Extension wall construction

[osprey]

I am planning a double height, 5x5 m extension to my 1920s detached house (50mm cavity brick). The architect says open cavities are 'best practice' and is planning for 100mm, half filled cavity, with additional 50mm Celotex inside the inner leaf. Outside will be rendered.

 

I have concerns, among them if this would make it difficult to hang cupboards, TVs etc, as well as possible construction errors, and wonder if this is the best approach (being used to brick under thin plaster walls!)

 

My plan for the existing part of the house is eps beads in the cavity, and I could add ewi to some elevations later. The most exposed elevation will be half extension, and half existing wall.

 

So I could override the architect, and full-fill the cavity, or leave that whole elevation cavity unfilled (I guess). 

 

Any thoughts? I am reluctant to override the professional...

 

 

[Redbeard]

Don't know what the claimed U value is, but my rough 'n' dirty calc (assuming 2 skins of brick, where yours will be 1 x brick and 1 x block) doesn't even get the Bldg Regs (England) U value of 0.18. It might if you substitute lightweight block (and maybe, as often happens, if you assume that the air gap has still air in it - which it almost certainly won't!) but it'll be a close thing. Also the 'spec' is incomplete. 50 PIR inside the block skin is fine, but what is it finished in? If it's insulation-backed plasterboard then (a) it is 50 + 12.5mm and (b) the VCL will only be perfect on the back of the boards. The joints won't have one. If worried re loads on the wall you could do 50 PIR, taped tightly at all joints and perimeters, or add a plastic VCL treated similarly, screw 25mm ply to the wall and then plasterboard (or even Fermacell if you don't dislike cutting it as much as I do).

 

It's a faff, though. If it were me I'd consider getting the cavities built empty (and I'd be tempted to go 150 - 100 is not too exciting - and carefully (no rubble and cr*p inside) then get the cavities fully filled with graphite EPS beads when you get your old cavities filled.

[osprey]

Maybe I was unclear. Proposed wall would be externally rendered (so block-block), cavity 100mm, of which 50mm would be 'some form' of insulation. 50mm gap. The 'room-side' of the inner leaf would have 50mm Celotex.

 

Yes, I was wondering why not 150mm gap, and/or full fill with beads. Seems there is still a lot of caution about not having a clear gap...

[HughF]

Rendered externally then you can easily go full fill cavity.... Might as well go cellcon/concrete as there's no need for brick internally.

[Tims]

We have a similar build up to your architects proposal. 

Rendered Block

50mm clear gap 

50mm PIR

100mm medium dense block

50mm PIR taped and sealed

38mm battens fixing PIR to walls, giving a service void and cross noggins placed for kitchen units and TV etc.

12.5mm palsterboard

 

gives a U of 0.16 (not including the ventilated gap and outer block in U value calc)

 

I've got no issues with hanging TV's etc as I thought about this u front. I considered using a 150mm cavity but it was easier to tie into the existing house and maintain the existing 50mm cavity (less faff). This approach also helped me with getting the new structure air tight. 

 

I also considered 100mm cavity with 90mm taped PIR but I didn't trust my bricklaying enough not to fill it with snots and leave paths for damp tracking though to the insulation, it would probably have been fine. Other down side of 90mm fill is regs at the time (now still?) call for a 50mm clear gap and I would have had to convince my BCO that my brick laying was up to par and that I have achieved the manufacturers specs exactly.

Edited January 22, 2023 by Tims
added PB

[osprey]

Thanks all!

I like the idea of 150 cavity, and eps bead full fill, but seems a bit 'new' around here.