wall insulation in new extension

20 Nov 2012
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United Kingdom
Dear All, we are doing a ground floor extension on the house (side and rear) next year. I wanted to ask what the norm is when insulating cavity wall (brick outside and block inside). The cavity is 100mm - is it normal to fill the cavity completely with insulation? architect is specifying full fill cavity 100mm dritherm 32. I have read (online, books, etc.) that it might be better to have 50mm insulation for example and leave a gap in the cavity in case water were to penetrate the outerleaf of the cavity wall. I just want to make sure that the specs are all in line with what the normal approach to doing a standard home extension is. Many thanks in advance for any replies and advice.
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You can use full fill if you like. The batts are designed with the fibres going one way to stop moisture penetration. Some builders prefer part fill, so it's a matter of personel choice.
Nowt wrong with your architects spec. As mentioned you can do either provided the right insulation for each scenario is specified, which has in your case.
Aah OK, many thanks for your reply. When a builder does go part fill, I guess they still need to ensure that the insulation effectiveness is still the same as a 100mm batt right? Is the insulation something that I can rely on bulding control to check (to make sure builder is using correct spec of part fill insulation)? Thanks
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Check it yourself. Part fill will be insulation boards. Building Control can miss stuff. You might not see the BCO too often.
thanks, so what u value or specification is required to be compliant with today's building standards? (so I can check whether the insulation being used is not inferior to the insulation provided by a 100mm batt. Many thanks
A 50mm board, such as Kingspan or Celotex will be roughly equivalent to the fullfill batts in terms of insulation performance.
Personally I would use full-fill; as Stuart has advised, they are designed to repel water.
Rigid boards have to be carefully fitted to be effective and they are less forgiving of indifferent workmanship.
Thanks All for your input, we'll use full fill then - seems like the standard thing to do.

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