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Can 100mm polystyrene sheets serve as cavity wall insulation instead of rockwool?

Hi - I have a build about to start and my builder is wanting to use Rockwool slabs in the cavity wall. I have read that Rockwall or mineral wool can sag and even act as a straw to bridge the wall and create damp - and - once damp actually reverse the insulation and make the house colder. I expressed my worry to the builder but he said he has never had a problem. Hoever - Im still worried about the rockwool touching the outside wall and bridging the cavity. So... My gut feeling is to buy Celotex type product as boards which are foil backed/vapour barrier but they are twice the price. However - my research has led to me wondering if Polystryene boards are of use. I do understand the insulation quality will be less than celotex initially. Can it be used? Eg - something like this:

1 Answer from a MyBuilder Insulation Installer

Best Answer

you can use a slab type insulation like kingspan or celotex and 25mm would be sufficient . 100mm cavity is too wide in reality your cavity is usually no more than 70mm


Answered 5th Dec 2016

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