Foam over wall sheathing: Typar under foam?
cconti
| Posted in General Questions on
Hi all,
We are doing 2 inches of rigid foam on top of (i.e., outside of) our wall sheathing (with fiberglass in between the studs). The lumberyard specified Typar between the sheathing and the foam, but the builder is suspicious of having two vapor barriers next to each other, and is concerned about moisture being trapped and eventually compromising the sheathing.
What is correct here? Do we use the Typar, or skip it? If we skip it, do we tape the seams of the foam?
Thank you very much for your help!
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Replies
Chris,
First of all, Typar is not a vapor barrier. It is engineered to be very vapor-permeable.
Second, even if Typar were a vapor barrier, having two vapor barriers adjacent to each other won't cause any more problems than a single vapor barrier would.
Third, every wall needs a water-resistive barrier (WRB). You can use Typar as your WRB if you want. You can instead use a layer of rigid foam as your WRB if you prefer to go that route. However, you always need a WRB, and you have to know which layer is your WRB, so you can integrate your window flashings with your WRB.
For more information on these issues, see these three articles:
All About Water-Resistive Barriers
Using Rigid Foam As a Water-Resistive Barrier
Where Does the Housewrap Go?
Typar is more vapor-tight than other housewraps, but still no worse than the very high end of class-III vapor retardency, and can't/won't create moisture trap. It's more vapor-open than #15 felt under most conditions, and about 10x as vapor open as 1" XPS foam.
Taping the seams of the foam is a always good idea no matter how air-tight you've detailed the housewrap & sheathing.
Hey Chris -
I remember from an earlier thread that you are in central Maine - same area I'm in. Check out Martin's article on calculating the minimum thickness of rigid foam sheathing; two inches is cutting it close in our zone as far as keeping the plywood sheathing above dew point. The cold climate housing research center has a great manual on building remote wall assemblies that you might also find interesting. I'm no expert and could easily be missing something about your wall assembly - just my two cents in case it's helpful.
Thank you everyone for all of your advice.
It sounds like perhaps the best idea for my case would be to skip the Typar, and instead thoroughly tape the foam seams (that's the builder's preference as well).
Jim, good memory! I am indeed in Central Maine (just outside of Augusta). As far as the foam thickness, I was operating under the assumption that ANY foam on the exterior of the sheathing was an improvement over standard, traditional building practice of no exterior insulation, with all the insulation inside…
Am I missing something?
I'll take a look and see if I can find the Cold Climate Housing Research Center material you mentioned!
By adding foam sheathing you're restricting the wall's ability to dry to the exterior - which is fine, but you need to make sure the wall can dry to the interior (no poly behind the drywall or finishes that will prevent drying) and you need to add enough foam to keep the plywood sheathing warm enough to prevent moisture accumulation. For a 2x6 wall in zone 6, r11.25 is the minimum listed by the IRC, as pointed out in Martin's article "Calculating the Minimum Thickness of Rigid Foam Sheathing." The potential problem is compounded a bit with insulation in the stud cavities, because it further isolates the sheathing from the conditioned space. If I remember correctly, the REMOTE wall manual from cchrc calls for a maximum of 1/3 of the insulation value of a sheathed wall assembly to be inside the sheathing. Like I said, I'm no expert. I'm planning a build using this assembly, so I have done some reading - but that's it. I'm sure someone will jump in if I'm steering you in the wrong direction here.
Chris,
You wrote, "I was operating under the assumption that ANY foam on the exterior of the sheathing was an improvement over standard, traditional building practice of no exterior insulation."
You're wrong. If your exterior rigid foam is too thin, it won't keep the sheathing above the dew point during the winter. This is the worst type of wall, because the sheathing will accumulate moisture during the winter, but it will be unable to dry quickly to the exterior in the spring.
To learn more about how to design a wall with exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Ah ha, I understand.
Thank you all for your help. Back to the drawing board.
Chris
"It sounds like perhaps the best idea for my case would be to skip the Typar, and instead thoroughly tape the foam seams (that's the builder's preference as well)."
I'm not sure how you're coming up with that interpretation. You still need a weather resistant barrier & drain plane that ties in with your window flashing. That's easy to do with Typar or similar, but requires a lot more detailing to make that work with taped foam. See Martin's WRB discussion in response #1.
If you use rigid rock wool on the exterior in lieu of foam you are improving things substantially, since the wall can dry readily to the exterior (even through Typar). But at 2" (R8-ish) you would still need to lower the vapor retardency toward the interior in winter. While 6 mil polyethylene would work, it is an extreme vapor barrier that would block all drying toward the interior, and potentially cuase condensation inside the cavity in summer when air conditioning is running.
A better solution than 6 mil polyethylene would be "vapor barrier latex" primer under the interior finish paint, which brings the vapor permeance under 1-perm, which low enough to limit wintertime moisture diffusion to manageable levels, but leaves at least some drying capacity toward the interior. Better still would be to use a variable permeance vapor retarder such as 2-mil nylon (eg Certainteed MemBrain) or an even lower permeance smart vapor retarder such as Intello Plus instead of 6 mil polyethylene. (The 2-mil nylon approach is pretty cheap, about 2x the material cost of 6 mil polyethylene vapor barrier, about the same labor cost.) The smart vapor retarders become vapor open if the proximate air has a sufficiently high relative humidity, but are vapor tight when the air is dry. In winter as the sheathing adsorbs moisture from the air inside the cavity the RH next to the membrane is very low. But in the spring as sheathing warms up, releasing the moisture from the wood the smart vapor retarder becomes vapor open, allowing that moisture to pass toward the interior of the house. As a result, moisture transfer from the conditioned space to the sheathing is very slow, but moisture transfer from the sheathing toward conditioned space is 5-10x faster, which is why it's so much better than 6-mil polyethylene or half-perm latex.