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In my renovation I have ripped off the window sill around my bay window and exposed the cavity underneath. It's had some failing glass foam insulation in it, so have emptied it all out. For context the wall is a traditional brick and block build, with the window sat on the outer brick leaf. I will seal to the window with insulation and air tightness, but am unsure what to re-fill the cavity with The cavity varies in size between 60 and 35mm, would seem obvious to use EPS beads. I thought maybe use an old bin to mix the beads and PVA and then out them in? Anyone got any experience or better ideas? It's only 1m high, and about 2m along.

Not yet- but working on it as part of renovation/ extension. Aim to be when I'm done. Mvhr going in as part of the same work.

Been searching and thinking and can't think of / find / work out how do air tightness well for my extension other than taping everything in sight like @plockhart is. The extension is going to be a warm flat roof attached to an existing cavity wall, with a new full fill cavity wall. Airtightness on new walls achieved with parge coat and old walls with wet plaster. Roof buildup so far is: - plasterboard - joists - OSB - VCL - Insulation - EPDM Can see how I can easily join the VCL to either wall, and therefore am I better just omitting it, and ensuring the OSB is vigorously taped to all joists / itself / the walls, or doing something different? I don't like the idea of the VCL below the OSB, as the OSB is then effectively sandwiched between 2 airtight layers and has a moisture risk. Grateful for wisdom / pointing me to the answer elsewhere.

I was pleased with the collingwood ones we had in our last place: https://www.collingwoodlighting.com/product-category/indoor/downlights/

Dodgy ubakus calc below. No idea what the aerated concrete blocks are I selected, and I changed the lambda value of the PIR to 0.019 to match k108. Note kingspans calculator suggests this as a way to hit u values, but no details on actual materials except the K108. @Caroline I'm assuming the celotex you were quoted is still a PIR board, so even though it's a bit cheaper, you still have a load of issues. As you say slightly bigger cavity. I.e. If you have a cavity of at least 130mm and do EPS or mineral wool batts you will save loads and have decent insulation.

It does with super light blocks. You can technically hit 0.18 I value.... Sounds like a rubbish idea though hence I changed my spec.

Yeah my builder specced the same. If you do a full fill cavity batt at 125mm you get basically the same u vale on paper: https://www.insulationsuperstore.co.uk/product/knauf-dritherm-32-125mm-cavity-wall-slab---2184m2.html * Note you might need to do something stupid like 130mm to pass building regs. Or just widen to 150mm. Side benefit, even at 150mm it's much cheaper than k108. At least half price. The same is true for EPS. Only issue I can see with EPS is the cosmetic element of holes. Not an issue if you are rendering though. Also see my discussion and calcs on it here: https://forum.buildhub.org.uk/topic/43093-cavity-insulation-and-air-tightness/

Ref airgap size: No idea. I'm a DIY builder and planning a warm roof. Just know an airgap is required. Aerogel: lambda value ranges from 0.015 to 0.023. So not much different to PIR. Another option is Vacuum insulation panels (aka VIP). They are a lambda value of 0.007 I think, made to measure and approx £200 a square meter. Would mean you stay between joists. Check out kingspan OPTIM-R. Re Q3. Not sure. Maybe. Re q4, it would be ok on 400centres. Much better than just taped (wet skim coat)

Mineral wool / EPS would increase your insulation depth required by 1/3. I.e. a total of 185mm. I think you are stuck with a cold roof buildup, so will need a ventilation gap no matter what.

Quick play using PIR if you just fill between rafters gives u value of 0.399. To hit the required Building Regs U value you need another 65mm of PIR. This assumes PIR of 0.023 R value. As I'm sure others will comment, it's great on paper, but really hard to do well unfortunately.

Here's my Ubaqus calcs, including a batten (wrongly sized) for metal. Metal first Then wood. Sizes based around 40*63 CLS. Also annoyingly discovered my existing cavity is 50mm, not 100mm, and the existing insulation in there isn't in great shape.

Are you just using PIR foil tape? Or a different air tight tape? Interested as probably going to have to do similar. Context: our extension build hasn't started yet, but can't work out how to join the VCL above OSB to parge coated blocks on walls. In some ways I think why bother with the VCL.

For awareness: Done some quick u-value calcs. Using metal studs looks to be worse than wood studs due to thermal bridging. 0.2 vs 0.19

Awesome thanks for the detailed reply. Yeah was skeptical, hence the question. I got BPC to do a design, and they specced the semi flexible ones, didn't like the idea, so sought wisdom here. Thanks for validating my thoughts! Yes have oversized unit: it's capable of 375m3, but my max requirements from BR are approx 170, so hopefully running quiet. I'll probably try and get the 900mm version of these in then: https://www.blauberg.co.uk/blauberg-attenuating-silencer-duct-mounted

Not an expert by any stretch, but detail library lets you buy BR compliant cad drawings for something likely £3 a go that is good for checking: https://detail-library.co.uk/product/mr7/

Putting together my mvhr system, and want a sanity check on silencers. For context I'm planning a radial system. The questions: - where do you place them. I assume I only need 2, each fitted between the unit and the supply / exhaust manifolds. - anyone had experience of the semi flexible BPC ones? If so any good? There also look like there are some ok ridgid ones from other suppliers. And before anyone says, yes aware I need to factor in pressure losses etc 🙂

I have ordered one of these from this seller. Lets see how it goes!

1. New layer 2. Select the various menu 3. Select the air type

As you should! 🙂

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.

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

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?

Agreed - actual plan is either 63mm CLS or metal c / I stud. Just drew it badly as 2 bits on the air gap drawing, and didn't correct the non air gap one.

Circling back to this. One final thing I am confused / not sure about: My current plan after advice is to keep the existing plaster for airtightness, adding airtight paint at top and bottom where it is missing / between floors, and tape round windows. Then add mineral wool with no vapour barrier. Material profile below (not airgap size / service cavity size TBC). However, from reading others posts this may have air convection losses with air travelling up in front of the mineral wool as I am not planning to install a vapour barrier / membrane in front of the insulation. Are the losses going to be significant? Or is this not really an issue. This resource suggests I should avoid doing what I am doing: https://www.eurima.org/uploads/files/modules/articles/1607345579_DEF_2020_eurima_howtoinstall.pdf My easy alternative is just to remove the air gap as shown below. But that means electricians have more of a pain running cables / boxes etc, and likely require thicker cables. Thoughts appreciated!

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.