Please could anyone help with the following, re. airtightness barrier. We are carrying out a restoration of a 1950s church hall, rendered cavity, brick and block construction. We have a Kingspan Quadcore composite steel roof, fitted with additional seals. The floor build up is 300mm concrete, 200mm PIR and 70mm Cemfloor screed, painted at the floor junction with Pro Clima Aerosana Visconn to continue the radon barrier under the concrete. The task is to make the walls airtight. There is a 200mm EWI system on the outside of the building. It is twin skin cavity, but having been filled with concrete due to the poor state of the block work. I was going to continue the Pro Clima Aerosana Visconn from the roof panels down to the floor. I’ve used the airtight paint but it seemed a little frail for our site conditions. e.g. we used it on some of the wall sections, where the joists run parallel to the wall and would be subsequently difficult to access to plaster. But having had a few knocks with bits of flooring etc. the paint has peeled off the wall. There are lots of maybe’s…. The EWI maybe reasonably airtight at the external render surface. The concrete in the cavity maybe airtight where it is continuous, but it is impossible to know. The cement render on the internal brick/blockwork maybe airtight.
So my question is, would a plaster parge coat be a better bet and if so what plaster? The plaster will lap airtight tapes or paint at the window/door junctions. Please note, this is only to provide the airtight barrier. A decorative board will create a services cavity to run pipes, ducts and cables etc. British Gypsum obviously make a full range but they don’t have any real world figures regarding airtightness. I spoke to their technical support and they recommended their Soundcoat Plus. However, they can’t say that it is 10 times better than their other products, since no one has conducted any tests. The Passive House Guide To Airtightness (my Bible!), suggests 6mm of wet plaster is airtight.
I wanted to use the Thistle Universal OneCoat plaster to get a good build up and reduce the labour costs. Has anyone had a similar situation and achieved good results with this solution. Any comments welcome. Cheers

I don't really understand the situation with the airtightness paint. You say it has been damaged, but IIUC when everything is finished it will be at the back of a service cavity and so protected from any further damage. So why not simply repaint the damaged areas?

You say it is difficult to know whether any of the layers are airtight, which is true, but it is relatively easy to test for airtightness and to identify any leaks whilst doing so. So maybe it is worth doing a test before spending too much time reworking the airtightness layer?

You don't say what your goal is, either overall or in terms of an airtightness result? Is the hall being restored as a hall, or as a dwelling?

I don't see any problem with gypsum, indeed in some ways it's better than lime since it expands when setting, but it is also more rigid than lime and cracks more easily. But do you need a plaster parge coat?

Posted By: tuftythesquirrelThe aim is to use the house as a dwelling and get it as close as possible to Passive House (0.6 AC/H) but since it is a refurbishment I’m still getting an idea how difficult this could be. The airtight paint layer seems to get damaged on a daily basis e.g. when installing our posi-joists, moving materials around internally etc. Plus in some cases the original masonry is very poor with cracks right through to the cavity, so painting isn’t a great option.

OK, thanks, that gives an idea of your goal. I suspect you'll have to work quite hard to get a refurb to a 0.6 ACH level. The EnerPHit standard has 1.0 ACH as its target I think so you could use that as a backstop.

I was going to suggest that you leave the airtightness paint until nearer the end, when there's less chance of it being damage. I think that's probably still the case, or maybe a better plan is to paint it as soon as its practicable and then plan to touch it all up near the end just before it is covered up. i.e. Don't obsess in the middle period.

I'd certainly suggest patching up any holes or major defects in the walls before doing anything else. You can always combine airtightness techniques - we used our internal lime plaster, plus loads of membrane in the roof and a few other areas and miles of tape at junctions such as windows.

I think it a bit too early to consider an airtightness test as we will undoubtedly fail pretty spectacularly.

One possibility is to make yourself an airtightness fan unit. You can use that at any time to find leaks and get some idea of how bad the problem is, then just get a proper test at the end. To make one you just need an electric car radiator fan and an adjustable power supply for it, plus a pressure gauge (the analogue manometer type is adequate).

I think I’m going to go with the plastering option. I’ve put mesh in the first coat for durability, which is also cheaper than the airtight paint. I can get a good 8mm layer on using the BG Thistle Bonding Coat for the first coat with mesh then, the Universal OneCoat for the topcoat. I’m still using the airtight paint where the masonry meets the windows and roof etc.

Plaster tends to crack at corners, so make sure to do a good job there. You can also put a bit of plaster-in membrane/tape behind the plaster at corners, as insurance. You can always use paint afterwards of course.

Best of luck with your project. Whereabouts are you, if you don't mind me asking?

Just a quick point on the DIY pressure tests.

You don't need to get particularly good seals around the fan unit, or indeed any of the partially complete work. You're not looking for a flow rate at a given pressure differential. You just need to get that pressure differential, and then use a smoke/vapour generating device to show where the leakage paths are.

I always do this sort of test on the houses I build - before plasterboard, even with the odd door missing (sheet of OSB fixed over the gap). If you wait till the house is finished, you may have difficulty getting back to the leakage point, and be chasing air movement going towards that leak.

I've found some howlers over the years, where the joiner just didn't bother doing what he was instructed to do, or the spark/plumber didn't bother using the rubber seal patches I supplied.

As regards sealing the walls, sounds like you have a firm, flat substrate. Could you not use a self adhesive VCL, by the likes of Glidevale or Proctor. I suspect the challenge will be less about the walls, and more about the junctions (ie around doors/windows, corners, wall to ceiling/roof. I say self adhesive, as that would be easier to install, but if you batten over the walls to install the finishing boards, then any VCL layer can be mechanically clamped by the battens, and if in doubt, use the proclima goop.

Posted By: tuftythesquirrelI like the idea of the home-made fan, the biggest issue from my point would making it seal properly in the door/window.

It's not too difficult. Mount the fan on a lump of something flat (ply perhaps). Then cut another piece of the flat stuff sized to fit in a window. Screw the smaller piece with the fan attached to the larger piece. You don't have to do this the first time, it's really about how to use it again in a differently sized window. Then fit the larger piece in the window.
You just open the window and push the flat plate into place. Then seal the joints between the fan and flat stuff and flat stuff and itself and flat stuff and window using copious quantities of tape. Use some tape that will come off again, not pukka airtightness stuff! You can use sticky sealants or compressible window draught excluder instead. Whatever works for you.

The merchants and builders are regularly telling me they’ve never seen it done that way and as a result, have next to no stock of the more sophisticated components used in a Passive House. I just can’t believe Jewsons, Travis Perkins etc. don’t stock a single airtightness product. It seems to indicate how far we have to go before our housing stock starts to improve.

Yes I found the same thing six years ago. It doesn't sound like things have moved on much. But at least the specialists are helpful and do seem to be pretty competent.

I have two airtight layers in some places and have been grateful for it. The primary barrier in the areas where the structure is timber frame (ring beam, awkward corners) is a membrane but then there's a render board and lime plaster over that as well. But those are exactly the areas where we've had some small cracks so it's been nice not to have to worry about them until we got them filled this year for aesthetic reasons.