U value of cavity wall

[deuce22]

Hi.

I have a 110mm cavity wall, 100mm dense concrete block on both the inner and outer leaf. 60mm of insulation inside the cavity.

I have been told by the BCO that I need to either use a thermal block for the inner leaf or a dense block with an insulated plasterboard.

I need to achieve 0.26 for the U value.

I have just put the figures into the Celotex calculator and it is coming up at 0.25.

I am not sure of the correct name of the block, so I have gone with a Hanson evalast, which has the worst rating.

Is this correct as I'm confused why he is telling me that I need more material to achieve this value.

Thanks.

 

[jeds]

I'm wondering why you have to reach 0.26. Are you working to a calculated SAP or is it a standard extension? The standard for extensions is 0.28. (unless there is a sap calc that requires something different)

 

[^woody^]

Why not just put 100mm quilt in the cavity?

110mm for a cavity is an odd design in any case

 

[deuce22]

I am working to SAP calculations for a new build.

I have found out, that if I use a 100mm cavity with 50mm Celotex I only achieve 0.28, so I would then have to include some other material on the inner leaf (thermal block or insulated plasterboard).

However if I increase the cavity to 110mm then I can use 60mm Celotex and that gives me a U value of 0.25, so that is all that is required.

I was told that a 50mm breather gap in the cavity is required, so I could use a 100mm cavity with 60mm of insulation.

Cheers.