75mm cavity & insulation materials

[jesteh]

We have an extension with 75mm cavity which is currently being built. Because the outer skin has been built and the inner one is down, we can no longer use the part fill rigid foam insulation (building inspector won't let us).
What are the options in terms of full fill insulation?
Rockwool has minimum depth requirement of 100mm. We can't make the cavity that wide as its built on existing narrower foundations.
Are there any full fill materials other than mineral wool/fibreglass that will satisfy both the thermal reqs and the 75mm depth?
wall is outer brick inner aerated blocks.

 

[jeds]

Full fill cavity batts with a minimum thermal conductivity of 0.036 and 100mm Thermalite Turbo blocks just scrapes a U-value of 0.30W/m²K

Assuming you are on old regs.

 

[jesteh]

Already got the blocks! Normal Celcon ones.

 

[RonnyRaygun]

I don't know if you can do this but it may be possible to install further insulation to either the internal or external face of the wall. I expect 25mm insulation backed plasterboard plus the cavity insulation would give you a sufficient U-value.

 

[noseall]

As RR has said, you may need to fit additional insulation (rigid foam board) to the face of the inner skin.

This could turn out to be a reet pain particularly if you intend doing a lot of fixing to said wall, like a kitchen extension for example.

 

[theoldun]

If you go this route.
Celcom standard blocks are 0.15 lambda. Assume you are not trying to obtain latest U values from, Oct 2010 but wish to obtain 0.30W/m2K, then this will give you 0.30
Brick 75mm cavity full fill Dritherm 32. 100mm block with lambda of 0.11. such as Durox Super block.
As you have only got 0.15 lambda, you may just scratch in with 15mm lightweight plaster, or 12.5 dot and dab, but we have our doubts whether the man will accept it, so you may have to dot and dab with insulated plasterboard. Suggest you ask Hitler whether he will accept 22 or30mm insulated plasterboard.
If you go this route hang blockwork out 10mm to increase cavity to 80mm.
oldun

 

[jeds]

Already got the blocks! Normal Celcon ones.

In that case you will need something extra. Knauf do make a cavity batt with a K of 0.032 (Dritherm32) but you won't get it off the shelf. You'd need to phone Knauf to find a supplier. And then you'd need to fly the blocks over by 10mm to take 85mm batts. OK as long as the loads aren't excessive.