What size cavity for domestic rear extension

22 Aug 2010
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United Kingdom
Hi guys,

Can anyone tell me what the best cavity size is for a standar rear extension? I know that the lintels for example come in sizes for anything between 50 - 110mm. I'm keen to ensure I have as much internal space in the extension as possible, but don't want to compromise on insulation.

Is there a standard gap you need between the celotex and external skin or can it be a very small gap from building regs point of view? i was under the impression that it would be 100mm block - 100mm cavity - 100mm block, but reading into it more, the cavity can be smaller.

Thanks in advance
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The cavity is just one part of the wall, so thickness depends on the insulation and type of block used

The thinnest type of wall will be a timber frame, then a solid lightweight block wall, and then a cavity wall. So from 150mm to 300mm thick
Thanks woody, but my question is aimed specifically at the cavity only. I'm building the skins from 100mm concrete blocks, but believe that the cavity itself can vary in width and so to can insulation etc. I suppose I'm asking what the most common cavity and insulation width is for a cavity wall.
You can't think of the cavity in isolation.

If you have a 100mm cavity then and fill it with quilt then you will still need a certain type of block on the inner wall

Or you can have a 70mm cavity and different insulation and blockwork

There is no standard cavity, and it is the wall construction and materials which are relevant

The most common cavity width is 100mm as its easy for whoever draws the plans
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That makes sense then woody. I never thought of it like that. So am I right in thinking that 2 skins of dense concrete block requires high rated insulation than if I used aerated inner skin?

Is it a valid question to ask what cavity wall construction you would advise? I wanted dense concrete, purely because they feel more solid. Would you recommend 100mm cavity with a celotex/kingspan type of foil backed insulation? And what thickness of insulation would be best. Sorry if my question isn't as straight forward as I thought.
With 2 skins of medium density block you'll need something like 100mm Dritherm 32 in the cavity. Lightweight internal blocks will allow you to use a standard cavity insulation which will probably make it overall cheaper but will still be a 100mm cavity. But it depends where you buy your stuff and how good a deal you get. Insulation costs and deals go up and down like yo yo's to prevent anybody settling on one particular brand.
Thanks for all the advice guys and Garyo, that document is a very useful reference guide for floors, walls and roof, which will be very useful for my self build extension.

Interestingly, I have been following a side extension build at the bottom of our road. I just walked down to see progress and take more photos and while I was there I took measurements of the walls and insulation. The builder has used 100mm celcon block - 100mm cavity - 100mm celcon block. The insulation is foil backed Quinn therm that is 60mm thick. Based on Garyo's guide this is more than is required. Is that because the guide gives min u-value requirements and so this is simply better?

I'm beginning to wonder if I should use celcon blocks rather than dense concrete. Perhaps concrete on the outside and celcon on the inside....
Are you drawing the plans yourself or doing it under a building notice?

If not, just tell whoever that you want the walls as thin as possible and to select the insulation and blockwork accordingly.

For that extension down the road, that may well be a lazy designer or builders, for using a 100mm cavity and 60mm foil board insulation and celcon blocks.

The minimum spec to meet b/regs would be 40mm insulation 25mm cavity and normal celcon blocks internally. Celcon blocks externally should be avoided IMO due to increases risk of cracking and render falling off, and as Tony said above, IMO foil board insulation is hard to use and easy to mess up for DIY building
Woody, I've had an architect draw up plans that I will submit under building notice. I don't see the benefit of getting full planning review. I've also decided to use a private building inspector rather than the council. A fraction cheaper, but according to the architect and the inspector, it is possibly better for a self build, in so much as they can be a bit more reasonable and less down the line, if you know what I mean. Saying that, I by no means intend to cut corners.

I'll speak to the inspector in time about the specifics. If you don't recommend foil board insulation for self build, would you mind telling me what product you would suggest. If rock wool type insulation, does this simply fill the entire cavity?
Just realised that I may have wrongle described the build at the bottom of our road. The insulation is 60mm and the remaining cavity is 40mm making the 100mm cavity I mentioned.

Any advice on the alternative to rigid foil insulation would still be appreciated.
Foil board is OK if you can do a good job to stop it popping out all over the place and can keep the cavity clear.

I tend to go for a 75mm cavity with one of the high efficiency Dritherm or Isover full fill quilts and then either a 100mm standard or super/solar block internally
Thanks woody. Just having a look at the dritherm range. Out of interest, what is e thickness of the slabs you tend to use. I noticed that the higher u value slabs are 100mm thick and you suggested a 75mm cavity. OR have I got this wrong again, are you suggesting 75mm cavity + 25mm dritherm to make a 100mm spacing between the blocks? Sorry for sounding dumb, just wanting to make sure I understand this correctly.

Any quilt will be full fill with no air cavity

I think 100mm cavities are crap, so I prefer where possible to have a 75mm cavity with higher efficiency insulation, or better internal block

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