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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.

Nice! Stupid question: surely there is some cold bridging from the upright steel you insulated with aerogel, to this horizontal steel as they are touching? Or is it so many minimal it's best ignored?

Did you do anything for the horizontal steel above? (Got a similar set up going in soon).

Yeah, at same time I'm installing UFH with lots of insulation below. Flow temp at -2 would be 30degrees (ISH). Emitters are too small upstairs for open flow system, so planning on replacing them in 2-3 years then and putting in a heat pump then. For now will mix down the flow for the UFH.

If you do this then surely you have to breach the VCL multiple times when you run cables / pipes from your service void to your internal face. You want your VCL behind your battens, I'm from of your first layer of insulation (working from inside).

I'm sure you could. 2 different approaches: Estimate and simplify: just add a new source and exhaust on opposite sides of the building, and add estimated flow in and out at each point. External model: model the outside of the building, and see how much air travels through. Bit harder as you have to do temperature as well. However simscale (the platform) I have been using does do thermal modelling with airflow so I'm sure it's possible.

Yup. Complete overkill, but interesting. Takes about an hour to model a house, and then 20 min per run. Been playing with either optimising age of air in the house (low), or creating longest flows through house. Obviously whilst ensuring coverage. Not sure what's best. But, not really needed to just a play thing. Post is here on it:

Cool. Assumed as much. Assume that having the boards run continuously, then studding in front is slightly better, but still not great. Thanks. I'll look properly with WUFI at some point. Good to know the limitations. Super. That's basically my plan. MVHR is planned, just playing with CFD to work out flow etc.