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17th Century timber framed house - from inside to out will have: lime plaster, 15mm woodwool board, OSB as airtightness layer, 100mm thermafleece between studs, 100mm Steico woodfibre breathable EWI, propriatary lime render. Question is this - the ring beams goes around the room at eaves height and protrudes into the room further than the studs. Originally the wall was lime plastered on laths over the studs, but the ring beam was left exposed. I want to do this again. I've shown a picture of the ring beam coming through the OSB layer. The plan will be to complete the airtightness layer by sealing around the ring beam with airtightness tape. This will be crucial to stop air leakage. Has anyone tried doing this, and do they have any tips? How effective will it be? Thanks!
could you do that on the outside? I would worry about movement, expansion of the sheet materials, thermal movements, old building type movements all trying to open up cracks and gaps. will there be a vapour control layer?
It is better to hermetically seal air tightness barriers together and to overlap them, there are questions about the long term adhesion of tapes and they dont stick well to old dusty surfaces and need a wider.
You could make the decorations the air barrier and reseal or re caulk them to the ring beam every few years?
I'd suggest you drop an email enquiry to Siga's technical service department they have a number of sealers and primers that may be appropriate for this job and I am sure they have plenty of experience with airtightness around old beams in Switzerland and Germany.
That Siga tape (well one of them) is dead sticky stuff, although I would worry about long term (like 30+ years) performance as well as sticking it to an old dirty, wobbly bobbly beam.
Depending on sizes of gaps between the OSB and the beam, a deep fill of flexible mastic (bathroom sealant type) first, then a tape over the top might be a good belt and braces approach. The ultimate long term performer is something that is mechanically restrained/fixed.
I would hope that the OSB is sufficiently dry that there should be no great movement from it once the building is heated. At most there may be 2 mm of pull back away from the beam, but I would guess that in reality you won't see anywhere near that ammount. I also assume the beam is 'dry' so that shouldn't subject to much, if any, movement.
What I had in mind Timber was Siga Primur adhesive which comes in a cartridge like mastic, it remains elastic and will take up any variations in the surface. But I haven't used it myself so hence would advise contacting Siga to see what they recommend.
can you put gasket on to the beam, and mastic/gasket onto the OSB and then squash both of these with a batten screwed into the OSB? You could use sheet material as your batten to allow you to scribe it to follow the beam's shape.
I'd prefer to have the airtightness layer on the outside, because then I could reveal more of the timber internally, but the manufacturers of the woodfibre EWI say that the sheathing board should go on the inside and the woodfibre should go direct against the studs. Do you think I could reverse this and put the sheathing board on the outside?
No plan for a vapour control layer, as it's a fully breathable construction.
I agree it may be more 'comfortable' to have the air-tightness layer somewhere other than right on the outside, but if the wood-fibre board is T & G, could you not detail the boards and lime render to *be* the air-tightness layer?
Zebra - a few points to clear up first. You have a vapour control layer in your current construction. It is the OSB. Remember VCL doesn't mean plastic bag!
You can put the OSB on the outside, with a high resistance membrane on the inside but I wouldn't do that with a direct applied render system. Too risky, for many reasons.
Air tightness needs to be both inside and outside. If outside only, bulk air movement into the structure (through gaps around the beam) may move lots of moisture vapour into the wall which MAY lead to condensation.
Timber, you're quite right of course, what I had meant was that there would be no additional vapour control layer such as a membrane. So I think you've answered that question, if you think it's not a good idea to have airtightness layer on the outside of the frame only. A membrane on the inside would be to no advantage because I'd still have to seal around the beams. So I must persevere with this system, OSB as airtightness and VCL on the inside, with the wood fibre on the outside. Nick Parsons, I don't know how effective the woodfibre is as airtightness. You can certainly blow through a piece of it, I don't think the manufacturers recommend it without a separate airtightness layer in any case. I have scribed the OSB around the beams as well as I possibly can to minimise the gaps. Will await answer from Siga then, and try using a mastic underneath the tape too. Thanks!
Hopefully Siga will get back to you promptly. Without having any practical knowledge of it I'd expect that the Primur (mastic type stuff) on it's own should be sufficient to give you an airtight seal without the need for a tape over the top. The only question I would have is whether a primer would improve the seal further.
I see the stuff you mean Chris, I'll see what Siga come back with. Of course, it being a historic building I'm also hoping that whatever sealant I use will not permanently mark or damage the old beams, I hope that's not asking too much!!!
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