Cavity insulation and air tightness

[mistake_not]

So considering options for insulating the cavity of our extension.  Have come to the conclusion that using PIR ends badly from an air tightness and gaps perspective. Learnings from this thread:

Cavity is currently planned at 125mm and would prefer not to tweak. So thinking my options are:

 

- full fill batts

- closed cell foam

- EPS

 

In terms of what's easiest for builder not to get wrong, and not need another sub contractor, and VFM full fill batts seems the way to go. However, is there any potential issues with air tightness with using them? 

 

To note inner leaf will be parge coated for air tightness and some IWI added (mineral wool) before plasterboard.

Edited March 4 by mistake_not

[Dave Jones]

150 cav is much easier to source batts for and is also the minimum cavity to meet new build spec when using batts. just because your extension doesnt have to meet new build regs, i'd still not want a poorer standard.

 

no need for any internal insulation then.

[Canski]

@Dave Jones 125 mm bats are readily available and (depending on the block used) can easily meet the new regs. 

[mistake_not]

Main reason for 125mm cavity is aligning to existing walls that will need IWI. However yes can easily tweak cavity size. 

Given you can get EPS and Bats at same lambda value (0.32) any thoughts on best filler?

[MikeGrahamT21]

EPS will have the same results as PIR if poorly installed.

 

You could also consider blown bead once its built too.

[Nickfromwales]

On 04/03/2025 at 22:10, mistake_not said:

However, is there any potential issues with air tightness with using them? 

Airtightness is not achieved at the cavity   You’re referring to “reducing the draughtiness” of the structure here. Not splitting hairs but there’s a huge difference so just want you to be aware of choices vs remit.
 

To achieve airtightness you need an airtight house, pointless making this exceptional if the house is is bolted to is ‘ok’, so remember the balance of things when investing here. 
 

I’d say go blown bonded beads, and fully fill the cavity. Detail the insulation in the roof to overhang the cavity, and those will serve you well.

[mistake_not]

2 hours ago, Nickfromwales said:

Airtightness is not achieved at the cavity   You’re referring to “reducing the draughtiness” of the structure here. Not splitting hairs but there’s a huge difference so just want you to be aware of choices vs remit.
 

To achieve airtightness you need an airtight house, pointless making this exceptional if the house is is bolted to is ‘ok’, so remember the balance of things when investing here. 
 

I’d say go blown bonded beads, and fully fill the cavity. Detail the insulation in the roof to overhang the cavity, and those will serve you well.

Cheers.

 

Good to confirm thoughts; have detailed the warm roof insulation to go over the cavity with an overhang.

[Nickfromwales]

2 hours ago, mistake_not said:

Cheers.

 

Good to confirm thoughts; have detailed the warm roof insulation to go over the cavity with an overhang.

It's detailing such as this which will reap the biggest rewards tbh.

 

Remember not to cover the cavity completely, so whatever gap you have for the breathable part of the cold roof void, above the insulation, should be the same at that junction.

[mistake_not]

1 hour ago, Nickfromwales said:

It's detailing such as this which will reap the biggest rewards tbh.

 

Remember not to cover the cavity completely, so whatever gap you have for the breathable part of the cold roof void, above the insulation, should be the same at that junction.

 

I'm now confused again.

 

I was planning on doing something like below (image from NHBC), but full fill rather than partial fill cavity obviously.

 

[lookseehear]

You're specifying a warm roof, so no ventilated void, therefore the quoted post doesn't apply (unless I've missed something).

[Iceverge]

Is the construction a warm roof is the question I think. 

 

The following are all warm attics but not all warm roofs 

 

Is your insulation :

 

A. All above the rafters? ( Warm roof)

B. Above and between the rafters? (Hybrid roof) 

C. Just between the rafters?

D. Between and below the rafters? 

 

[Nickfromwales]

8 hours ago, lookseehear said:

You're specifying a warm roof, so no ventilated void, therefore the quoted post doesn't apply (unless I've missed something).

Yup. Sorry @mistake_not, I assumed a cold ventilated roof.
 

I will punish myself by forcing beer down my neck this evening until I feel I have suffered sufficiently.

[Dave Jones]

23 hours ago, Canski said:

@Dave Jones 125 mm bats are readily available and (depending on the block used) can easily meet the new regs. 

 

sure about that ?

 

[MikeGrahamT21]

4 minutes ago, Dave Jones said:

 

sure about that ?

 

 

Change your glass fibre on to 0.032 and the blockwork to 0.09, currently the best values available for these, not that i'd want to use any more AAC blocks, but they're an option nonetheless

[Dave Jones]

2 minutes ago, MikeGrahamT21 said:

 

Change your glass fibre on to 0.032 and the blockwork to 0.09, currently the best values available for these, not that i'd want to use any more AAC blocks, but they're an option nonetheless

 

link to the blocks with 0.09 U ?

 

aerated conc are crap for air tightness and the normal ones are 0.15U

 

still a fail for regs.

[Dave Jones]

[MikeGrahamT21]

https://www.thomasarmstrongconcreteblocks.co.uk/airtec-aerated-concrete-blocks/airtec-xl

 

 

Aiming for 0.18W/m2K for an extension, so still a fail, but very close

[Dave Jones]

2 minutes ago, MikeGrahamT21 said:

https://www.thomasarmstrongconcreteblocks.co.uk/airtec-aerated-concrete-blocks/airtec-xl

 

 

Aiming for 0.18W/m2K for an extension, so still a fail, but very close

 

that calc looks wrong. its the uncorrected U value by the looks.

 

try https://www.changeplan.co.uk/

 

[MikeGrahamT21]

1 minute ago, Dave Jones said:

 

that calc looks wrong. its the uncorrected U value by the looks.

 

try https://www.changeplan.co.uk/

 

Think i'll stick with ubakus to be fair after looking at that site! Looks like it was designed in the early 2000's lol.

 

https://www.ubakus.de/u-wert-rechner/?

Its made by someone who studied Physics, not some random IT Support company. I'd advise using a more accurate tool.

[mistake_not]

5 hours ago, Nickfromwales said:

Yup. Sorry @mistake_not, I assumed a cold ventilated roof.
 

I will punish myself by forcing beer down my neck this evening until I feel I have suffered sufficiently.

As you should! 🙂