Insulation behind uPVC cladding

25 Jul 2007
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United Kingdom
My 70s house is part clad in uPVC shiplap. This is mounted to wooden batterns which are mounted to the breezeblock, and then internally this is plastered. I'd like to insulate behind the uPVC as the wall is facing north and north east and is very cold.

Can I fix the celotex type insulation direct to the external breeze block wall, then the cladding over that, or do I need a vapour barrier or ventilation somewhere? To my knowlegde, the cold side will be the exterior of the celotex, directly behind the uPVC so any condensation will be there rather than close to the block work, so this method should be ok.

Also, will I be able to remove the uPVC and refit it, or will it break/weaken etc. so that I'll have to fit new cladding.
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Do you like the UPVC? I have something similar on a house I am working on. I plan to replace the UPVC with external insulation and smooth render.

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It's not great, but I don't mind it really. Every house in the street has it, so it would look very odd if we had something different.
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If anyone has any other info about this it would be great, as I've been doing a lot of reading and still not got the perfect answer. Some cladding manufacturers say there should be an air gap behind the cladding of 25 mm, but my existing installation seems to be blockwork, 19mm battens, then cladding. I reckon I can fit some Kingspan K5 type insulation which is 20mm between the battens. This seems to be external grade, so shouldn't need a vapour barrier, but it would mean the cladding is touching the insulation. What I don't know is if this matters? If condensation forms, wouldn't this be on the back of the cladding, or on the insulation face behind the cladding? Is this a problem?
20mm of PIR isn't worth the trouble of pulling the cladding off.

If you don't have an air gap then the battens won't be able to breathe, which isn't ideal. The PIR and UPVC cladding should be fine though.
Is there any reason you can't have another 2-3cm when you reattach the cladding? 20mm will halve the U-value but its not going to be noticeable. 50mm is really the minimum you should be thinking about. It will bring the wall up to 1990s standards.
Can anyone shed anymore light on this?

I'm in our similar situation. The timber cladding on our 70's home as seen better days and needs replacing. I was hoping to insulate behind if possible when replacing this year.

Would it be breeze block, vapour barrier, battens, 50mm celetox or similar, allow 25mm air gap, cladding.

External insulation and render. It will need building control and may need planning but it will transform your home
External insulation and render. It will need building control and may need planning but it will transform your home

I only plan on changing the timber cladding like for like.

Just wanted to know the correct way to insulate behind if possible.


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