the frame (no air gaps) and 25mm minimum across the face before plasterboard.
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Thanks fellas.

Having had a poke around it seems that condensation from within is the least of my worries. Roll on Spring I think I'll be having a fair bit of rotten wood and leaky render replaced...

For the time being I'm going to use 100mm celotex in the gaps as I have some arriving tomorrow which I was going to use elsewhere. That and some insulated plasterboard should more than meet building regs U-values.

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