I am retrofitting a 1960s detached house.

I am trying to decide how best to deal with airtightess at the upstairs ceiling level. It's not terrible with just the existing plasterboard but it has been hacked-around where the loft hatch got moved. I've put some blowerproof on the joints but interesting that hasn't worked perfectly. Seems like 2 coats is a good idea.

My main question is about putting up a membrane, and the pros/cons of plastic sheet, breathable roofing membrane or smart intello-type membrane.

Obviously best practice would be pull down all the ceilings, take off the internal wall-tops, make a truly epic mess and put a continuous intello sheet across the whole ceiling. (Or decide to fit a warm roof). I'm not doing that :-)

However I am considering putting up a membrane in each room between the walls (wet plastered block so reasonably airtight) under the existing ceiling, tape it to the walls then put in a new ceiling with a ~20mm service cavity for light wiring.

I have already done this in the main bedroom a few years ago. I just used cheap green polythene sheet from the builders merchant, held up with orconF during installation (then screwed battens). So there is no vapour permeability. The question is how much does this matter in a building with MVHR and a ventilated loft?

A vapour-permeable membrane would be more robust long-term as it would allow drying down as well as up (in the summer at least when the loft is warmer than the house - not sure if any vapour would move down towards the warm side in winter?

So I _could_ buy a roll of intello but it's quite expensive and I'm not sure there is much point in this application.
I do already have a roll of Cromar Vent3 roofing breathable membrane which is specified as wind-tight and breathable with an Sd of 0.02 m
https://www.cromarbuildingproducts.com/products/vent3/
What it doesn't specify is whether this stuff is deemed airtight.
I guess I could do a test.

Has anyone tried using these roofing membranes as airtightness membranes? Is it a bad idea for some reason?

Apart from being 2.5 times cheaper than Intello, I can easily go to screwfix and get some more if needed (£114 vs £249 for 1.5 and 50m)
I also still have loads of green plastic which is very cheap, and wider so fewer taped joints.

I think the impermeable plastic is probably fine under normal circumstances. If there is any condensation then it'll go into the plasterboard and diffuse then evaporate into the loft. I know my loft is well-ventilated due to a major leak a few years ago (in Jan, which dried out by about March without any significant harm occuring to timbers). A leak like that again would presumably dry out more slowly with a vapour-impermeable membrane in place (but also with a vapour-open one and a second ceiling).

So what do we reckon. Is just plastic really a bad idea here? Any reason not to try something like Vent3? Are you all going to tell me to just go any buy a load of intello?

I don't recommend air testing like this:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16060

But it did reveal that plasterboard ceiling was pretty airtight, compared to the wall-ceiling corners, electrical fittings and loft hatch, which were not.

So adding membrane to your existing plasterboard might not add much, but if in the process you permanently address those kinds of leak points, then that could be good, though maybe there's easier ways to do that?

If the ceiling is well insulated above the membrane and the rooms and loft are well ventilated, then I wouldn't worry about breathability.

Are you thinking of rewiring, to bring the lighting circuits 'inside' the a/t membrane - if so, will you have to open the walls to run the new droppers to the switches and pass the ring from room to room? You might have to meet new regs if you rewire (metal CU, mains smoke alarms?)

OK. Glad to hear everyone else thinks polythene is fine too.
By taping to the walls that should deal with any leaks round the edges (I don't have cornices).

Yes I am pondering whether to move some/all of the lighting wiring inside, as opposed to having two penetrations per room as is it at the moment (switch drop + lamp). My lighting circuit is old enough not to have an earth in it so really needs re-doing anyway (although I have removed all the metal fittings as having those on a no-CPC circuit seemed a bit dodgy even to me :-) So I have to do a load of re-wiring anyway, but moving it inside commits me to re-doing all the ceilings, which as Will says may not actually be that leaky.

The main advantage of leaving it in the loft is future accessibility and not having to re-do the plastered-in switch-drops. It's not that hard to put grommets on the wire entries and goop down the switch drops if they leak. I've not tested how leaky all the existing stuff is yet. If it's really bad then that's an incentive for changing it all. I have the last three old CFL pendants to get rid of anyway and turn into LEDs. The LED driver unit really needs to remain accessible somewhere as they don't last forever. I hate having one tiny, inefficient, driver for every lamp (which is how they tend to come), so like to fit one current driver and 2 or 3 emitters per room. Drivers in the loft are easy to replace. Ones in a service cavity rather less so. Some kind of neat hatch-fitting is one possibility, or put them all in a cupboard and wire everything back to there. Normally keeping the LV cabling short is a good thing, but ultimately it's ~6-12W per room and ~500mA in the LV cables so it doesn't really matter, especially with constant current drivers.

I'm perfectly happy to replace the lighting circuit wiring without changing the CU. I'm ignoring part P until everything is finished and there is no 1960s wiring left. Then I'll get an EICR. Going through the electrician rigmarole every time you do any job is just ridiculous. Especially for lighting now that lighting circuits are all LED and dramatically lower power than it was designed for. It'll be a hell of a lot better than it was, as opposed to being old and compliant, but a bit 'C2' (no CPC). I'd have had about 12 lots of £several-hundred by now doing it by the book. And I don't trust electricians any more than I trust plumbers or car mechanics to actually do it right. One does have to be a bit more careful with EV charger circuits :-)

That would work, but I like a nice simple mains switch that costs a couple of quid over the fancy kinetic switches at £30 per switch+receiver pair.

OK. So I've thought about this a bit more and have realised that there are some issues with having mains wiring in a narrow ceiling void. My battens are 25mm PAR so you end up with about a 20mm service void (I don't want to lower the ceiling height excessively). Loose mains cable in there is not going to comply with the '50mm clearance for screws/nails' rule. It's deemed fine just lying on plasterboard ceilings when the joist height is 70mm or more because there is plenty of room for the cable to just move should someone try to nail it from below, but with a 20-25mm cavity that's not the case.

And I wanted loose cables so one day in the future it would be relatively easy to thread more wires or change things around. However, the only way to do this in compliance is presumably to put the cables in metal conduit? And once done that way there will be no practical way to re-thread wires without taking down the ceiling.

Is there a rule for ceilings like the rule for walls where there are perpendicular routes in which it's OK to route cables less than 50mm from the surface, with just thin plastic/tin covers. I presume not so it's proper nail-proof conduit or nothing (or trunked wiring outside the void hidden by a cornice?)

And the 20mm conduit couplers will be fatter than 20mm so I'm not sure how well it will actually fit?

How do people do the ceiling/lighting wiring in normal passivehouses, with a nice membrane over the whole ceiling?

Posted By: wookeyHow do people do the ceiling/lighting wiring in normal passivehouses, with a nice membrane over the whole ceiling?

Not sure. We simply didn't put any lights or sockets in the airtight layer. Downstairs the ceilings are fair game; upstairs all the lights are on walls or on the top of dwarf internal walls (the tops of two built-in wardrobes). Maybe the airtight layer is at roof level so you don't care about the first floor ceiling in a conventional design? Or use deeper battens? Or Instaabox or similar airtightness 'gloves'. I guess other people run the cables in the joist space, along the centre of the joist and seal them where they penetrate the airtight layer.

The 50 mm depth, RCD and metal rules are for cables that are not in safe zones.

Posted By: tonySmall hole through membrane, sealed from above before insulation and below after terminating wires with acrylic sealant

Not in a passivhaus, no way. Either a grommet (e.g Pro clima or Dafa) or my favourite, short lengths of stretchy airtight tape (Siga Rissan is my favourite) which can be used for cables, pipes and ducts: any size penetration.

"One does have to be a bit more careful with EV charger circuits :-)"

No more so on a 7kW charger than a 10kW shower

"the only way to do this in compliance is presumably to put the cables in metal conduit?"

Not the only way. Massive nuisance as the conduit has to be fully and properly earthed and that's a big job. One does not put a wire down a pipe or cover it with a bit of tin, as penetration with a metal fixing through the conduit could connect just the love core and the conduit, meaning the entire conduit is live

If you're really bothered about it you could use properly earthed SWA but most people would just use an RCD or RCBO (about ten quid more than an MCB), or use a penetration patch (eg kaflex) and strive to run cables with a 50mm clearance from the finished surface

"I'm ignoring part P until everything is finished and there is no 1960s wiring left. Then I'll get an EICR."

It's not the same; you're engaging in notifiable work. Notify properly or engage someone who can, otherwise it's illegal. An EICR does not prove compliance