How to fill 150mm cavity

[allthatpebbledash]

What’s the best way to fill a 150mm cavity with insulation?

 

Should I partially fill it and go with 100mm PIR with 50mm gap as is being suggested?

 

Wall build up intention here;

Clayworks plaster or paint

Plasterboard dot dab 

100mm block not specced yet

150mm cavity

100mm block not specced yet 

Render by Baumit I think

 

Theres scope for a wider cavity to be honest but architect is saying going wider would increase costs as majority of ties and other ancillaries or all geared for 150mm cavity’s.

 

I’m not keen on blown EPS beads.

 

Editing to add, the roof is a pitched roof, gable end to end.

 

Intention is a cold roof, with loft roll above joists then boarded over for loft storage.

 

 

Edited Wednesday at 18:10 by allthatpebbledash
Roof info

[JohnMo]

Poly beads.

[ADLIan]

Built in mineral wool slabs. 150mm now a standard thickness

[Mr Punter]

Dritherm 32 cavity slabs.

[Iceverge]

Mineral wool batts.

 

Absolutely not put boards.

 

They're next to impossible to install properly in the real world 

[Andeh]

1 hour ago, Mr Punter said:

Dritherm 32 cavity slabs.

This! 

[ETC]

Pumped insulation.

[Oz07]

Ive had pumped before and slabs. Id be interested as to what is better around cavity trays. Do pumped beads or dritherm work their way around all the folds and lintels? Pumped used to be cheaper at one point but then slabs became cheaper again. I find its easier to keep cavity clean of mortar with slabs. The compo just drops down onto last row of bats and clean it off. When you have an open cavity it drops right down to the bottom. 

Edited Thursday at 05:42 by Oz07

[allthatpebbledash]

21 hours ago, ADLIan said:

Built in mineral wool slabs. 150mm now a standard thickness

Which ones?

 

20 hours ago, Iceverge said:

Mineral wool batts.

 

Absolutely not put boards.

 

They're next to impossible to install properly in the real world 

 Which ones?

 

21 hours ago, Mr Punter said:

Dritherm 32 cavity slabs.

Why this?

[Redbeard]

1 hour ago, allthatpebbledash said:

21 hours ago, Iceverge said:

Absolutely not put boards.

 

They're next to impossible to install properly in the real world 

 Which ones?

I suspect 'put boards' should read PIR boards.

[Iceverge]

1 minute ago, Redbeard said:

I suspect 'put boards' should read PIR boards.

 

Correct . Thanks.

 

 

[Mr Punter]

16 minutes ago, Iceverge said:

 

Correct . Thanks.

 

 

 

Proper rough job.  It would still be rough without the rigid insulation, which elevates it to farcically bad.

[Mr Punter]

1 hour ago, allthatpebbledash said:

Why this?

 

The Dritherm 32 slabs are easy for the bricklayer to fit, easy for wall ties, decent u-value, don't make a mess if the wall gets drilled and easy to clean up mortar droppings.  EPS beads are good too but you said you are not keen on them.

[allthatpebbledash]

23 minutes ago, Iceverge said:

 

Correct . Thanks.

 

 

Wow that’s shocking.

 

Can you help what wool batts I should be looking at?

[JohnMo]

XPS poly beads, every gap is filled around the whole building, any damp gets in the poly beads allows it to drain away. Easy 

[DannyT]

I’m a bricklayer.

Just had planning approved for south Scotland. Was told rigid boards (PIR) is the standard with 50mm air gap. I sent a email over to building control about having Dritherm 32 fulfill along with the BAA certification.

They replied saying I can go ahead with it as long as installed to manufacturers standards.

There was no way I would do PIR. Even on my own build taking all the time in the world it would be a nightmare to install. Looks great on a drawing but it just don’t happen in real life and when the bricklayers not getting paid much to take the time you end up with a right mess as shown in photos above.

It repels moisture too so won’t get through the cavity, even in exposed locations like mine.

 

[Iceverge]

It depends on the U value you're after re the batts. 

 

They get a lot dearer once you get better than a k value of 0.035W/mK but I'm told they are easier to fit.

 

We used EPS bonded beads. Works fine.

 

 

[allthatpebbledash]

On 02/04/2026 at 19:57, DannyT said:

I’m a bricklayer.

Just had planning approved for south Scotland. Was told rigid boards (PIR) is the standard with 50mm air gap. I sent a email over to building control about having Dritherm 32 fulfill along with the BAA certification.

They replied saying I can go ahead with it as long as installed to manufacturers standards.

There was no way I would do PIR. Even on my own build taking all the time in the world it would be a nightmare to install. Looks great on a drawing but it just don’t happen in real life and when the bricklayers not getting paid much to take the time you end up with a right mess as shown in photos above.

It repels moisture too so won’t get through the cavity, even in exposed locations like mine.

 

What was the cost difference between using PIR and the wool batts? Are there any alternatives to this specific brand?

 

15 hours ago, Iceverge said:

It depends on the U value you're after re the batts. 

 

They get a lot dearer once you get better than a k value of 0.035W/mK but I'm told they are easier to fit.

 

We used EPS bonded beads. Works fine.

 

 

I want to ideally beat the new wall construction u-value of 0.18w.

 

I did some calculations online using that German website I seen linked on here. Keeping a cavity at 150mm, the PIR on paper appears better. But that’s the topic being discussed here, theoretically it does. If construction doesn’t meet proper standards, how much of the supposed r value is lost?
 

PIR achieves 0.15W

Mineral wool batts gives 0.18W

 

I can build a wider wall of course and throw in more mineral wool? Then for the lintels use split lintels and avoid the catnic thingies?

 

 

 

[allthatpebbledash]

On 02/04/2026 at 19:52, JohnMo said:

XPS poly beads, every gap is filled around the whole building, any damp gets in the poly beads allows it to drain away. Easy 

What if they don’t? Like you’ve built the walls, and then the beads don’t go all over. Then you’ve got uninsulated pockets of walls. I don’t like the idea.

[Mr Punter]

When the cavity is fairly wide the risk of voids is almost eliminated with blown eps beads. Probably the safest choice. Also if they miss, say an entire elevation it can be filled later.

[allthatpebbledash]

5 hours ago, Mr Punter said:

When the cavity is fairly wide the risk of voids is almost eliminated with blown eps beads. Probably the safest choice. Also if they miss, say an entire elevation it can be filled later.

Hmm I can see the advantages definitely. But that leaves me having to find someone in the North who can do this and to a good standard.

 

What about using Recticel 140mm with 10mm nominal gap as a way of rigid PIR and having a fully filled cavity?