Ralph:
I'm not saying I'm in favor of open-cell
foam, just agreeing that closed cell foam serves as a vapor barrier and saying
that it needs to be taken into account. I think that if you do you use it as a
vapor barrier you can get around some of the problems of where the moisture is
going to condense (as in not in the middle of your fiberglass insulation). In
some instances that feature can be used as a strength, not as a
detriment. The wrong thing to do is to put a 4'x8'x1/2" sheets of it on
the outside (up here in the north) under your vinyl siding (which porbably
breathes okay) and not realize that in doing so it means that the
condensation will then take place in the fiberglass in your 2x4
walls.
I should also say that I am not an expert
in this area, just that I have given it some thought and found myself
confounded by the issue at times (most of the time).
I have an associate, a fine cabinetmaker
who also does high dollar home additions and improvements, who swears
against using modern building materials such as house wrap, foam insulation,
etc. in favor of tar paper (or maybe just kraft paper) and fiberglass because
he feels you'll never win in the moisture migration problem so he'd rather
just let the house breathe. I'm not in favor of throwing our hands up and
taking that approach but would rather try to work with the modern materials to
achieve a tight house and then do energy recovery in a ventilation unit
designed to get a healthy amount of fresh air into the house.
I hadn't considered the issue of the
off-gassing. I would would imagine that the closed cell foam does that to some
degree as well but maybe much slower (after an initial high rate as the
surface releases the gas)?
Bruce