Yes avoid blob and dob like the plague, With wet plaster you can see any cracks and deal with them on redecoration maintaining air tightness.

If you do decide to use insulated boards make sure that you are air sealed EVERYMHERE first.

Posted By: arthurWould it be better to use wet plaster on the inside side of the insulation too?

Probably not - the moisture will take a long time to escape if your house is airtight. Though I believe typical practice in the UK is to wet plaster on top of drywall/gyrproc/plasterboard - whereas standard practice over here is to just tape and finish the joints - skim coating is virtually unheard of except in certain immigrant areas (typically Italian).

Paul in Montreal.

Hi Arthur, See http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1070&page=1#Item_24 for my method and other comments on this subject

Not necessarily, depends upon the vapour permeability of the wall itself- many are permeable

If in doubt, contact one of the technical helplines who will do a condensation risk analysis for you.

As far as I understand the airtightness barrier always goes on the outside of the building between the external rainscreen cladding (no matter what the material is: brick, wood, cement board) and the internal structure that contains the insulation, the purpose being to prevent air entering the insulation and diminishing its effectiveness. The airbarrier is also vapour permeable so that the internal structure has a path for moisture to escape - this is particularly important for timber frame buildings since the timber may have got wet during construction and you want it to be able to dry out. The vapour barrier goes on the warm side of the insulation (inside in a heating climate, outside in a cooling climate) to prevent moisture from the occupants getting into the structure of the building, condensing and causing rot. If you have a solid structure (concrete etc.) then it's not so important to have a vapour barrier (see ICF construction for example where the concrete is sandwiched between two layers of expanded polystyrene (which is not a vapour barrier)). However, your insulation layer may also be an air barrier - just saw a house recently that was built as poured concrete and then sprayed over its entire surface with polyurethane foam before a decorative facade was built out of nice cut stone. No other air or vapour barriers in place as far as I can see - all internal walls regular plasterboard on steel studs.

Paul in Montreal

Paul -- I dont think we know where the air tightness barrier goes yet in the UK. Majority of house construction is extreemly air leaky and in masonry construction there is never an air barrier outside the cavity insulation (unless there is either render or paint) so do we have any hope? Even with timber frame the air barrier is often still the same one as the vapour barrier.

That's right Nigel but on balance my vote goes to inboard air barrier - wet plaster when the inner leaf is masonry, or a membrane when it's stud. Much easier to make it continuous, inboard. The only real problem then is floor joist ends wanting to go out through the barrier - that's about all. Paul's wish to put the air barrier outboard is to prevent outside air blowing around within the insulation - fair enough - but outboard vapour permeable sheathing should do a fair job at that, even tho not qualifying as a full air barrier.

The outboard vapour permeable insulation-containment sheathing can be Panelvent-type fibreboard, cheap moisture resistant plasterboard - or even OSB maybe, tho there's confusion as to its vapour permeability. I'd like guidance on the latter - if OSB is permeable enough to be used in this outboard position, then that's the structural sheathing plus insulation containment sorted in one go. If not, then I need to do a re-think, as the structural sheathing has to go inboard and has to be duplicated by Panelvent or plasterboard outboard.

This http://www.greensteps.co.uk/tmp/assets/1163178050906.pdf which someone posted previously here (can't find the thread - anyone), represents the hardline fuil-breathability with hygroscopicity camp, and gives warnings that have certainly got me worried about previous jobs and re-thinking future ones.

This is good, Paul. Can I get a few things clear:

Posted By: Paul in Montrealever seen a system with sheathing outside (but before the insulation is fitted)? No matter have carefully installed, there are always gaps

You're saying serious gaps? - and that 'fair job' isn't good enough to prevent outside air blowing around within the insulation, that 'full air barrier' is reqd for that purpose as well as for preventing internal airchange?

Another approach, with inboard air barrier to take care of inboard airchange, is to allow the wind to blow over the outside of the insulation, but the insulation is closed-cell e.g. Cellotex, and is bedded all round and at joints with expanding foam. What about that?

Actually, that's a big no-no according to http://www.greensteps.co.uk/tmp/assets/1163178050906.pdf because any such use of impervious insulation between studs/rafters forces any construction moisture or any interstitial condensation that may happen, to take the only route out, via the timber. So, instead of Cellotex, how about blown-in Warmcel? behind insulation-containment OSB etc, breather membrane, or just tough scrim, which the Warmcel boys use to close any gaps before blowing-in. Anyway, not relying on the OSB or the scrim as air barrier to prevent infiltration of the insulation, just on the compressed gap-filling quality of blown-in Warmcel. Warmcel is very vapour permeable, as well as hygroscopic, so protects the studs/rafters from that having to be that moisture-escape channel.

Having said all that about OSB etc as a 'fair job' air barrier (which you say it won't be), of course it will have breather membrane outboard of it. Would you say that such breather membrane, with or without OSB etc, is still not good enough to prevent outside air blowing around within the insulation, unless scrupulously done to full air barrier standard - in which case why duplicate that with an inboard air barrier?

Posted By: Paul in MontrealPlasterboard would never work

Hm, my 1991 Brit Gypsum White Book has a whole section on MR plasterboard as external sheathing behind tilehanging etc, incl 'exposure to external conditions during construction' and I've used it as such with no problems. However current BG advice is only for 'external soffits in sheltered positions'. So I'd better forget that option - pity, as it has 3 times the vapour permeability of even Panelvent, regardless of its resistance to liquid water.

So I'd welcome comment on changing my stud/rafter construction from:

plain plasterboard on service-space horizontal battens on careful airtight membrane (also acting as near-full vapour barrier) over studs/rafters of minimal structural size, Cellotex between bedded in expanding foam, thus immune to outside air infiltration. Outboard, tilehanging, boarding or render on rendermesh on ventilated-space vertical battens on breather membrane on OSB structural sheathing on the studs/rafters;

to:

plain plasterboard on service-space horizontal battens on careful airtight membrane over OSB structural sheathing studs/rafters of deeper section, blown Warmcel (apx 1.5x as thick as the Cellotex) between, hopefully immune to outside air infiltration (?).Outboard, tilehanging, boarding or render on rendermesh on ventilated-space vertical battens on breather membrane on the studs. That is, the breather membrane acts as the container of the blown Warmcel. (I've never yet found the need for Panelvent). I should consider changing the inboard air barrier from being a vapour barrier, to airtight but to vapour permeable.

Yes - that's the way to do it.

Tom, I don't see how the following cannot be airtight. I have done this on several jobs and I know it to be very effective. So much so that the internal doors wouldn't stay closed until I cut a nice gap underneath them. Not a million miles from what Arthur is proposing

1. Ensure the wall is free of cracks and missing beds/perps
2. Parge coat of 6:1:1 sand:Cement:Lime. [Nice and flat]
3. Dot and dab PUR insulation backed platerboards, taking extreme care that the perimeter is covered with a consistent, uninterupted band of adhesive. Bands of adhesive also at 400mm ctrs
4. Ensure a gap between floor and boards of say 30mm
5.When adhesive is dry, fill the gap between the floor and the plasterboards with expanding foam.
6. Fix skirting, then plaster walls.
7. Allow timber to acclimatise then seal the junction of floor and skirting with a mastic.

What do you think is missing? My one concern would be regarding the vapour barrier. I have been told by the the manufacturers technical department that the PUR boards laminated with a foil backed plasterboard do amount to a Vapour Control Layer, Though I have read some comments here that suggest foil backed baords are not effective in this way.

Anyone clarify?

Hi Mike

I expect your method would work fine, if having to put lightweight insulation/dry lining inside a masonry wall. I haven't done that for a while, for all the thermal massiveness reasons we know, but if I did ...

Your method is real belt and braces but would you say it's principally the dry lining that lifts the air sealing from 90% to 99%? and the perp filling and parging just gets you to the 90% mark, which isn't enough by itself? AFAIK you agree with me that it's permanently closing *all* the pinholes and hairlines that gets you from mere 90% to useful 99%? Left to itself, wet plaster (or parging) will initially and continually produce such hairline cracks, and unless regularly stopped in, paint filled or maybe wallpapered over, will degrade in airtightness, e.g. if inaccessible. Disagree?

I've been impressed by the advice at AECB conference seminars etc given by pressure-tester Paul Jennings of http://www.stroma-ats.co.uk, that sticky tapes, adhesives, mastic and foam gradually lose bond, if only in a few places, and that's enough to hit that vital top 10% of airtightness for six, after a few years. He says that only continuous mechanical clamping is durable, and that's how I've been proceeding. I'm sure that manufacturers will eventually develop patent products that make effective airtightness easy, just like they did for through-ventilation after about 5yrs of struggling with battens and flyscreen mesh. The sooner the present crop of so-called airtightness tapes and gunges disappear, the better.

I'm hoping that blown-in Warmcel, which is highly gap-filling and hopefully remains in elastic compression forever, amounts to a continuous mechanical closing of its surface and clamping of its perimeter to the materials around it, and so would not require a scrupulous airtight membrane outboard of it, to prevent outside air infiltrating its surface and body. Any comment?

Posted By: Mike GeorgeNo, I have not heard of warmcell having this kind of property.

http://www.unitedinsulations.co.uk/Warmcel-brochure-2001.pdf

Warmcel 500 combines high levels of insulation and air-tightness with excellent hygroscopic qualities.

The air-tightness is one reason I really like the idea of using Warmcel.

I have used Warmcel once and it was and seems to continue to be very satisfactory. Its gap-filling ability is remarkable, the way it finds its way tightly into unanticipated odd spaces. The installers come well prepared with scrim to quickly fix over any uncontained areas - the blowing-in requires absolute containment.