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
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
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
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