There's a LOT of discussion on this if you look back... sorry to be "that guy".

Trouble is, no one brand is OK - airtightness varies from batch to batch.

AFAIK Smartply was working on this from a quality control POV, to achieve reliably airtight but vapour open uncoated OSB - but sadly instead went for a coated product that's proundly advertised as an airtight vapour barrier.

I heard Peter Warm, one of UK's 3 (?) PH Assessors, say that he's never yet met an airtightness problem due to the OSB itself - its joints yes but not the body of the material.

Isn't that why you tape the expansion joints with a decent airtighness tape. It allows movement, keeps the wind out and lasts decades.

Posted By: SteamyTeaa decent airtighness tape. It allows movement, keeps the wind out and lasts decades

Movement? I'd say the only chance of longevity requires absolutely no movement, clamping tight, ultimately as a passive gasket. 25/50/100/200yrs is an awful long time for a bit of plastic to not harden/fatigue.

Keeps wind out? even short-term, let alone long-term depends on workmanship, as MarkyP outlines nicely.

Lasts decades? a) not long enough; b) what other bit of plastic have you met that's still performing without deterioration after 25 let alone 50/100/200yrs. Not credible, uinknown.

I'm with Tom Foster on this one. My experience of adhesive tapes is they will fail either, through drying out, or warmth and humidity causing adhesive failure, or mechanical stress. Add to that workmanship issues and you'll be lucky to get any more than a few years.

What can you do ?

Get a perspective of airtightness as part of a whole - just how much air is passing through OSB at a joint line on a stud or similar that is nailed at the correct centres and with say a 3mm expansion gap.

Like ST, other than on a very small building I wouldn't be ignoring expansion gaps - and on a roof, I wouldn't ignore them on any building

There are plenty of materials out there to do the job ranging from externally applied tapes to bedding strips captured between the board and the stud

Longevity? - what's the design life of the building answers that question - most domestic buildings rarely have a design life exceeding 60 years (and often less)

Regards

Barney

What can you do?

Posted By: barneyGet a perspective of airtightness as part of a whole - just how much air is passing through OSB at a joint line on a stud or similar that is nailed at the correct centres and with say a 3mm expansion gap.

Do you have any figures for that, Barney? My instinct is that there would be quite a lot of leakage in the situation where the pressure was blowing the board away from the stud. Especially after a couple of years or so.

There are plenty of materials out there to do the job ranging from externally applied tapes to bedding strips captured between the board and the stud

What do you mean by 'externally applied tapes'? The only thing I can think of is airtightness tapes, which I thought were already being discussed? I believe in them enough that I've committed to them.

By bedding strips, I presume you mean compression gaskets. I would agree with you that they are good - probably the safest of all where practicable. The safest way to use silicone beads IMHO is as a compression gasket rather than an adhesive sealant (i.e. apply to one element, let it go off, and then press the other element up against it). But there are many rubber gaskets as well that may be better.

I think professionals who talk about a design life of only 60 years for a dwelling are conning themselves and their customers. There's no sign that all the houses built before 1956 have all been knocked down, or even a small fraction of them. Indeed, it seems that to the general population, houses become more desirable as they become older.

Posted By: SteamyTeaPossible not sensible to have a large structure that is not allowed to expand safely. It can cause buckling and secondary damage

The larger, the more it has to be deliberately designed to expand/contract as a whole without 'leaving behind' any rigid bits.

Nothing wrong with a 'monocoque', like a plywood dinghy, or a stud framed shell with continuous OSB skin glued and screwed. If it's all-timber like that then it will expand and contract all-over without any strain/stress differential concentrations.

Trouble comes when that continuity isn't done consistently all over, then individual weakly-connected bits may move differentially. Expectation of that inconsistency is why panel manufs recommend sloppy gaps all over the place, to disperse the damage.

Movement has to be allowed for where the monocoque meets say an existing masonry wall, or its foundations if masonry/conc (another good reason for all-timber floor/ground beams).

Where say fibre cement panels are fixed to timber framing, that's something else - there will be continual 'working' between the two types of material, the pull-push making the panels move relative to each other even though they don't themselves expand/contract much. Hence zero chance of making the sheathing stay airtight for long, if ever, in my opinion.

If not worried about airtightness, or not taking advantage of/relying on monocoque-integrity, then you can get away with a lot - e.g. non-expand/contract plasterboard with brittle skim fixed to timber framing without a thought of expansion gaps.

Posted By: ringihow does nailing the OSB to the stud provide for expansion?

Incalculable, not to be relied on, except getting progressively sloppier over time.

Posted By: djhMy instinct is that there would be quite a lot of leakage in the situation where the pressure was blowing the board away from the stud. Especially after a couple of years or so.

Too right, or just where board and/or framing shrinkage leaves the nails a bit looser that when installed. That's why clamping a batten over a tape will go slack soon enough, leaving the tape at mercy of a lifetime of wind-buffet forces (which, remember, can drive a yacht at 30 knots!). The only way to clamp, is over an expensive elastic gasket whose compressed thickness is many times the size of the potential movement - and then pray it doesn't go rigid/crumbly over a lifetime.

Posted By: gravelldWhen people say to use bubble glue, isn't that the stuff that sets hard and brittle? Can't see how that would work for air tightness.

Why not, if part of a monocoque, glued and screwed (or power-nailed)? If the joint is rigid enough to not even let any movement begin, then it's a lot less challenged long term than a piece of tape, whether clamped or not, that has thickness and elasticity which allows frequently reversing micro-shear movement. Even though Polyurethane bubble glue is no more tried and tested for longevity than the tapes, I feel it's fundamentally less challenged, more a passive gap filler.

Posted By: djhI think professionals who talk about a design life of only 60 years for a dwelling are conning themselves and their customers. There's no sign that all the houses built before 1956 have all been knocked down, or even a small fraction of them. Indeed, it seems that to the general population, houses become more desirable as they become older.

Exactly - now that do-able insulation/airtightness standards have reached an optimum beyond which it's pointless to 'improve' forever, we should be building for seriously long life.

The beauty of putting the OSB outboard of the wall/roof stud/rafter framing, as pic above, is that it forms an unbroken airtight 'tea cosy' not penetrated by joists etc.

And no need for a batten-space inner service zone to prevent wiring penetration of an inboard airtight OSB - just plasterboard/skim straight inboard of the framing, which the electrician is at liberty to puncture.

Fill the stud/rafter space with blown-in cellulose (e.g. Warmcel) with precautions against slump, which in itself is fairly airtight, helping with airtightness deficiency (if any) of the OSB - and you have foolproof, easily-made in-depth airtightness which is quite robust against future ignorant penetration.

Unbroken EWI tea-cosy over the outside of the OSB tea-cosy means that there's no wood-based stuff within the outer 40% of the insulation sandwich - a good safety margin against any interstitial condensation causing liquid wetness, which will happen outboard if at all.

fostertom,

If 3by2 studs are used with glued OSB, can the studs be drilled for wire and pipes etc? If so how much benefit is there from filling the studs with Warmcel rather then just putting a little more EPS on the outside?

(However I still like the sound of using Icynene to get the air tightness so you don't have to get trade people correctly gluing the OSB.)

However I expect that once EPS is fixed to the outside of OSB there are no airtightness issues with the OSB apart from at joints of boards.

So - if you use glued and nailed OSB outside of your joists with say EPS outside of that for a airtight teacosy plus fiberboard between the joist for mass and additional insulation - would a bco be convinced that a VCM is not required between the plasterboard and the joists?

Posted By: ringiIf 3by2 studs are used with glued OSB, can the studs be drilled for wire and pipes etc

I'd use 4x2s or 6x2s where structurally necessary (paired with 150 or 100 EPS respectively, or maybe make that 200 or 150 if really going for it). Note that '4x2s' are likely to be 89x38 CLS or 95x47 sawn regularised - '3x2s' only 63x38 CLS or 70x47 sawn regularised. '4x2s' should be safely drillable, on stud CL for pipes, edge-notched for wires.

Posted By: ringiIf so how much benefit is there from filling the studs with Warmcel rather then just putting a little more EPS on the outside?

Drilled or not has no bearing on that. If you don't fill then you need to add 100 to the EPS thickness.

Posted By: ringiusing Icynene to get the air tightness so you don't have to get trade people correctly gluing the OSB

I wouldn't rely solely on Icynene for airtightness, but Icynene or blown Warmcel as valuable robustness/augmentation of the OSB airtightness, organic-origin Warmcel also providing marvellous humidity-stabilising hygroscopicity provided no-one ever applies standard plastic paint to the plastered face.

Expanding/gap-filling polythene bubble-glue-and-screw/nail OSB is really easy and pretty foolproof, apart from getting it all over hands/clothes - buy packs of vet's rubber gloves!

Posted By: ringiI expect that once EPS is fixed to the outside of OSB there are no airtightness issues with the OSB

Wouldn't rely on that at all.

Posted By: goodevansfiberboard between the joist for mass and additional insulation

Won't contribute at all to airtightness, unless foaming all edges and gaps all over - costly. Mass? do it some other way. But organic-source would be good for hygroscopicity.

Posted By: PeterStarckIf using Icynene as the insulant then the Icynene company carry out an interstitial condensation analysis for your construction and tell you if a VCL is required. It is one of the selling points of Icynene.

Not a good reason to prefer Icynene - get architect to work it out - but don't settle for the old-fashioned Glaser steady-state method - dynamic simulation inputting your actual local climate data in WUFI or similar - though standard rule-of-thumb constructions are becoming more confidently applicable.

Then no problem with Building Inspector accepting 'breatheable' in lieu of fail-safe (but often troublesome) 'traditional' internal VCL.

Would this be any use for compression gaskets?

130 rolls new rolls armaflex type fridge insulation tape
http://www.ebay.co.uk/itm/130-new-rolls-armaflex-type-fridge-foam-insulation-tape-class-0-rated-/112284248189?_trksid=p2141725.m3641.l6368

I like good old fasioned Compriband http://www.compriband-tape.co.uk - "open cell polyurethane foam heavily impregnated with an acrylic based, UV and water-repellent resin", tho I'm a bit dismayed to find the impregnation is no longer bitumen - that seemed to promise infinite durability, the new resin maybe less proven.

It also expands powerfully (but quite slowly) once released from the roll, to fill all gaps.

Amazed to see it's also 'breatheable'!

If you use a bedding strip does the screw go to the side of that, so two strips per board interface?

Seems very labour intensive?