# Cavity Wall Insulation - what's best?



## telekon (30 Mar 2010)

I've read a lot of threads here and elsewhere about cavity wall insulation, they've been very helpful and definately ruled out some options, but i'm still not sure on what's best.

The architect has drawn up plans with a 125mm cavity, fitted with 80mm Kingspan TW50. Now, since then i've been reading a lot about thermal looping and the u values that you can get with 80mm of kingspan, so i've decided that partial fill cavity walls are a no-no.

It also seems that all the partial fill vendors suggest that you dry-line to get decent u values. A lot of forum contributors suggest this as well, but i thought there would be issues with cold bridging at the junctions of internal and external walls, and condensation due to changing the dew point of the wall? Also, won't you have a better finish to the wall with 10mm plaster on the blockwork rather than layers of plasterboard skimmed over?

So, my engineer has suggested that we just pump fill the cavity afterwards with bonded beads. Advantages being that you fill the cavity, and it gets everywhere so you can be sure the cavity is properly insulated. I'm not sure about this approach either, and i had actually thought that one of the issues was that you can't be sure it gets everywhere? However one advantage of this approach is that we could use xtratherm thin-r range - i like the idea of tongue & groove panels and special corner fitting panels.

There were two other options i had found to avoid the thermal looping problem. One is a product from aeroboard, aeromark platinum. A 120mm board would give a wall with something like 0.22u - this wouldn't be as good as what xtratherm would say you could get with a partial fill, but in the real world is it ever possible to get partial fill to work as advertised? The other option is a semi-rigid glass mineral wool or rockwool bat. I was looking at Knauf Earthwool DriTherm 32. Again 125mm full fill is supposed to give 0.21/0.22.


Does anybody have any experience of the aeroboard or the knauf products?

With a tidy brickie, proper site management, cleaning up, using cavity boards, etc. - is it ever possible to avoid thermal looping? Would pump filling afterwards negate the thermal looping problem?

Does anybody think there's an issue with full fill in Ireland with our driving rain? (Outer leaf will be rendered)

I've seen reference to an aerated block for the inner leaf - is this a good suggestion for whatever solution i go with?

So - what's the best solution / best u value i can get with a 125mm cavity? I think i should try to get the cavity pushed out to 150mm if there's a good benefit, but it's a tight site.


I don't want to get into a discussion of alternatives to the cavity wall at the moment, and i know there are a lot of other factors to consider besides just the u value of the wall insulation, but i'll start other threads for those to keep things simple.


All opinions welcome!


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