How to fill 150mm cavity

[Iceverge]

5 hours ago, DannyT said:

build houses everyday and there is no way I’d have PIR in the cavity

 

Preach. 

[allthatpebbledash]

Using a dense block external leaf, and lightweight block internal seems to achieve 0.18 with 32 batts in the cavity I just seen. Are the lightweight blocks terrible as I’ve heard of them cracking and crumbling easily.
 

Using dense blocks both leafs and 32 batts gives 0.19 which is so close. Would it not be possible to do this and say increase u-value of floor or roofs? Or windows? Anything to compensate for such a small nominal. 

 

It’ll allow for using the much cheaper and better dense blocks for walling and go with fully filled cavity mineral wool. For now taped PIR at 140mm is being planned. 
 

 

[Oz07]

On 12/04/2026 at 20:27, DannyT said:

I'm with you I've never used pir in cavities either. I've done traditional with dot and dab and got great air test score. Parge coat prior and linked to airtight membrane on ceiling. I don't see how the pir helps you with this?

[Iceverge]

6 hours ago, allthatpebbledash said:

Using a dense block external leaf, and lightweight block internal seems to achieve 0.18 with 32 batts in the cavity I just seen. Are the lightweight blocks terrible as I’ve heard of them cracking and crumbling easily.
 

Using dense blocks both leafs and 32 batts gives 0.19 which is so close. Would it not be possible to do this and say increase u-value of floor or roofs? Or windows? Anything to compensate for such a small nominal. 

 

It’ll allow for using the much cheaper and better dense blocks for walling and go with fully filled cavity mineral wool. For now taped PIR at 140mm is being planned. 
 

 

 

Wider cavity. 

 

It's not rocket science. We have 250mm.

[Oz07]

6 hours ago, Iceverge said:

 

Wider cavity. 

 

It's not rocket science. We have 250mm.

I think OP said he was restricted to 350mm total due to foundation. My point is by the time you spend upwards of 2k on 25mm pir, 25mm battens and lose 50mm of internal space it doesn't seem economical