Cavity Wall Insulation Top Up When Replacing Window Cills

12 Jan 2010
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United Kingdom
Hi All,

I'd appreciate some advice please

I am replacing a window cill in an old house it has a 75mm ish cavity, there no cavity wall closures so will stuff in a bit of spare celotex I have left over.

Looking down the cavity it's above the first floor and under the bedroom window, it has cavity wall blown wool type cavity insulation many years ago (brick and block wall), which has either not been done 100% or settled,, there is a gap of a few feet, I cant get celotex all the way down as it has wall ties...

An insulation firm suggested I cannot top this up and I need to suck it all out and have the cavity wall insulation re done which I don't really want to do.

I have some left over Knauf Ekoroll Loft Insulation which is a sort of re-used glass fibre roll type material,,

Can I just push some of this in before I add the new window cill is this ok or does it need to be special waterproof stuff??

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Glass fibre is OK, but if you leave it at 4" thick when you come to a wall tie, it will be problematic,a s you will have to push it down either side of any tie. it might be better tearing the GF into small pieces and stuff them down with a garden cane or such like.
On the basis that you have a brick outer leaf, the existing cavity "could" be subject to damp, particularly across the wall ties. This risk I believe is trying to retrospectively instilling insulation that could bridge the cavity. You may end up with a better u-value by adding insulation, but risk damp penetration. A cavity with no insulation is better than a cavity with damp insulation.

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