Loft insulation celotex

23 Aug 2013
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United Kingdom
Hello, parts of my loft are boarded for storage without the leg raisers which allow you to go for thicker loft insulation.

I’d like to take the floorboards up, then put celotex insulation down, then screw boards back down.

What thickness celotex board do I need to achieve the recommended equivalent to 270mm wool insulation?

What I’d then like to do everywhere else is lift up the wool insulation, replace with celotex, then lay the insulation back on top (except where there’s floor boards already)

This is for insulation on the loft floor between the joists, not the roof.

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My advice: do not use celotex.
Rockwool is perfect for the job and a lot cheaper, so i wouldn't replace it.
If you really have to, use quinn board insulation which is not flammable.
Celotex burns like petrol.
If you don't believe it, take a piece from any broken slab in large builders merchant and try.
I did on a friend's suggestion and I was shocked that celotex is still on the market.
They must have changed the formula since the grenfell tower disaster.
I tried it myself and it caught fire like a haystack soaked in petrol.
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I believe PIR performance is around double that of mineral wool - leave 100mm wool between the joists and put 100mm celotex over. Top up the existing wool with the left over where you're not flooring.

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