Cavity wall insulaation for new extension

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the cavity of the walls on my new extension is 100mm,
I have been looking for 95 or 90 mm celotex broads but unable to find on ebay. but there are plenty of 50mm. I appreciate help with the following :-

what is the best cavity wall insulation ?
is 50mm celotex is equivalent to 100mm knauf slabs
 
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you can't use 100mm celotex in a 100mm cavity, you need 50mm space on the cold side as standard. Or minimum 25mm at a push. You can use wool slabs and full fill though which would be the equivalent.
 
the simple reason is because it's not approved for use in that way bt the manufacturer.
The real reason I assume is both because celotex needs to be kept dry, and water could track across any joints to the inside leaf. The outer leaf of the caity needs to be able to drain, as it can get water penetration and possibly even condensation.
 
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If you look at the websites of insulation companies they have technical data sheets giving their recommended types of insulation and 'U' values for different cavity wall situations. Your building regs application should specify the insulation required for your new extension.
 
Yes, every bricky will run a mile faced with having to use full fill rigid stuff. Bonkers.
Agree with that, although it's quite popular round here on Random rubble stonework of different thicknesses as it saves using a backing wall.
 

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