Vapour barrier – Double stud wall
Hi,
Building a new home in Zone 7a, with double stud wall design. Currently we have (from exterior to interior) Typak, OSB Board, 2×6 framing with R22 Roxul, with 1.0″ rigid insulation to follow and then 2×4 framing the interior wall with R15.
I’m getting differing opinions on the vapor barrier and where it should be placed. Our architect advised that the V.P. should be on the warm side of the 2×4 wall between the r15 and drywall, however our building inspector has asked us to put it on the warm side of the 2×6 wall between the r22 and the rigid. I would normally follow the inspectors suggestion, but my understanding is that would cause more moisture issues.
Typak – OSB – 2×6(R22) – V.P. – 1.0″ XPS – 2×4(R15) – Drywall
vs.
Typak – OSB – 2×6(R22) – 1.0″ XPS – 2×4(R15) – V.P – Drywall
We already installed the V.P. as the inspector suggested but are questioning moving forward at this point. Much easier to redo the V.P. than wish we had.
The architect also mentioned that the inspector might be considering them as two separate walls (like a timber frame), but is that viable? Obviously we’d like to NOT redo if there is a viable solution.
Thanks for reading!
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Replies
For your climate, I wouldn't locate a vapor retarder any more outboard than ~1/3 of the R value from the interior. Any more and it will probably see some time below condensing temp - while you can get away with some sorption/condensation, why have any? Also put a good interior side air barrier before this point.
Forgive my lack of knowledge, is the "vapor retarder" synonymous to vapor barrier or the 1" XPS in our case? Can you explain "any more outboard than 1/3 of the R value from the interior" in different or simpler terms for me.
Appreciate the response!
A simplified answer is that with the first option, you may get a little ice forming and then water running down on the outside of the XPS (mostly due to air leakage). Why risk it if it is easy to fix?
Code requires a Class I or Class II on the interior side of a frame wall. Yours is basically in the middle.
If 45% of your R-value is outboard of the vapor barrier (and air barrier!), it appears that you would be following the rules in this article: https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
The article is talking about rigid insulation outboard of sheathing, rather than fluffy insulation outboard of a vapor barrier, but I believe the situations are fairly comparable.
There is no need for a vapor barrier on the interior side: there is very little potential for condensation on the interior-facing side of the XPS, as the bulk of the insulation is toward the exterior. The part of the double stud wall outside of the XPS is functionally equivalent to insulated sheathing, and that sheathing has an R value of about 27. By code, so long as you have more than R10 of exterior sheathing in climate zone 7, you only need a class III vapor retarder on the interior. Say you'll use latex paint on the drywall as a Class III vapor retarder, and your inspector should be happy. If not, including a vapor barrier won't do any harm, it'll just cost a little extra.
As an additional note, that one inch of XPS is functionally equivalent to a class II vapor retarder, so putting a vapor barrier over it is a little silly (EPS would be class III, foil faced polyiso class I). I recommend taping the seams of the XPS, as it will serve as a good air barrier and make for a more robust and moisture resistant assembly.
The greater risk for condensation is on the exterior-facing of the XPS, but even then the risk is very small. Plus, there should have ample opportunity for that condensation to dry toward the exterior, and so the assembly should be perfectly safe. Tape the OSB to create a second air barrier to limit air infiltration, and that will further reduce the moisture risks.
To summarize, I recommend:
Tyvek – Taped OSB – 2×6(R22) – Taped 1.0″ XPS – 2×4(R15) – Drywall with latex paint
This will give you a high-performing assembly that meets code requirements.
The building inspectors recommendation will also meet code, and will perform just as well as my recommendation so long as you tape the seams. Your original proposed wall might not technically meet code, but I still think it will be safe.
First off, thank you so much for taking the time to spell that out and summarize. Really appreciate the input.
Just to clarify, if we have started on the inspector's recommendation:
Typak – OSB – 2×6(R22) – V.P. – 1.0″ XPS – 2×4(R15) – Drywall
you would recommend taping the OSB and XPS?
Yes, I would. Including an air barrier in the assembly is key to having good performance, otherwise the cold air will just blow through all your insulation. Taping both the OSB and the XPS is a little extra, but I think the redundancy is worthwhile to ensure the assembly performs well, especially given how little extra it would cost.
And I have to apologize, I had mixed up the inspectors recommendation and your architect's recommendation in my response. To me, the architect's wall is actually more in line with what the code requires, so I am a little confused why the inspector wants the vapor barrier on the XPS. Again, it is still perfectly safe though.
Tacking on to my response again, see if your inspector would be happy with you using kraft-faced fiberglass batts to meet his vapor barrier requirement, as the paper counts as a class II vapor retarder. Alternately, consider using polyiso, which is a class I vapor retarder (the shiny face should make them happy). The polyiso is also better for the environment than XPS.
Can I assume "V.P." is a typo for V.B. (=Vapor Barrier)?
Get rid of the XPS, use (far greener and higher performance) foil faced polyisocyanurate instead, as Aedi suggests. XPS is made of higher impact polystyrene, and is blown with a mixture of extremely powerful greenhouse gas HFC blowing agents (predominantly HFC134a, about 1400x CO2 @ 100 years.) As the HFCs escape the foam performance drops by about 15% over several decades. Polyiso uses a less impactful polymer chemistry, and is blown with a mixture of hydrocarbons, predominantly variants of pentane, with a global warming potential about 7x CO2 @ 100 years.
If the inspectors insist, it's fine to use kraft faced batts in the 2x4 wall, or 2 mil nylon (MemBrain) on the interior side of this stackup, though it's not really necessary for dew point control at the interior face of the foam board.
And yes, tape both the OSB and the foam board.
Thank you! I guess one piece that I may not have mentioned, is we have already secured the V.B. to the 2x6 (R22). Should that be removed or can it be left in place?
It's fine to leave it in place. The first condensing surface is the interior facing side of the foam board ( on the 2x4 wall side), but the foam is not significantly susceptible to moisture condensation or asdorb (unlike wood), and is fully protected if it has a foil facer on both sides (like foil faced polyisocynurate.
At 1" thickness even XPS is pretty close to the NBC definition of "vapour barrier", only ever so slightly more vapor open than the NBC prescribes, but not so vapor open that it is going to load up with moisture with a polyethylene vapor barrier on it's exterior side. But with foil faced polyiso each of the facers is FAR more vapor retardent than prescribed by code- an order of magnitude more vapor tight. There is effectively no vapor diffusion passing through the foil facer on the 2x4 wall side into the foam, let alone passing through to be trapped by the polyethylene vapor barrier. Taping the seams with foil HVAC tape and carefully sealing the top; & bottom edges will prevent air transported moisture too.
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