Minimum Cavity Size

noseall · 4 Jul 2020

frutbunn said:
Not dodging it, a simple way of showing compliance is to use the U values stated in a table, however you can adopt other approaches ie average U value calcs, this allws you to compensate for reduced insulation in one element by increasing other elements, but for such a small extension I would make a judgement, for example increasing the insulation in the roof of the proposed work, topping up the roof of the existing house or having insulation blown in the wall of the existing walls. Its about reducing CO2 emissions.
What you have to appreciate the approved documents are a method of showing compliance, they are not regulations.

100mm cavities are not just about how much insulation you can get in them. There is a practical side to them also.

frutbunn · 4 Jul 2020

noseall said:
100mm cavities are not just about how much insulation you can get in them. There is a practical side to them also.

No B reg stipulates it, and until relatively recently 50mm cavities was the norm from their inception.

noseall · 4 Jul 2020

frutbunn said:
50mm cavities was the norm from their inception.

Correct, and (with some builders) a pile of pants. I'm fed up with fixing NE elevation masonry due to poorly constructed 50mm cav's.

frutbunn · 4 Jul 2020

noseall said:
Correct, and (with some builders) a pile of pants. I'm fed up with fixing NE elevation masonry due to poorly constructed 50mm cav's.

???? no idea what you're saying

stuart45 · 4 Jul 2020

Nice to know some ones putting all my old work right.

noseall · 4 Jul 2020

frutbunn said:
???? no idea what you're saying

Cold bridging due to poor construction and narrow (50mm) cavities.

frutbunn · 4 Jul 2020

noseall said:
Cold bridging due to poor construction and narrow (50mm) cavities.

Sorry last reply was done, then I think you edited it after I replied!
Nothing wrong with 50mm cavities they were used almost exclusively for 100 years. Poor tradesmen will still bugger things up whatever cavity, thats poor work not poor design.
Just for reference I worked in the second highest part of the UK in the Pennines from 1981 when 50mm cavities were still the most common method and very porous stone was the traditional material, I've been to some very exposed work, not a problem with most of the local lads.

stuart45 · 4 Jul 2020

Cavities were 2-3 inches before modern times, but a lot of sites I worked in used 10.5 inch walls to allow to some inaccuracies. With a 10 inch wall you lost 1/8 due to the brick size, and 1/4 if bricks were used on the inner skin. Cavities could easily end up 1.5 inches with 10 inch walls.

theDIYkid · 21 Jul 2020

wow this got going eh nice one, the small room is not attached to the house its seperate outbuilding...Im using 50mm dritherm into 50mm cavity, thanks.

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Minimum Cavity Size

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