Using 75mm-wide blocks for inner skin, instead of 100mm?

[ectoplasmosis]

We're about to start building a single-storey rear extension to a Victorian semi, with a flank cavity wall which butts up to a neighbour's existing wall.

 

Would it be possible to use 75mm-wide medium-density concrete block for the inner skin of the cavity wall, instead of the normal 100mm? This would allow us to increase the cavity size and use full-fill Dritherm 32 insulation instead of PIR, eliminating the risks with using rigid board inside a cavity.

 

 

 

[Mike]

Building Regulations Approved Document A requires a minimum thickness of 90mm, so to go thinner than that you'd need Structural Engineer's calculations to prove that it's adequate.

 

Edited December 13, 2023 by Mike

[Russell griffiths]

As above, they are for internal walls. 

If your tight on space look at a different build bethod, sips panels and a block outer skin, or timberframe and brick/block outer, cavity construction takes up a lot of room for very little insulation value. 

[Conor]

Good case for timber frame with block outer skin.

[joe90]

  On 13/12/2023 at 10:40, Conor said:

Good case for timber frame with block outer skin.

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Yes then insulation within the timber frame as well 👍

[Jane W]

  On 13/12/2023 at 00:06, ectoplasmosis said:

use full-fill Dritherm 32 insulation instead of PIR, eliminating the risks with using rigid board inside a cavity

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We are planning a newbuild atm but have the same quandary. Can insulated plasterboard or insulating render bring the u value up to building regs if you go 100mm medium density blocks and dritherm?