Wall Thickness 2024

[meet_the_smiths]

My extension has had strip footings and block & beam laid already.

My builder is concerned that the cavity wall is 100mm. My designs clearly state 90mm kooltherm106 with a 10mm gap.

Building control have seen and not questioned the cavity. Everyone I speak to says it needs to be 150mm. Why would my designs have specified 100mm - the guy is very awkward to talk to and basically every question I've ever asked is met with either 'Building Control will tell you' or 'structural engineer to specify'.

I'm not even sure why I have paid him £1500 to submit to Building Regs - from what I can see he has made a guess at where radiators might be (I'll decide that I told him not to) - and put some doors in (again I'll decide that when I build my studded walls) but produced nothing of any use.

Can anyone advise - why would he have specified 100mm - it's of no benefit as far as I can tell.

 

[freddiemercurystwin]

A 10mm residual cavity will be a PITA to build, as your builder is surmising, so increasing the cavity will be easier (and less prone to problems down the line if your builder is not fastidious), if you're in a coastal or area of high exposure then 50mm cavity is the norm anyway. But if 90mm achieves your U Value and you're not in an area of high exposure then the Regs (and Mr BCO) will be satisfied. Beware that if you increase the overall wall width BC may expect the foundation to be wider. He's probably just a lazy/crap designer. That said you can't very well submit plans for Building Regs that don't show some doors or radiators, normally there's a note (or should be) saying client to confirm exact positions etc.

 

[noseall]

My extension has had strip footings and block & beam laid already.

My builder is concerned that the cavity wall is 100mm. My designs clearly state 90mm kooltherm106 with a 10mm gap.

Building control have seen and not questioned the cavity. Everyone I speak to says it needs to be 150mm. Why would my designs have specified 100mm - the guy is very awkward to talk to and basically every question I've ever asked is met with either 'Building Control will tell you' or 'structural engineer to specify'.

I'm not even sure why I have paid him £1500 to submit to Building Regs - from what I can see he has made a guess at where radiators might be (I'll decide that I told him not to) - and put some doors in (again I'll decide that when I build my studded walls) but produced nothing of any use.

Can anyone advise - why would he have specified 100mm - it's of no benefit as far as I can tell.

The rules changed in June 2022, whereby when say using the old full fill 100mm Dritherm cavity batts, the new thermal resistance reg's meant that you would now need 150mm to achieve the numbers.

However, you can still opt for 100mm cav's as long as you use the correct (rebated PIR) full fill slabs. These are very expensive (comparatively) and can be prohibitively so.

 

[noseall]

A 10mm residual cavity will be a PITA to build,

The proprietary full fill slabs are now rebated, so that the slabs can touch the outer skin. The 10mm cavity is just a fitting gap, really. It maters not whether you build the inner or outer skin first, either.