Celcon 2.9n block for inner leaf?

[Lears]

Hi,

 

My architect has spec’d 100mm dense block for outer leaf and Celcon 2.9 n/mm2 for the inner leaf with a cement mortar (3:1) for an extension we’re building on an old, solid stone home. 
 

For the rest of the main house, we’re going breathable with IWI wood fibre and lime throughout with a MVHR system. The extension currently has a 125mm cavity designed to have 100mm ecotherm insulation and surecav 25. I spoke to the architect about reservations of aircrete blocks having issues with cracking but he didn’t think it was an issue. 
 

I don’t want dot dab inside and planning parge coating and wet plastering but again, a bit concerned the light weight blocks will suck too much moisture out with that approach.

 

I probably can’t swap to a denser block without needing to change cavity size as current spec only just hits 0.17 u value so a bit torn. 
 

Any suggestions would be massively appreciated as feel I’m going in circles but really don’t want to regret a choice down the line! 

[JohnMo]

Why not replicate the existing house with wood fibre IWI? 

 

48 minutes ago, Lears said:

125mm cavity designed to have 100mm ecotherm insulation

Not a fan, too easy to make a dogs diner of it, and effectively make it useless.

 

Disadvantage of IWI is you make the build thermally light, so temperature can yo-yo over a short time frame. It very limited thermal capacity.

 

Or just go with a bigger cavity, heavy blocks inside and out, and either poly bead or mineral wool or similar cavity fill. Then just plaster, not sure what the parge cost brings to the party if your plaster anyway?

[Gus Potter]

2 hours ago, Lears said:

My architect has spec’d 100mm dense block for outer leaf and Celcon 2.9 n/mm2 for the inner leaf with a cement mortar (3:1) for an extension we’re building on an old, solid stone home.

The mortar strength is far too much for these blocks. 

 

Politely ask your Architect if they can explain to you how the old and new building are doing to interact in terms of day to day movement and cement base blockwork shrinkage, that starts from the founds up.

 

Then ask them to detail the movement joints in such a way that it also stands a good chance of meeting the Structural Engineers tying and stability requirements and explain to you how that all works. A competant Architect should take this in their stride.