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How do you deal with pipe work that falls below an airtight membrane? Is it a case of cutting and taping around said pipes?
This is on my to do list for next year. We have insulation but opted for no membrane as we’re having new heating put in and the membrane would have been hacked to shit. I’m mindful of all the pipework from sink/bath/kitchen plus rad pipes that will need to be navigated.
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There are special grommet patches that pipes pass through, which are then taped to the wall/substrate. I used a load of these in addition to Ilbruck FM330 airtight expanding foam. They're difficult to retrofit to existing pipework though, as you need to cut the grommet and re-tape. There are also special retrofit grommets, but I didn't achieve very good results with them.
Available here: https://www.ecologicalbuildingsystems.com/products/airtight-windtight-systems/pipe-and-cable-sealing/grommets
the best way would be to get underneath and use airtight tape (with primer) to seal between the masonry and whatever insulation is between the joists. Ideally you'd run an airtight membrance from wall to wall, sealed with tape at the joins. That also has the benefit of improving the warmth of your insulation as cold air will no longer 'windwash' it.
Next best is to take skirting off and seal between floor and wall, then replace skirting.
After that, you're probably looking expanding foam tape - along these lines https://www.airtighttapes.co.uk/store/Xpanda-600-Expanding-Foam-Tape-BBA-Approved-c34468280