Thinner Cavity Walls

4 Apr 2008
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United Kingdom
I'm in the process of designing my own lean-to extension and have been thinking about the possibility of gaining a bit of space by making the walls a bit thinner.
I have a few constraints that cavity wall construction is the simplest option, although timber frame may also be an option.
I want to use the same bricks as the existing property so using lightweight blocks is not an option, at least not for the outer leaf.
It seems that brick, rockwool, 75mm lightweight block giving a wall thickness of 277.5mm will give me a U-value of 0.275 so under the limit of 0.28 in Part L.
I was thinking that the cavity could be reduced in size a bit - I've discovered that full fill PIR insulation is now available, but it doesn't seem to be available in thicknesses of less than 100mm.
Is it likely Building Control would allow me to use an area weighted calculation and increase the floor and roof insulation to give me a bit of wriggle room.

Any other ideas?
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My question would be is it worth it for 22.5mm.
If you could get it to work with timber frame at 100mm, from a structural point of view (your department really) then you could get it all to work within 265mm plus plasterboard

100mm brickwork
50mm cavity
100mm timber frame (60mm celotex between)
15mm celotex overdraw

Comes in bang on 0.27

could save a further 3mm by going 70 between and 12mm overdraw, but that would be separate 12mm overdraw and plasterboard
100mm timber studs would work fine one a single storey extension, and although there's no reason not to do it, I would just prefer to add a masonry extension to a masonry house. That doesn't mean timber frame is out of the question though.

I'm just thinking that as Part L1B allows an area weighted calc to allow for additional glazing, could the same argument be used to reduce the thickness of the walls. So the walls don't quite reach 0.28 (as required in the approved doc) but the roof and floor compensate for this.

Just looking on the Kingspan website and they suggest two skins of 100mm lightweight block and K106 full fill insulation (10mm residual cavity). 250mm not including internal finishes. As I want to use brick on the outer leaf I'd probably need 50 or 60mm of insulation with a 75mm inner leaf. Best case scenario would give me 237.5mm excluding internal finishes.

I wonder if I could persuade Building Control...

Whether or not it's worth it is another matter. The truth is I want to match in with my next door neighbour as much as possible. Their extension is 3.5m. I would have liked to have gone out a bit further but would prefer aesthetically to match in. It will gain me around a square metre in total which is probably worth around £1500 these days!
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I had a similar scenario, and decided on timber frame and brick slips to match the existing.

15mm plaster board and scim
50mm celotex
89mm frame with 90mm celotex!
9mm cement board
25mm battens
9mm cement board
5mm adhesive
20mm slips

222mm total

You could reduce the slips to 10mm potentially. And this exceeds the requirement of L1, so the 50mm celotex could be reduced too - probably to 25mm

So that's potentially 185mm
maybe worth asking the question, but I would go armed with all the figures ready as deviation from the norm tends to scare them..
But shouldn't see why not, why should a masonry wall be treated any different from a glass one.
I'm actually coming round to the idea of timber frame. If I self build I can pre-fabricate the panels and bolt to the substructure which would be easier and a better use of my time than trying to lay blocks. Also cheaper than paying someone to do it for me...

I like the idea of brick slips - would be happy with 250mm but any improvement is worth it. Any idea of the cost of brick slips system compared to building a brick outer leaf?
Single skin brick with wind posts, 100mm internal insulation

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