I have mainly used tape and membranes in the jobs I have done (One full refurb, one large new-build and many partial jobs), but I suspect that many here would contend, probably rightly, that plaster may pass the test of time better. If you build in masonry and use hard plaster I am not certain the winds would move things enough to cause problems, but then in Sheffield I am quite a way from Atlantic gales!

On masonry, yes, hard plaster/render as the barrier - but still junctions etc where something flexible is necessary.

On timber framed construction, rather than membranes/tapes, airtight but breatheable board glued and screwed at all joints with gapfilling adhesive. OSB is the obvious candidate but there's disagreement as to whether OSB is airtight, not quality-controlled from manufacturer's batch to batch. I've put my faith in the words I heard from Peter Warm, one of UKs three (?) PH Assessors - 'I've never yet found a building that failed PH airtightness because of OSB'.

Glued and screwed OSB is very hard to install wrong, is within the skill range of everyday chippies, is robust in use - all the opposite of the dreaded tapes and membranes.

Posted By: nick1cDoes anyone know if the longevity of different methods of achieving good air tightness has been investigated and if so what the results were? It looks as if we may be in a position to build a house in a wet and windy site. I would love to construct it to achieve at least passivehaus levels of efficiency, but am concerned about the toll Atlantic gales will take over the years.

There have been investigations, yes, although I don't remember the names of the authors or papers unfortunately. The Pro Clima and the Siga products are good and have both good theoretical/tested results of up to 100 years and practical results exceeding 30 years or so to date. There may be other good products but there are also horror stories about some products so caveat emptor.

Plaster is good - it's my main airtightness barrier - but you need to think about where it is likely to crack and have a backup plan. Cracking depends on the substrate and the geometry. It's very likely at corners and junctions and having membranes and tapes behind the plaster (and keyed into it) at such locations is pretty much essential. Slab to wall, wall to roof, doors and windows, corners of the building etc. Some cracks can be seen and repaired but other locations will be hidden. Also think about penetrations etc. I just made a rule that there are no services in my external walls and nothing connected to it, except by glueing. Most people will want to find a different solution, I expect. Cracks form because of differential movement as the building dries out, no matter how solidly built, as well as movement due to wind.

Medite make OSB products that are guaranteed airtight; I don't know about other manufacturers. The guarantee applies when the panels are taped together. I don't know whether anybody has done any testing of glued panel structures, nor how they deal with the omission of expansion joints. I've no reason to suppose that the system Tom describes doesn't work though.

I got to justless than one ach/m2/m3/h at 50Pa on my masonry build, no tapes used.

I've thought about this too.

I dont understand why people dont like foams - around a floor joist in a retrofit, and not exposed to uv, its airtight and can take quite a bit of movement (apparently).

Re wet plaster, the good point about it is that cracks can be seen and fixes, I have virtually no cracks on the inside of my outside walls and only one or two minor ones on internal walls avowed doors.

I plastered the membranes and the “tony tray” into the walls by trapping them being expanded metal.

Posted By: fostertomOn timber framed construction, rather than membranes/tapes, airtight but breatheable board glued and screwed at all joints with gapfilling adhesive.

We build with SIPs, additional internal insualtion, internal airtight barrier and tapes. Our neighbour did timber frame with external OSB and gapfill adhesive. Having seen/done both I'd go for OSB/Adhesive if I was building again. A lot less faff and the external shape of a house is a lot easier to work with than the internal. Also seems much less chance of damage to the airtight layer from internal works in future

Posted By: Simon StillA lot less faff and the external shape of a house is a lot easier to work with than the internal. Also seems much less chance of damage to the airtight layer from internal works in future

Exactly, Simon.

Also, if the studwork voids are filled with blown-in cellulose rather than insulation roll or cut boards, the cellulose itself is pretty airtight (tho not ideally enough on its own) so any accidents/later punctures of the (well shielded) OSB is backed up by in-depth supplementary airtightness - so the overall spec is very robust longterm.

That's in addition to the great virtues of blown-in cellulose - first, being organic material, it's hygrocopic (a separate topic) and second, because it fully fills all gaps without joints and remains under slight elastic compression, the actual in-use insulation value it delivers is far beyond its modest nominal lamda, compared with the alternatives mentioned above.

''hygrocopic (a separate topic)''

Poetry!

Insulation across the nation

Is there any reason the same approach (external/sealant adhesive joints) wouldn't work on a SIP or CLT construction?

Posted By: Simon StillIs there any reason the same approach (external/sealant adhesive joints) wouldn't work on a SIP or CLT construction?

How would that work, when the outer skin stops at the edge of ea panel i.e. doesn't bridge unbroken across joints or between elements? In other words it's full of joints to be sealed by some method, vs eliminating such joints by making the outer skin continuous. The challenge is not the general surface of the panels, but the joints between them.