how thin cavity wall mainland UK

[wub]

Hi All, as part of a renovation project, i am infilling a approx 1.7 space between boundary wall and extension to use as a utility room. Given the narrowness of the space, every inch counts and i am wondering how thin i can go on the block work and cavity wall insulation, with high performance insulation to meet Uk mainland building regs? The building contractors are telling me with new regs the wall width is 375 mm (100mm block, 150 mm cavity, 100mm inner block) and then the inner finish render and skim. Given area will have hanging wall units etc i am going for high dense outer block, low dense inner blocks for good fixing strength.

Googling it seems i may still be able to get away with 100mm cavity if i use high performance insulation, but i cant find the definitive answer anywhere. And advice?

 

[wub]

Sorry inner skin is medium dense blocks.

 

[noseall]

Googling it seems i may still be able to get away with 100mm cavity if i use high performance insulation, but i cant find the definitive answer anywhere. And advice?

This is what we use, if we want to maintain a 100mm cavity.

 

[^woody^]

You wouldn't build a cavity wall unless a cavity wall is required.

Solid wall and suitable insulation with supports for the units, or an internal timber frame with enhanced internal sheathing for the units.