What vapor barrier should be used with interior / exterior insulation?
ranson
| Posted in Energy Efficiency and Durability on
I’m probably looking to build a house in Kentucky, IECC Zone 4 (not marine.) My general goal is durable, economical, energy efficiency. I’m currently thinking cellulose filled wall cavities with exterior foam or mineral wool sufficient to keep the sheathing warm. The structure will be air conditioned.
What sort of vapor barrier is called for, if any? I could imagine a fully breathable structure with Roxul, or a tightly sealed barrier under foam. Are there particular merits to any solution?
(I’m also interested in general comments on this approach.)
Thanks for the advice,
–J
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Replies
In Zone 4A Kentucky you don't need or want an interior side vapor retarder other than standard latex paint. That's true with or without the exterior foam, but it is especially important to avoid tight vapor retarders or vapor-barrier finishes (vinyl or foil wallpapers, etc) when you have low vapor permeance foam on the exterior of the sheathing, or when the house will have air conditioning.
For the vapor retarder requirements per IRC code, see:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
You will still need to make the house air tight, and it will need a WEATHER resistant barrier (WRB) like a waterproof vapor permeable/semi-permeable housewrap or # 15 felt to manage rain incursions, etc. Whether the WRB goes between the foam & siding vs. the foam & sheathing depends on how the windows are mounted & flashed. The flashing needs to be lapped properly with the WRB to direct bulk water to the exterior.
John,
If you want to read more on this topic, I suggest that you read:
Calculating the Minimum Thickness of Rigid Foam Sheathing
Do I Need a Vapor Retarder?
Since you wrote that you are considering "cellulose filled wall cavities with exterior foam or mineral wool sufficient to keep the sheathing warm," the first of the above articles is relevant. In that article, I wrote, "Because foam sheathing reduces the ability of a wall to dry to the exterior, all foam-sheathed walls must be able to dry to the interior. That means you don’t want any materials with a very low permeance on the interior of a foam-sheathed wall or between the studs. If you are building this type of wall, you should not include interior polyethylene or vinyl wallpaper."