Having finally got some screed on the floor, my extension is in need of some plaster on the walls to call it actually finished enough to get a certificate.

The design is such that the plaster is the airtightness layer.

I thought this would be easy - just get a plasterer in and tell him to get on with it, but the two I've had round so far have looked at the funny tapes (http://wookware.org/pics/house/extension/plastering/html/005-IMG_6879.JPG.html ) and gone 'what the hell is that'? Tell them it is (pro-clima Contega PV) airtightness tape for joining membranes to plaster and you get a very skeptical look, and 'Are you sure? - I've never seen it before - doesn't look like it will stick'.

One reckoned that it would be much better to plasterboard and skim over a) the block wall (to avoid cracking) and b) the ex-house and ex-garage walls (to avoid cracking). That's pretty-much all the surfaces involved, and of course plasterboard+skim is useless for airtightness.

Then there are various bits round the windows and next to the DPC with some plastic membrane exposed, and I can see that plaster won't stick to that very well. 'Put some plasterboard over that bit' is the standard answer, but that makes the airtight connection tricky again. I'll add some pics for these bits so you can see what I mean.

So. Firstly, is there such a thing as a plasterer who is familar with airtightness issues in the Cambridge area? How might I find this fabled fellow?

Secondly, is it really true that plastering on to ex-external brick is a problem? Same for heavyweight 7KN) blockwork. (Or is is only lightweight blockwork that is a problem). Given that both these need to be wet-plastered, what is the recommended way for least cracking (cracking being bad for both airtightness _and_ aesthetics).

Thirdly, how would you lot deal with the plastic bits on these reveals and the DPC-connection?

Window reveal: about 8cm of plastic sheet next to window:
http://wookware.org/pics/house/extension/plastering/html/003-IMG_6877.JPG.html
http://wookware.org/pics/house/extension/plastering/html/002-IMG_6876.JPG.html

Does plaster stick to tescon? Is putting that crinkly metal stuff over the plastic a good way to deal with it (at the expense of making holes in the membrane)? Or cover the reveal in plasterboard and make sure it's sealed to the tape/membrane, then what's the best way to tie it into the plaster? Standard fibreglass mesh tape?

DPC connection: membrane behind proclima tape: (I'm hoping that the tape will provide sufficient key (it would have been mart to put it on about 7cm higher so that furry bit wasn't below screed)
http://wookware.org/pics/house/extension/plastering/html/000-IMG_6874.JPG.html

This must happen regularly where a DPC connection has to be plastered-over at the bottom of a wall - how is it normally dealt with?

One other thing I'm not sure about:
http://wookware.org/pics/house/extension/plastering/html/001-IMG_6875.JPG.html
If you plaster under the step to connect the tape into the wall, how do you protect the top edge at the step? Just leaving it exposed it'll presumably get separated by footfall after not too long?

You mention plaster presumably you mean render and plaster? Or are you considering undercoat plaster such as Carlite?

I would take a look at the BBA's for the tapes as there should be complete details of what to do on them. I have not seen such tapes used for some of the applications you show....

I have been plastering for many years and have not seem plastic of any kind successfully covered with render/plaster.

Expanding mesh will work as tony suggests but that would need to be fixed through the tapes/plastic.

Posted By: wookey
'Are you sure? - I've never seen it before - doesn't look like it will stick'.

Pro Clima say that gypsum plaster will stick to it very well and other types need a bit of help. Whatever secret sauce my plasterer is putting into his lime mix seems to be working, although we haven't had an airtightness test yet!

Then there are various bits round the windows and next to the DPC with some plastic membrane exposed, and I can see that plaster won't stick to that very well. 'Put some plasterboard over that bit' is the standard answer, but that makes the airtight connection tricky again. I'll add some pics for these bits so you can see what I mean.

I found windows to be much trickier than I thought they would be. What we've ended up with is basically double layers, because I don't know which (or neither or both) is going to work. Wherever there is membrane, or wood, or anything else except straw (or blocks if I had any) there is Celenit board covering the non-sticky stuff. And wherever the non-sticky stuff comes to an end and turns into straw, there is a border of Contega PV. So in theory there's a continuous area of plaster that is airtight, and in theory there's a membrane that is airtight.

Wherever there's a tricky area, the plasterer also puts in some plastic mesh (similar to the stuff on the Contega) and there's fibre in the mix as well, but this is lime render on bales so it's probably different for gypsum.

So. Firstly, is there such a thing as a plasterer who is familar with airtightness issues in the Cambridge area? How might I find this fabled fellow?

Word of mouth via trade contacts? Look for somebody who specializes in historic house restoration, because they expect the unexpected and live for it. Look for somebody who also does lime and/or clay, because they're interested in the unusual?

Secondly, is it really true that plastering on to ex-external brick is a problem? Same for heavyweight 7KN) blockwork. (Or is is only lightweight blockwork that is a problem). Given that both these need to be wet-plastered, what is the recommended way for least cracking (cracking being bad for both airtightness _and_ aesthetics).

AIUI, the trick for different substrates is to modify the base coat with some adhesive or somesuch. Generally the solution to avoid cracking is to increase the tensile strength, which means adding mesh and/or fibres. How much and what type depends on the type of plaster and substrate. For lime-based wattle and daub there can be as much as 30% fibre in the mix.

Does plaster stick to tescon?

I think plaster will stick to Tescon Vana/Profil to some extent but probably not to the likes of Uni. Obviously it is supposed to stick to the various forms of Contega.

DPC connection: membrane behind proclima tape: (I'm hoping that the tape will provide sufficient key (it would have been mart to put it on about 7cm higher so that furry bit wasn't below screed)

This must happen regularly where a DPC connection has to be plastered-over at the bottom of a wall - how is it normally dealt with?

I'm not quite sure what I'm looking at there. The fleece part of the Contega is what is important for airtightness and it's supposed to be plastered into the middle of the plaster so both sides of the fleece are covered. I'm not clear whether that's what you've got. At the bottom of my wall, I've glued some Dasatop strip to the floor slab and fed it up past the wooden base that the bales stand on. Then there's Contega PV stuck along the top edge of the Dasatop that will be plastered in. I was lucky that the Dasatop was almost exactly the perfect width for my application.

One other thing I'm not sure about:
If you plaster under the step to connect the tape into the wall, how do you protect the top edge at the step? Just leaving it exposed it'll presumably get separated by footfall after not too long?

I'm not sure where your airtightness boundary goes there. In my case the door frames have a built-in sill that makes an airtight connection to the door leaf. I've just connected that sill directly to the floor slab with lots of strips of Profil, and some primer on the slab to ensure it sticks to that. In my case the finished floor will cover the tape, but if it was exposed then I expect I could plaster over it or paint it for appearance.

I agree the tapes are overpriced but for my project I didn't see any alternative to using lashings of it. I don't want to lose the ship for a haporth o' tar.

Posted By: wookey

The fleece part of the Contega is what is important for airtightness and it's supposed to be plastered into the middle of the plaster so both sides of the fleece are covered.

I hope you mean s/fleece/mesh/, otherwise I don't understand. The fleece is the sticky bit that attaches to the membrane, so it can't be plastered 'both sides'. The mesh is the bit that connects securely into the plaster.

No I mean what I said. If you think about it, the fleece must be the airtightness connection, because the mesh is full of holes. There's no way it contributes to airtightness. In fact it isn't present at all on Contega FC. The mesh is there purely to increase tensile strength and help to stop the plaster pulling away from the fleece.

The fleece has three areas across its width. The first is the region where the mesh is attached. Then there's a blank region and on the other edge is the region with the Duplex tape that is attached to the membrane. Only the region with the Duplex, and then only on one side, is covered and not available for bonding with the plaster. Two thirds of the fleece can attach to the plaster on both sides; one third on only one side. Also don't forget that the membrane should have slack where it joins the fleece to allow for future movement of the building.

I hadn't realised when I taped the plastic strips on the windows that it would be a problem at this stage. I had assumed that plaster could span a bit of plastic if there was something for it to stick to either side, but it seems that that's optimistic. Oh goody - more taping!

I can't speak for gypsum, but for lime if it crosses a change of material and/or something with no key, then something like hessian or plastic mesh is embedded to help hold the plaster together. If possible, some adhesive prep bond with perhaps some sand is applied to the smooth surface to provide a key. And if it's a big area (whatever 'big' means) then the surface is covered with a render board.

It should be fairly easy to experiment with bits of various materials and some gypsum plaster to see what works, I suppose. EBS in Carlisle are very helpful in answering questions. Perhaps you could try to find the plasterer who did the passivhaus that was being built in Cambridge?

I used some of the tape with the mesh attached but in the end mostly just used the same tape but without the mesh (can't remember the names of them all). Seems to work fine without the mesh - we are using lime throughout so don't know if that makes a difference. Bit of render behind the tape first then a little on top in the base coat.

lime also scores on being able to use the stone/brick later on in life!

The idea of spraying on the plaster sounds excellent - my arm is very sore after 5 days helping and learning from the plasterers. Only a 1/4 of the way through - I suspect I shall be doing a lot myself in the end as the money is running out!

Posted By: wookeyNeither Duckduckgo, nor google seems to have heard of 'EBS in carlisle' Can you give me a clue who they are? I'm guessing plaster manufacturers?

And you a Pro Clima fan!

Ecological Building Systems UK office.

requires some understanding from the plasterer

Indeed, and quite a bit of patience. Fortunately, mine is keen on getting a PH certificate for a straw bale house, so is willing to put in the time.

It is a web search site that does not track you/store information about you, pretty good.