Cavity wall insulation requirements

[bsr]

Hi All

What is the standard wall detail with the new part L requirements please?

I have a 2021 design with 100mm cavity bats and thermalite inner skin. I understand that's no longer enough?

I guess the three options are:
1. 150mm cavity with rockwool batts
2. 100mm cavity with Kingspan
3. EWI or insulated plasterboard

What are most people doing? Can you fit a 150mm cavity wall on a normal 600mm trench? I need clay board which may make the trench requirement wider?

Thanks

 

[noseall]

Hi All

What is the standard wall detail with the new part L requirements please?

I have a 2021 design with 100mm cavity bats and thermalite inner skin. I understand that's no longer enough?

I guess the three options are:
1. 150mm cavity with rockwool batts
2. 100mm cavity with Kingspan
3. EWI or insulated plasterboard

What are most people doing? Can you fit a 150mm cavity wall on a normal 600mm trench? I need clay board which may make the trench requirement wider?

Thanks

Were using the latest 90mm Unilin rebated stuff. Very expensive and shít to fit.

 

[bsr]

This stuff?

CavityTherm | Unilin Insulation (UK) | Formerly known as Xtratherm

CavityTherm is an innovative built-in insulation for traditional walls that achieves passive level U-values as low as 0.13 W/m²K.

unilininsulation.co.uk

Appears to be PIR like Kingspan with a textured surface to aid drainage on the external face?

 

[noseall]

This stuff?

CavityTherm | Unilin Insulation (UK) | Formerly known as Xtratherm

CavityTherm is an innovative built-in insulation for traditional walls that achieves passive level U-values as low as 0.13 W/m²K.

unilininsulation.co.uk

Appears to be PIR like Kingspan with a textured surface to aid drainage on the external face?

Correct. Louvered plastic face. Fits into a 100mm cavity. Horrendous to cut. Jambs are a pain. Corners are a pain.

 

[bsr]

But easier than a 150mm cavity?

 

[jono_h]

I’ve just done a 150mm cavity with 100mm PIR. BCO wanted a 50mm clear cavity due to the coastal location.

If I had a choice I’d do 150mm full fill with Rockwool.

 

[matlob]

When you say 2021 design was it submitted to BC before the change in regulations?

If so you can just go of those specifications and keep 100mm cavity I believe

 

[noseall]

But easier than a 150mm cavity?

There's nothing that more difficult in a 150mm Dritherm cavity v's a 100mm Dritherm cavity. You just make sure your suppliers are up to speed with wider lintels, wall ties, cav' closers, window boards etc. I'm pretty sure BC are still accepting 350mm wide masonry on a 600mm wide trench.

 

[noseall]

But easier than a 150mm cavity?

No.

 

[jono_h]

Mine wouldn’t accept a 350mm wall on a 600mm foundation, it had to be the full 650mm. Then when the SE specified a 140mm internal skin he wanted me to make it another 40mm wider.

 

[bsr]

That's one of the things I was wondering about. Is there a rule about footing width v wall width? You can build a wall offset on a foundation so presumably there isn't a minimum spacing to footing edge?

We will need clay board so potentially looking at a 700mm trench which I guess is another muckaway.

 

[Ivor Windybottom]

It's also not a digger bucket size. Perhaps someone needs to invent an adjustable bucket?

Seems a bit petty to demand 50mm wider, will make approximately bugger all difference to strength and end up a lot less clean as a result of soil getting in from a second scoop.

 

[noseall]

It's also not a digger bucket size. Perhaps someone needs to invent an adjustable bucket?

It can be an issue, if all the plant suppliers have is a 600mm wide fella. It's fairly easy to widen a trench for the first couple of feet, depending on the power/size of the machine. Much more difficult and time consuming to do at depth. Especially in desiccated clay.

 

[bsr]

Sounds like it might be easier to go for a 750 bucket rather than scraping trench walls, especially if clay and going to 2m.

But any thoughts on my question?

Is there a rule about footing width v wall width?

 

[jono_h]

Well Part A of the Building Regs (pg 37) gives minimum foundation widths in varying ground conditions for differing loads. But, apart from building on rock, the table doesn’t refer to a wall width.

If you want to build a 350mm wall on a 600mm foundation you’re either going to need to give your BCO some chat, or get a SE to prove it’s OK.