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Q.While building a new home in upstate New York, we installed R-15 kraft-faced insulation in the walls, but the local building inspector then made us install 4-mil poly over the kraft paper facing. We've been told by other builders and inspectors that this is not a good practice. Is this double vapor barrier likely to cause problems later on?

A.Bruce Harley, technical director of Conservation Services Group in Westboro, Mass., and the author of Insulate and Weatherize, responds: Your building inspector was mistaken. The New York code calls for a "... vapor retarder having a maximum permeance rating of 1.0 perm. ... The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation."

A lot of people call this a "vapor barrier" requirement, and believe polyethylene is required by the code, but kraft facing on batt insulation (which has a permeance of about 0.4) meets this code requirement all by itself.

More than likely, installing an additional layer of poly over kraft facing won't cause any problems — it's no more risky than if the wall had only unfaced batts and poly — and the risk of using poly on the interior is relatively low in a cold climate like New York's.

The risk is mostly one of summertime condensation on the poly, which is unlikely unless you have air conditioning in the home combined with a highly vapor-permeable exterior, or a moisture-reservoir cladding such as brick.

In my opinion, the only real problem here is that it was probably a waste of time and money to install both kraft-faced insulation and poly.

The common taboo against a double vapor barrier arises from a very real concern: If you have a Class I vapor retarder (less than 0.1 perm) on both sides of a wall, that wall has virtually no drying potential in either direction. So whether it gets wet from lingering construction moisture, a bulk water leak at a window or roof, a plumbing leak, or condensation, the wall will slowly turn into a soggy, stinking mess. That is why you should avoid poly on the interior if you are installing foil-faced rigid foam on the exterior of the wall sheathing.

Of course, if you have enough rigid foam, you don't need the interior vapor retarder — the foam keeps the sheathing warm, reducing the potential for condensation to form on the sheathing. How much foam you need depends on the climate.

In a hot climate — particularly a hot, humid climate — everything I've said above is reversed. The last thing you would ever want is polyethylene (or vinyl wallpaper, or any other nonpermeable membrane) on the inside of an exterior wall.