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Confused about vapor barrier with outsulation

Last year I undertook a whole house outsulation project in Coastal Connecticut climate zone 5 with 1 inch polysio. New vinyl siding and new construction vinyl windows were also installed during the project.

This was passed during our inspection and permitting approval process and thanks to the research on this site was able to undertake the project with confidence.

I am currently remodeling our kitchen down to the studs and replacing the insulation in the stud bays while I am at it. My inspector during a structural inspection told me I have to put up a vapor barrier or else "my walls are going to rot from the inside". I told him he previously approved the 1 inch on the outside (exceeds the minimum required for a 2x4 construction) and since that was more than required it would create a moisture sandwich. He responded with it didn't matter and to put it up anyways to pass inspection.

Of course I keep reading how dangerous it is to have this. But at the same time I have read how it's only really a problem IF moisture enters the cavity itself. Meaning if proper flashing procedures are taken into account outside and a vapor barrier on the inside (with good moisture venting in bathrooms) there shouldn't be a problem at all.

Ultimately I think the solution would be closed cell spray foam I think but my budget does not support this. My current project would have the R-6.5 Polysio outside my sheeting, R15 Roxul and then sheetrock. I feel like it's half a dozen one way and 6 the other.

Asked by Paul Krsiak
Posted Fri, 02/07/2014 - 23:35


1 Answer

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Your building inspector is wrong. You are right.

You have two choices:

1. You can tell your building inspector that your approach is supported by the 2009 International Residential Code, in section R601.3 and Table R601.3.1. If the inspector looks up this reference, perhaps he will concede that you are right.

2. You can conclude, as many builders have, that "you can't fight City Hall," and can install a "smart" (variable-permeance) vapor retarder like MemBrain on the interior of your studs to satisfy your ignorant building inspector. This will be a safer membrane to use than polyethylene, since a smart retarder will allow your stud bays to dry to the interior if they ever get wet.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 02/08/2014 - 06:59
Edited Sat, 02/08/2014 - 07:00.

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