Block n brick cavity wall insulation

[WWilts]

U value max 0.25 suffices for external wall, block inner brick outer. Airtightness 4.0 suffices. Plasterboard on dabs internally.

Cost control is important consideration. Likely to move from area in few years. Not against better thermal perfomance, of course, but with an eye on cost.

What inner leaf would you use? Why? Can some blocks compromise a SAP pass even though the external wall meets the 0.25 U value?
What insulation would you use and why?

[PeterW]

30 minutes ago, WWilts said:

Cost control is important consideration.

So why build something very unique if you’re not staying in it..? 
 

Medium blocks, 150mm cavity, blown beads. 
 

Quick, cheap, simple and meets regs with no additional effort. 

[WWilts]

Brick outer, 100mm cavity, block inner, plasterboard on dabs. U value 0.25 suffices

Debating between

Either Aglite or similar block 7N for inner leaf with enhanced EPS beads (0.032 lambda) blown in, or 
Concrete inner block 7N with Celotex 4000 90mm in 100mm cavity (0.022 lambda)

Perhaps some other cost-effective option for 100mm cavity and inner wall?

Edited February 27, 2021 by WWilts

[Makeitstop]

I'm also interested to hear opinions on this.

[Olf]

On 27/02/2021 at 17:58, WWilts said:

Celotex 4000 90mm in 100mm cavity

 

This on its own is a premium product.

Also go through the forum, there is a similar discussion and as good as rigid boards are, they require extra care/patience - adding cost if that is taken care of or losing performance if not.

[MikeGrahamT21]

On 27/02/2021 at 17:58, WWilts said:

Brick outer, 100mm cavity, block inner, plasterboard on dabs. U value 0.25 suffices

Debating between

Either Aglite or similar block 7N for inner leaf with enhanced EPS beads (0.032 lambda) blown in, or 
Concrete inner block 7N with Celotex 4000 90mm in 100mm cavity (0.022 lambda)

Perhaps some other cost-effective option for 100mm cavity and inner wall?

 

You can only use products which are BBA approved for the purpose, think you'll see that Celotex GA4000 isn't suitable for full or nearly full cavity fill.

 

Specialist products exist, Xtratherm CT/PIR and there is a Kingspan product which is also approved for this purpose (can't remember product), but they have special surface coatings, foil isn't suitable for the coating as it can easily be pierced.

 

If you want something easy and cheap, brick outer > 100 cavity with 35 or 40 wool full fill > whatever 7N block > insulated plasterboard

 

[WWilts]

4 hours ago, MikeGrahamT21 said:

something easy and cheap

Tks, great info & helpful to see it laid out so clearly. Easy and cheap without increasing wall thickness is the goal. U value 0.25 acceptable for SAP.
Celotex themselves clarified to me that their product requires an air gap of 50mm, or if building control accepts then 25mm. So, quite right

Given external wall thickness out to in:
102mm brick + 100mm full fill cavity + 100mm block 

Cavity insulation either Dritherm32 or enhanced grey EPS beads blown in (k = 0.032). Leaning towards the beads because it's possibly more reliable for insulation.
For inner leaf leaning towards Aglite Ultima 7N block

All becoming frighteningly real now, because specs and quotes are being discussed with prospective main contractors. 

[WWilts]

Aglite has relatively high thermal conductivity compared to Durox Supabloc. (see attachment)

So 100mm cavity filled with k=0.032 Dritherm32 or enhanced grey EPS beads + Aglite 7N inner leaf will fall short of U value 0.25

Either Durox Supabloc (k=0.17) or slight increase in cavity width required to get the wall U value to 0.25

Full fill PIR boards (Mannok Isoshield) tongue & groove, pricey. 97mm of that would easily take a 100mm cavity under U 0.25. But pricey

Aglite cavity wall U value calculation.pdf

Edited March 1, 2021 by WWilts