Patio Door Aperture

[BiscuitThief]

Finally have some responses. Not sure what to make of them but am waiting to see what the patio door installers say.

What's a 250x300x40 m/s plate?
SE made a mistake and changed this to 250x300x20mm

Why not use a 152x152x23 instead of 203 x 203 x 46?
Cost; 152x152x23 will need a plate to be welded to support the solid wall, 203x203x46 will not need it so it will be cheaper to use that

Additionally, the SE insists that the new boundary wall we'll build for the side infill extension can't be thinner than 365.50mm because "Building regulations changed last year in June, so minimum insulation needs to be 165mm to comply with the new regulations."

What is the thinnest external wall that can built which meets regulations?

I can't find anything showing those measurements but am seeing u-value numbers. Is there any validity in what he's saying about 365.50mm? At this thickness the room in the extended area would be incredibly narrow and not worth building.

SE's 0.18 W/m2k Wall components
Facing brick 103mm
Aircrete blocks 100mm
Rockwool insulation 100mm
Celotex PIR insulation 50mm
Plasterboard 12.5mm

 

[RonnyRaygun]

Finally have some responses. Not sure what to make of them but am waiting to see what the patio door installers say.

What's a 250x300x40 m/s plate?
SE made a mistake and changed this to 250x300x20mm

Still not necessary on such a small span.

Why not use a 152x152x23 instead of 203 x 203 x 46?
Cost; 152x152x23 will need a plate to be welded to support the solid wall, 203x203x46 will not need it so it will be cheaper to use that

Did he say why you can't just use a Catnic?

Additionally, the SE insists that the new boundary wall we'll build for the side infill extension can't be thinner than 365.50mm because "Building regulations changed last year in June, so minimum insulation needs to be 165mm to comply with the new regulations."

What is the thinnest external wall that can built which meets regulations?

No, you need 150mm of rockwool to meet regs, but there are alternatives. You can use 90mm Kingspan with 10mm residual cavity, so 315mm is achievable, or some architects are now specifying external insulation instead which would mean a 215mm wall would be OK.

SE's 0.18 W/m2k Wall components
Facing brick 103mm
Aircrete blocks 100mm
Rockwool insulation 100mm
Celotex PIR insulation 50mm
Plasterboard 12.5mm

Erm, is he suggesting 100mm rockwool and 50mm Celotex in the same cavity?! Never heard of that before.

 

[BiscuitThief]

Thanks @RonnyRaygun

Erm, is he suggesting 100mm rockwool and 50mm Celotex in the same cavity?! Never heard of that before.

From SE notes "Cavity fully filled with 100mm Rockwool cavity insulation per manufacturers' instructions and provide 50mm PIR insulation over vcl internally, e.g. Celotex GA4000"

I read this about June 2022's changes:
Under current requirements, an external wall U-Value cannot be higher than 0.30. When the regulations change, this limit is being reduced to 0.26. So, as long as the wall construction achieves 0.26 or lower, the wall will meet the heat loss requirements of the new regulations.

Why would the SE be trying to target 0.18?

 

[freddiemercurystwin]

Because he's an idiot.

 

[mrrusty]

One storey room with flat, timber + felt roof

To my mind there seems to be a lot of overkill in this thread. If I was doing this, I'd go on catnics website, use the lintel selector, and pick the result. The standard solidwall lintels go up to 3.6m length, so a 2.4 is well under max span, and the actual loading must be close to the minimum any lintel is expected to take. I expect even the most basic calculation will show actual loadings way inside the lintels spec.

 

[BiscuitThief]

I've asked the patio door installers to go with their surveyor's suggestion (which is catnic).

The more difficult job is getting the SE to budge on the extension wall thickness...

 

[RonnyRaygun]

Why is the SE specifying the insulation? Not really their job...

 

[BiscuitThief]

Why is the SE specifying the insulation? Not really their job...

My bad, the wall specifications came from the Architect initially and the SE agrees with them.

The two building companies I talked to for quotes sounded flexible on the thickness of the walls (mentioned they could look at masonry/timber walls and material substitutions etc.) but the Arch + SE have been inflexible citing regulations.

 

[RonnyRaygun]

Go on the Catnic and Kingspan websites. You'll be able to use their calculators and brochures for alternative options for insulation and lintels. The info you get off the calculators and brochures is suitable for issue to building control.

Then you can ignore what your architect and engineer are telling you.

 

[BiscuitThief]

Thanks for everyone's help so far.

Took all the advice and searched online for a solution and got one that the building inspector has approved at 280mm thickness:
https://www.multifoilsdirect.co.uk/solutions/superquilt-external-wall-ybs-superquilt/

Wall includes:
- external render
- insulated plasterboard
- 25mm batten
- 7mm SuperQuilt insulation
- 25mm batten
- 190mm AirTec Standard block

(In case it helps anyone in similar situation), exterior wall needed to satisfy two main conditions in the new regulations:
Part A: solid wall must be at minimum 190mm thick (page 16)
Part L: wall must be 0.18 u-value or lower (table 4.2, page 25)


My challenge now is finding somewhere that supplies 190mm Concrete Aerated Blocks (3.6N/mm2) in London - so far, no luck. Or I could double-up blocks/bricks that total 190mm but that's tricky too.

If anyone knows where I can find 190mm blocks like this then please let me know. Thanks again.