Choosing cavity size and selecting type of blocks

[garyo]

I'm just putting together some detailed drawings for the construction of my new porch. It's a front extension so needs to be thermally efficient. I've been reading the sample wall construction methods in the 'u value of elements' that the LABC publishes, and I can't really see much difference between the options.

Given that I'm making a room that's 4m x 1.8m, and expecting my walls to be about 300mm thick, and all I've committed to is that the outer leaf will be brick, what should I choose?

I'd be happy to pay a bit extra to make the wall 5cm thinner, but all of the options I see with a 50mm cavity rather than 100mm cavity all tend to have insulated plasterboard on the inside, the net effect being that you still have a ~300mm thick wall.

I intend using lightweight thermalites, because they're easiler to handle, good thermally, and seem to be pretty cheap, by the way.

Am I missing something?

Gary

 

[freddiemercurystwin]

You're not missing anything. You could use full fill cavity insulation and negate the requirement for an air gap but this type of insulation is less heat loss resistant so you need more of it so you still end up with 100/100/100. As you say when either insulating with partial fill insulation or internal insulation you always need a 50mm air gap for build regs compliance so horses for courses.

 

[garyo]

Thanks Freddy, that's good to know.

So really I should just go for the easiest construction method, using the most widely available and commonly used materials, which would imply good value through mass production etc.

As I say, I'm going for thermalites on the inner leaf, 100mm full fill dritherm cavity bats, and stock bricks on the outer leaf.

Gary

 

[DevilDamo]

I have just run a quick u-value calculation through Celotex's u-value calculator and it has produced this...

Brickwork (103mm)
Cavity (65mm)
Thermalite Turbo blocks (100mm)
Lightweight plaster (13mm)

The 65mm cavity is formed by providing 40mm Celotex CG4040 and maintaining a 25mm clear air gap, which has to be increased to 50mm if you intend in using NHBC or Zurich insurance for the works... which is highly unlikely.

So that construction gives you an overall cavity wall width of 281mm and it achieves a u-value of 0.28W/m²K, where the maximum (according to Part L) is 0.35W/m²K.

There are more efficient Thermalite blocks so if you let me know which ones you're intending to use, then I can run that through the calculation.

 

[freddiemercurystwin]

Hmm, where does it say in Part C that you can get away with a 25mm residual cavity? Whilst some manufacturers and their BRE Agrement Certificates suggest a 25mm air gap is acceptable it ought to be 50mm to allow for inaccuracies and the inevitable mortar that ends up hanging on the ties and in the cavity.

 

[jeds]

Personally I would go for a 275mm wall with brick, 75mm full fill cavity (superwall cavity slab) 100mm thermailite turbo block finished with dot and dab. That will give you a U-value of 0.30, is easy to build and will comply with everything without any hassle.

 

[DevilDamo]

Hmm, where does it say in Part C that you can get away with a 25mm residual cavity? Whilst some manufacturers and their BRE Agrement Certificates suggest a 25mm air gap is acceptable it ought to be 50mm to allow for inaccuracies and the inevitable mortar that ends up hanging on the ties and in the cavity.

I didn't mention Part C. I quoted Celotex's calculation/data sheet.

For those reasons you stated, I would and have NEVER noted up a 25mm residual cavity, but always a min. 50mm.

 

[freddiemercurystwin]

 

[garyo]

Thanks guys.

There's definitely no works insurance going on!

Personally I would go for a 275mm wall with brick, 75mm full fill cavity (superwall cavity slab) 100mm thermailite turbo block finished with dot and dab. That will give you a U-value of 0.30, is easy to build and will comply with everything without any hassle.

I'll have a look at that - cheers.

Gary

 

[DavidWP]

Hmm.
I think you (and prospective respondees) need to be clear on one point.
A porch is a non-habitable space and is exempt from BR's providing Part N is complied with.
If this front extension is a habitable space, and it would seem so from your words, then BR's will be applicable.
If in doubt speak to your LA Building Control Office or an architect.

 

[freddiemercurystwin]

Hmm.
I think you (and prospective respondees) need to be clear on one point.
A porch is a non-habitable space and is exempt from BR's providing Part N is complied with.
If this front extension is a habitable space, and it would seem so from your words, then BR's will be applicable.
If in doubt speak to your LA Building Control Office or an architect.

I think you need to be clear on one point, whilst he does describe it as a porch the OP has intimated that it is classed as a front extension not a porch and that he is building it to regs whether it is required or not.

 

[garyo]

It's definitely a front extension that I'll be using part of as a porch, but it's not a porch in the sense that it's being built under the 3 square meter rule, or under permitted development. I've been granted planning permission.

I'm removing the old front door and opening that out too, so it'll be classed as heated space, although in reality there'll be a well insulated wall and door separating it into 33% and 66% chunks.

My LABC has sent me a leaflet which describes the common construction methods that they approve of. It's called 'LABC Berkshire - L&F for builders' The thinnest wall contruction in these is 285mm, based on 85mm drytherm. They did say that the regs changed in 2006 and that if I use any of the other LABC's 'u value of elements' PDF, then there's a danger they'll be out of date if they were published in 2002 (which some are)

Strangely, this 2006 document says that 75mm drytherm, thermalite turbo, and brick, is fine: http://www.greenspec.co.uk/documents/materials/insulation/UvalueOfElements.pdf

Gary