Insulating behind UPVC Cladding

30 Sep 2008
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United Kingdom
At the front of my 1970's house I have a UPVC clad section of wall I would like to insulate.

Currently it's just a 2x4 frame with no insulation on the external leaf and a blockwork internal leaf. I'm replacing the windows in the cladded section so figure I may as well add some insulation as part of the job.

I'm looking for a bit of advice on how to go about doing this. Do I just fit 100mm Insulation boards so that they are flush with the front of the timber frame and maintain the cavity behind, then put the cladding back on? Do I need to add any kind of vapour layer before the cladding goes back on?
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Well, I am no expert, but I think you need to be careful when adding vapour barriers as in some houses the walls need to breath. I did read up on external v internal insulation and while external was more effective, it costs more and can lead to moisture problems if alternative ventilation is not available.

As for cavity, sounds like you want them fixed to the blockwork wall with little to no air penetration, otherwise the cold air will just get around them? Lots of foam.
The rest of the house has a 50mm cavity, which is why I'm unsure on full filling the void or maintaining this cavity.
Any vapour barrier goed on the inside behind the plaster board. A breathable membrane (or polythene for a small area) would go on the outside of the frame.
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I mean't breathable membrane, I've a huge roll of Tyvek that wasn't used on a roofing job I assume would be ok to use?

What about the insulation board, should it be fully filled back to the internal blockwork or should I maintain the 50mm cavity?
If the cavity is continuous with the other walls then it should be maintained. If it's a distinct section closed off at each side it can be filled, but this will rely on good detailing on the other face of the frame and above, to stop any water getting in.

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