I need to airtight the area between joists on the north wall of the house. There will be EWI going on so on the inside it just needs to be airtight.

Given the need to tape round joists, what do people do between the joists? The existing wall is wet-plastered to a depth of about 15mm, so there is a step where the plaster stops at ceiling level.

I have previously discovered that taping round joists where there is a step is a massive PITA so it's a really good idea to make the surfaces flush so that means a parge layer about 15mm thick to match the plaster.

When doing IWI in the past I have used gun foam to fill the area, then put the insulation on, then taped (Tescon) to the foil facing. Works great.

But just using gun foam here would leave the cut-off foam surface which is very rough so hard to stick tape to well. One could use an orconF bead under the tape to help ensure airtightness, but I thought just parging might work better.

So what should one use for parging? Conventional seems to be a 1:3:1 cement:sand:lime mix, but not normally 15mm thick. Does it matter doing it thick? Also cement doesn't dry all that smooth in my experience so I wonder how well tape sticks to it?

What about plastering? I guess I'd need the conventional 2 layers: base coat and finishing to get a nice smooth surface to tape to. Should work, but it's going to be quite fiddly between joists. Maybe just use base layer and tape to that as 'smooth enough'?

Any other suggestions? A few layers of jointing compound (much easier to work with than plaster in my experience)

What would you do? This must be a common-enough problem on retrofits but I failed to find a discussion in the archives.

Here is a pic. The 15mm step at the top of the plaster is not obvious here but you can see where it is.
And yes the floorboard above will need to be cut back 20mm and removed for taping. (There is a wall on top of the two close joists, just to make this more fafftastic).

I'd be tempted to try the airtight paint - blowerproof, or whatever its called.

I use 3:1 grit sand/NHL 3.5, but any plaster would probably do, as would EWI adh/render. You will probably need a primer (I use Pavatex or Pro Clima) for the joist and plaster. The fabric-based tapes 'mould' better at the corners. A colleague who has recently done a house to EnerPHit standard speaks very highly of the 'Blowerproof' product.

Posted By: wookeyIt's a good use for all those tiddly bits of plasterboard offcuts one tends to end up with too :-)

That's true. What I did with all my 'waste' plasterboard was glue it on the reverse of internal walls to add a bit more mass, using my favourite AC57.

Don't be tight as in put on several coats, or make them thick? Or both?
Presumably blowerproof is kind of rubbery? How long is the shelf-life of a tub/cartridge? i.e should one wait till one has a specific need or is it reasonable to buy some on the assumption that it'll be useful at some point? I've found OrconF goes off after 2-3 years, for example. How much better is it than conventional parging? Can you paint over it - stick tape to it?

I've used a bead of OrconF under tescon tape to get a good seal on the rather rough surface of timber joists, and that works very well. But I guess the proper primer would be better on cement. Can one just goop this blowerproof stuff across joist joints (with potential for relatively large movement) and expect that to work? If it does it could be a lot less faff.

Tony tray for new build, and the link for retrofit

You all seem happy with tapes, but how long will they last in the real world?

Posted By: tonyYou all seem happy with tapes, but how long will they last in the real world?

I hope they last longer than plaster does [without cracking] in my house.

I parged between with sand cement and then Orcon F round the sanded joists and then tescon vana tape, same as DJH. Had a lot to of joists to do in my house and got 0.225 Ach on the airtightness test.