Rigid foam sheathing moisture accumulation
David Kenesson
| Posted in GBA Pro Help on
I would like to add 2″ of exterior rigid foam to a 2×6 wall in climate zone 6. This is a new build. After reading your article “Calculating Minimum Thickness of Rigid Foam Sheathing” it sounds like with only 2 inches I am risking moisture accumulation behind the rigid foam. I’ve seen that Dupont offers a “Tyvek Drain Wrap” material specifically designed to go behind exterior rigid foam that allegedly allows any condensation behind the foam to drain. Does anybody have any experience with this particular material? Does it allow you to get away with using thinner (maybe 1″) exterior rigid foam without having a moisture issue?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Drain wrap is good for dealing with bulk water that gets behind the foam on the exterior side, and forms a (weak but acceptable) capillary break from exterior liquids, but does nothing to mitigate condensate & adsorb from interior moisture drives.
Once you have foam on the exterior limiting the drying rate toward the exterior, key factor to limiting the amount of moisture adsorption from interior drives becomes the average wintertime temperature of the wood. The only thing you can do on the exterior to raise the temperature of the sheathing is add more insulation. Drainwrap on the exterior can't magically draw that adsorbed moisture out of the sheathing. (Putting smart vapor retarders on the interior side can help though, as long as it's air-tight.)
If you're going to put less than the IRC prescriptive insulation on the exterior, make it something highly vapor permeable and use back-ventilated siding, so that the insulation doesn't impede drying toward the exterior, and use a smart vapor retarder on the interior. You can get away with 1" rigid rock wool on a 2x6 wall in zone 6, as long as you have both the rainscreen and a smart vapor retarder. You might even get away with 1" unfaced EPS. But 1" XPS or any foil faced foam wouldn't have sufficient wintertime drying capacity toward the exterior to keep it dry.
David,
You didn't mention what type of rigid foam you intend to use. Your goal is a minimum of R-11.25 of rigid foam. You can achieve that with 3 inches of EPS or about 2.5 inches of polyiso or XPS.
To make sure that you don't have moisture accumulation in your wall assembly, I think that it's a good idea to stick to these design principles.
Dana,
I understand that interior smart vapor retarder would help prevent moisture from getting into the wall assembly from the interior, but why does it help draw the moisture out of the sheathing? Also, wouldn't an interior vapor retarder slow down the drying process from the interior? Thanks for your help and quick responses!
During the drying season when the sheathing warms up, it begins driving off some of it's moisture into the cavity raising the relative humidity of the entrained air. As the relative humidity of the air next to the smart vapor retarder rises it becomes more vapor open, allowing the moisture to diffuse through the wallboard into the conditioned space. When the sheathing cools off it re-adsorbs some of the moisture inside the cavity air, and the RH of the entrained air drops, making the smart vapor retarder more vapor tight, limiting how much of the conditioned space moisture comes back in.
This goes on some even during the winter, but is more pronounced during the spring, when overnight sheathing temps are below the indoor dew point temperatures, but above the dew point temperature during the day. Under those conditions during the warmer hours it dries as rapidly as if there were no vapor retarder, but takes very little of that moisture back during the cooler hours. It's not fool proof, but it's better than half-perm paint as a vapor retarder. The drying rates through half-perm paint are not temperature or RH dependent. Half perm paint dries about an order of magnitude more slowly than through 2-mil nylon (eg: Certainteed MemBrain), even if the uptake rates when the sheathing is cold are comparable.
Putting that in perspective, a single day of warmer weather in the early spring would allow as much moisture out of the smart vapor retarder wall as a week or more of warmer weather through the wall using half-perm paint as the vapor retarder.
David, you really don't want to use less foam on the outside of the wall. Stick with the Drainwrap and two and a hap to three inches of foam, investing your money into that. It gives you Rvalue. Then you can skip the smart membrane, knowing you spent you money wisely on your foam insulation and your dense pack cellulose and air sealing.