Cold climate vapor barrier
olaire | Posted in Building Code Questions on
I live in Alaska. Do I need to have a poly vapor barrier to meet code requirements?
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First of all, it's always good to know your climate zone. Most of Alaska is in Climate Zone 7 or Climate Zone 8.
According to recent versions of the IECC, exterior walls in these climate zone require a Class I or Class II vapor retarder — in other words, kraft facing or polyethylene — on the interior side of the wall.
Three of the exceptions are listed in section 402.5 of the IECC, which notes that vapor retarders are not required on a basement wall, on the below-grade portion of any wall, or on a wall constructed of materials that cannot be damaged by moisture or freezing.
Further exceptions are allowed in section 402.5.1, which states that in climate zones where a Class I or Class II vapor retarder would normally be required, a less stringent vapor retarder — a Class III retarder like latex paint — can be used under the conditions listed in Table 402.5.1. Only certain types of wall assemblies are worthy of this exception; they must have either an adequate layer of exterior foam sheathing or “vented cladding.”
For more information on code requirements for vapor retarders, see Vapor Retarders and Vapor Barriers.
Now that I have explained some provisions of the IECC, it's important to say that the only code that matters is your local code. In some parts of Alaska, I imagine that there are no building code requirements at all.
If you want to know your local code, you have to contact your local building department or local government office for more information.
On the Alaskan Gulf Coast (particularly in the panhandle) it's a very different climate and answer than in the interior. (Juneau's 99% outside design temperature and average winter temperature is warmer than parts of zone-5 southern New England, despite a much longer wetter heating season and 40-50% more heating degree-days.)
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