Critique this wall build up please

[LiamJones]

The logic behind some choices;

- the 70mm cavity, this is to allow the extension to match the existing cavity and avoid a tricky detail/thermal bridge between new and existing.

- the external wall insulation, this will be being added to the existing walls, so it makes sense to run it through flush on the extension.

 

N.b, plasterboard to be dot and dabbed to aerated concrete block, struggled to model it.

I've also not included weep vents, i don't think i need them?

 

 

[Russell griffiths]

Use an icf block instead, one of the eps based ones, then you can run it inline with your ewi and render straight over all of it, no bricklayers needed, no silly aerated blocks, no dot and dab, and highly airtight. 

[Conor]

Wot he says.

 

You've basically made an ICF analogue but with none of the benefits.

 

I wouldn't be using EWI on block, it's probably the most expensive way of doing it. Either ICF or standard cavity with the face of the block (plus scratch coat) in line with the EWI, with the one render system over the lot.

[MikeGrahamT21]

Another method, you could go 215mm block and 150/160mm EPS on the outside.

 

you risk interstitial condensation with that build up, and would need some kind of check on the inside to avoid, I think you must have specified some wrong values on u-wert for it to not show.

[LiamJones]

Thanks for your comments @Russell griffiths @Conor @MikeGrahamT21

 

I want to use the same build up for the whole building, putting ICF on the end of a block cavity bungalow complicates things. It's only a small extension and i;ve got brick layers lined up to start in a week. I originally posted this more to check block choices and the dpc/weep vent requirements. To me, it just seems simpler to keep it all the same. Especially given i need to solve this problem on the existing bungalow.

 

Regarding interstatial condensation, i modelled it using 0c/20c with no condensation. -5c/20c does have some condensation, but appears to be significantly less than the existing and within acceptable ranges, fully evaparorating in the drying season. 

 

 

If i swap the aerated blocks on the inner leaf to solid blocks, i can further reduce the condensate, and move the condesation point to outside the cavity. This reduces the uvalue by 0.02, but within regs (0.18 for new elements on exisitng dwellings)

 

 

I'm no expert on interstatial condesation, but it all looks okay and within range. what am i missing?

 

Thanks, Liam.

[LiamJones]

FWIW, i'm also installing a whole house MVHR.

[LiamJones]

I've done a bit more reading, and a thread on green building forum seems to suggest it'll be fine regarding interstitial condensation.

[Russell griffiths]

Ok ditch the celcons, should you have a tray dpc to shed any water to the outside???

[LiamJones]

34 minutes ago, Russell griffiths said:

Ok ditch the celcons, should you have a tray dpc to shed any water to the outside???

I’m not sure? In theory, the bottom of the cavity is way below damp, so it doesn’t really matter which way it goes?

[MikeGrahamT21]

If you swap out the aerated blocks for dense blocks on both leaves, then you won't have any risk of condensation at all.

 

Or the alternative method, with 215 blocks:

 

[LiamJones]

Hi, thanks for all your opinions and advice! 
 

I’ve decided to go with medium density blocks to both skins. Combined with the EPS bead cavity and EWI gives me a very small condensate point way out in the render and a u value of 0.16.

 

Here it is going up 

 

 

[LiamJones]

Oh, and BC advised happy with full full cavity to the foundations with no tray or weep vents, given I was EWI and render to prevent driving rain.

[LiamJones]

I landed on the medium density blocks as a happy middle ground

 

lightweight/aerated

Low thermal conductivity

Low sound attenuation

lightweight for the bricklayers, less labour

more expensive material

more fragile, less ability to fix things to.

less airtight

 

heavy density

High thermal conductivity

high sound attenuation

heavier for bricklayers, more labour

less expensive material

High ‘thermal mass’

Better airtightness 

 

medium density’s sat in the middle for all of these properties and are a good balance in my opinion.

 

Edited October 4, 2023 by LiamJones
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