Plywood as a vapour barrier
abaerlo
| Posted in Green Building Techniques on
We’re building a passive house in New Brunswick, Canada (climate zone 6). Our exterior wall consists of 2×6 stud wall (w/batt insulation), OSB wall sheathing (as the air/vapour barrier with joints sealed/taped), 2×10 wall furring (w/cellulose), 1/2″ plywood sheathing, Tyvek, and siding.
We’re thinking of using plywood instead of OSB for the air/vapour barrier. Our designers recommend OSB because it’s less susceptible to moisture infiltration. They said if we wanted to use plywood they would recommend adding 6mm poly or Intello on the interior wall. Just wondering how much of a risk it would be to use plywood without the additional interior poly/Intello?
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Replies
abaerlo,
The problem you will run into isn't a practical one, it's the building code. Houses in New Brunswick are required to include a vapor-barrier on their walls [9.25.1.1(2)(ii).]. Plywood is a variable perm vapour-retarder. You might have to use a vapor-barrier paint to get it to conf0rm. In practice the plywood will work very well. I would run this by your building inspector and see what they will accept.
I would also consider including a rain screen gap to aid drying. Double stud walls, where the sheathing remains cold, benefit a lot from them.
The same code requirements apply equally to OSB. It comes down to the inspector. I think the same rules apply in Ontario, but our building inspector accepted OSB as the vapour retarder without much fuss.
tplb,
Yes and it's complicated by the fact that in US codes a vapour-barrier is defined by being under 0.1 perms, while in Canada it's under 1 perm.
Ok, got it, thanks. So it's more of a building code issue. I forgot to mention a rain screen gap is in the plans as well.
abaerlo,
For a mid-wall air/vapour barrier I don't think it makes too much difference whether you use plywood or OSB. Neither should see any appreciable wetting. With that layer I don't see any point in including an interior vapour-barrier or membrane if they will allow you not to.
It sounds like the concern is moisture potentially being trapped on the inside of the outermost sheathing layer during the colder months.
abaerlo,
Both OSB and plywood act as variable-perm vapour-retarders. Plywood is around 1 perm when dry and can open up to 20 when wet. OSB starts around 2 perms and goes up to 12 when wet.
With a good air-barrier, this mid-wall vapour-retarder, cellulose in the outer wall acting as a moisture buffer, and a rain-screen, this wall is safe and doesn't benefit from an interior vapour-barrier. If your designers insist, use a membrane, not interior poly as it will stop any drying to the inside.
https://www.greenbuildingadvisor.com/article/the-mythical-threat-to-double-stud-walls
Thanks for all the advice! They aren't insisting on an interior vapour-barrier, just recommending it. Just trying to judge if we really need one or not.
When dry, plywood and OSB are pretty close in perm ratings, but plywood "opens up" more when damp and holds up better to repeated wetting. If you're considering poly or Intello anyway, what purpose is the plywood serving?
Please do your research prior to saying, "Yes," to OSB over plywood. Plywood absorbs moisture more quickly than plywood. However, OSB holds onto moisture longer than plywood. Both products swell when wet. Plywood returns to almost its exact shape when it dries. This is not the case with OSB. At the end of the day I think it is a personal preference. I encourage everyone to make the best possible educated decision when it comes to choosing building materials. Every situation is different. Not one building is like another given all the variables that come into play.
I definitely would prefer plywood over OSB.
One important note is that not all OSB is guaranteed to be airtight. So if you go that route, better to test a piece first.
Otherwise, if you are worried about moisture, definitely include a ventilated and drained cavity behind your siding. If you are using a reservoir siding, consider changing the Tyvek for something around 10-15 perms
It's unclear how widespread an issue this is? There seem to be plenty of passive houses built with an OSB air barrier that end up being very airtight.