mistake_not Posted March 4 Posted March 4 (edited) 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 Posted March 10 Posted March 10 150 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 Posted March 10 Posted March 10 @Dave Jones 125 mm bats are readily available and (depending on the block used) can easily meet the new regs.
mistake_not Posted March 10 Author Posted March 10 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 Posted March 10 Posted March 10 EPS will have the same results as PIR if poorly installed. You could also consider blown bead once its built too.
Nickfromwales Posted March 10 Posted March 10 On 04/03/2025 at 22:10, mistake_not said: However, is there any potential issues with air tightness with using them? Expand 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 Posted March 10 Author Posted March 10 On 10/03/2025 at 16:05, 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. Expand Cheers. Good to confirm thoughts; have detailed the warm roof insulation to go over the cavity with an overhang. 1
Nickfromwales Posted March 10 Posted March 10 On 10/03/2025 at 19:05, mistake_not said: Cheers. Good to confirm thoughts; have detailed the warm roof insulation to go over the cavity with an overhang. Expand 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 Posted March 10 Author Posted March 10 On 10/03/2025 at 21:11, 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. Expand 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 Posted March 10 Posted March 10 You're specifying a warm roof, so no ventilated void, therefore the quoted post doesn't apply (unless I've missed something). 1
Iceverge Posted March 11 Posted March 11 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 Posted March 11 Posted March 11 On 10/03/2025 at 23:33, lookseehear said: You're specifying a warm roof, so no ventilated void, therefore the quoted post doesn't apply (unless I've missed something). Expand 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 Posted March 11 Posted March 11 On 10/03/2025 at 12:44, Canski said: @Dave Jones 125 mm bats are readily available and (depending on the block used) can easily meet the new regs. Expand sure about that ?
MikeGrahamT21 Posted March 11 Posted March 11 On 11/03/2025 at 11:53, Dave Jones said: sure about that ? Expand 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 Posted March 11 Posted March 11 On 11/03/2025 at 11:58, 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 Expand 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. 1
MikeGrahamT21 Posted March 11 Posted March 11 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 Posted March 11 Posted March 11 On 11/03/2025 at 12:05, 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 Expand that calc looks wrong. its the uncorrected U value by the looks. try https://www.changeplan.co.uk/
MikeGrahamT21 Posted March 11 Posted March 11 On 11/03/2025 at 12:08, Dave Jones said: that calc looks wrong. its the uncorrected U value by the looks. try https://www.changeplan.co.uk/ Expand 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 Posted March 11 Author Posted March 11 On 11/03/2025 at 08:04, 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. Expand As you should! 🙂 1
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