How to fill 150mm cavity

[Iceverge]

On 04/04/2026 at 08:35, allthatpebbledash said:

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

 

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?

 

 

 

 

 

That "mineral insulation panel" has a k value of 0.045W/mK.  try it again with a "mineral wool 035" or "hk 33" for the EPS beads. 

 

 

 

 

[Oz07]

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.

 

You can still full fill up there then? I thought it went on exposure zones? The stuff is deffo water repellent. An offcut can sit in a puddle for a week and is no heavier for it. 

[Oz07]

On 05/04/2026 at 07:57, JohnMo said:

Or don't bother with cavity - block wall and external insulation? You can see exactly what is going on.

Not as good for finance or future sales purposes I think?

[JohnMo]

2 hours ago, Oz07 said:

Not as good for finance or future sales purposes I think?

People say the same for timber construction, which is standard building method in Scotland. No one has issue with finances or sales.

 

External insulation has been given out free for ages by the government, so not seeing the issue there either.

 

Personally I wouldn't do block or cavity build, it would be ICF or twin stud timber frame, as I could do both myself.

[Oz07]

23 minutes ago, JohnMo said:

People say the same for timber construction, which is standard building method in Scotland. No one has issue with finances or sales.

 

External insulation has been given out free for ages by the government, so not seeing the issue there either.

 

Personally I wouldn't do block or cavity build, it would be ICF or twin stud timber frame, as I could do both myself.

I think timber frame and block cavity are easy with finance. You'd have to check with CML or uk finance whatever they're called now make sure the majority of lenders are happy

[Oz07]

29 minutes ago, JohnMo said:

People say the same for timber construction, which is standard building method in Scotland. No one has issue with finances or sales.

 

External insulation has been given out free for ages by the government, so not seeing the issue there either.

 

Personally I wouldn't do block or cavity build, it would be ICF or twin stud timber frame, as I could do both myself.

 

[-rick-]

2 hours ago, JohnMo said:

External insulation has been given out free for ages by the government, so not seeing the issue there either.

 

Pretty much everyone who lives in a city knows someone who has been affected by cladding issues related to Grenfell. A lot of those are EPS EWI related. So even if mortgages don't mind there are a pool of buyers who may be put off by EPS EWI. Mineral wool EWI doesn't have the same problem but is a lot more expensive and frankly I doubt many buyers who are nervous about 'cladding' are going to worry about the distinction.

 

* Yes I know most buyers don't even know enough to think that deeply about what they are buying but even a subset of city dwellers moving out of the city is not a pool of people you want to put off IMO.

[Iceverge]

3 hours ago, JohnMo said:

People say the same for timber construction, which is standard building method in Scotland. No one has issue with finances or sales.

 

External insulation has been given out free for ages by the government, so not seeing the issue there either.

 

Personally I wouldn't do block or cavity build, it would be ICF or twin stud timber frame, as I could do both myself.

 

I could probably do any build method myself, masonry included. 

Unfortunately I would be long dead before I had finished. 

 

I think stick built has great advantages. 

 

1. Speed

2. Many options to source materials locally 

3. "Standard" TF constitution these days for mortgages and insurance etc. 

4. No big deposit to find.

5. Easier airtightness + thermal bridging details.  

6. No mega machines needed. 

 

Masonry is good but to make it work well you really need a simple box design, good masons and plasterers, a longer time scale and time for tricky airtightness and thermal bridging detailing. 

 

It's not impossible but for a high performance building (we did it) but it's more touble than it's worth. If you're will to accept something average it's fine.

 

ICF and twin wall factory frames are excellent when done well but carry the risks of higher up front cost, waiting times, deposits, scarcity of extra materials if needed. Big cranes on site.  Sometimes awkward insurance and mortgaging. 

 

Pick your poison. 

 

[allthatpebbledash]

On 06/04/2026 at 13:13, Iceverge said:

 

 

That "mineral insulation panel" has a k value of 0.045W/mK.  try it again with a "mineral wool 035" or "hk 33" for the EPS beads. 

 

 

 

 

Ah. Yeah so with the “035” wool at 150mm thickness it gives a 0.15w/ wall which is good. But it also shows a heavy set of rain drops suggesting huge condensation risk.

[ADLIan]

Condensation is often predicted with cavity walls. That’s the problem with taking the calculation at face value as in this instance any condensation risk is probably irrelevant. 

[DannyT]

11 hours ago, Oz07 said:

You can still full fill up there then? I thought it went on exposure zones? The stuff is deffo water repellent. An offcut can sit in a puddle for a week and is no heavier for it. 

Yes. The BBA certificate is for all exposure zones.

 

 

 

[Oz07]

10 hours ago, DannyT said:

Yes. The BBA certificate is for all exposure zones.

 

 

 

Presumably full fill too? You can buy those cavity sure trays if not now which give you a guaranteed 25mm or something clear cavity. 

[Iceverge]

15 hours ago, allthatpebbledash said:

Ah. Yeah so with the “035” wool at 150mm thickness it gives a 0.15w/ wall which is good. But it also shows a heavy set of rain drops suggesting huge condensation risk.

 

I wouldn't worry about it. The calculator takes a snapshot of a temperature and assumes it stays -5 deg forever. Not very realistic unless you live in a valley somewhere that never gets any sun.

 

Masonry is very tolerant of any damp. 

 

 Masonry wall with mineral wool is somewhat vapour open so drying does occur both back into the house and put through the wall .

 

 

 

 

 

[ADLIan]

30 minutes ago, Iceverge said:

The calculator takes a snapshot of a temperature and assumes it stays -5 deg forever.

Not so. Current version of BS does not use this number. It uses monthly weather data based on the building location

[Iceverge]

1 hour ago, ADLIan said:

Not so. Current version of BS does not use this number. It uses monthly weather data based on the building location

 

ubakus?

[allthatpebbledash]

On 08/04/2026 at 10:50, Iceverge said:

 

I wouldn't worry about it. The calculator takes a snapshot of a temperature and assumes it stays -5 deg forever. Not very realistic unless you live in a valley somewhere that never gets any sun.

 

Masonry is very tolerant of any damp. 

 

 Masonry wall with mineral wool is somewhat vapour open so drying does occur both back into the house and put through the wall .

 

 

 

 

 

Ice I’ve had a go at using the calculators on rockwool and knauf websites and I can’t seem to replicate the results.

 

I get 0.19w and no where near the number shown here. 
 

I’ve already said to the architect we should go with 140mm Recticel with 10mm residual as well as I couldn’t replicate the numbers using products available. Ooops.

 

[Iceverge]

I think this was on a forum here recently but I can't remember for who.

 

0.18W/m²K is just about achievable with a mineral wool 32 batt and 150mm cavity and some kind of light weight block inside.

 

In reality I think a 200mm cavity and a cheaper 0.36 batt is better. 

[Dave Jones]

avoid the tongue and groove celotex like the plague.

 

massive waste as it has to be cut when it comes to windows.

 

brickies hate it

 

taping nonsense every joint.

 

dritherm 32 every time. no waste, bulletproof and superior install.

[DannyT]

On 10/04/2026 at 05:30, Iceverge said:

I think this was on a forum here recently but I can't remember for who.

 

0.18W/m²K is just about achievable with a mineral wool 32 batt and 150mm cavity and some kind of light weight block inside.

 

In reality I think a 200mm cavity and a cheaper 0.36 batt is better. 

So I’m going a different route because our situation.

we purchased with concrete strip foundations already in place and can only go to a max 350mm overall to allow for required bearing on concrete.

Having the 150mm full fill with Dritherm 32 but on the inside we are lining the walls and ceilings with 25mm PIR and taped for air tightness with a service void.

That should achieve around 0.15.

Just the 150mm full fill is a tad over 0.18

 

 

 

[Oz07]

Ask AI what the payback period will be 25mm PIR is expensive for what you get. 

[DannyT]

1 hour ago, Oz07 said:

Ask AI what the payback period will be 25mm PIR is expensive for what you get. 

Not worried about the payback period. 
And for what I get that’s better than the insulation itself is surface I can tape and seal. You can’t get that with plasterboard dabbed onto thermo blocks.

Edited 44 minutes ago by DannyT