Dimensional Q

[ban-all-sheds]

What total thickness should I allow for a cavity wall? Each leaf will be lightweight blocks, outside surface rendered, inside plastered.

I'm trying to predict how much interior space there would be inside an extension where the positions of the walls are constrained by boundary lines, 45° rules etc.

 

[freddiemercurystwin]

Typically 330mm though can be thinner.

 

[ban-all-sheds]

Thanks for the prompt reply.

If 330mm turns out to be a bit restrictive for me, how tricky is it to make it thinner? More expensive but better performing materials? Different form of construction altogether?

There'd only be about 12m of single-storey wall, so set against the total cost of the build and fitting out, a higher material cost might be trivial overall.

 

[tony1851]

If they are lightweight insulating blocks both skins, you could probably get away with a 75 cavity with someting like Dritherm 32 filling.
Stick 9.5mm plasterboard on the walls.
If it's single-storey, could you consider an inner skin of timber studwork, and a cavity down to 25mm?

 

[ban-all-sheds]

If they are lightweight insulating blocks both skins, you could probably get away with a 75 cavity with someting like Dritherm 32 filling.

I was assuming that the cavity would be filled with insulation

Stick 9.5mm plasterboard on the walls.

To what end, vs just having it plastered?

If it's single-storey, could you consider an inner skin of timber studwork, and a cavity down to 25mm?

Thanks for the last suggestion, but stud & plasterboard "walls" are the work of the devil

 

[ban-all-sheds]

Typically 330mm though can be thinner.

So - as a rough guide?:

20mm external render
100mm block
90mm fill
100mm block
20mm internal finish
--------
330mm?

And I could shave a bit with high-performance insulation? Possibly quite a lot if I specify Aerogel?

A few 10s of mm might seem pointless, but if it means that the internal space becomes a nice multiple of kitchen cabinet sizes that would be handy, as it would permit a quite different, and much more useful, interior layout.