OK - I'll be careful about the canned foam over-expanding. I've seen
low-expansion stuff in cans at Menards or somewhere, so I'll try for some of
that. However, I tried some low-expansion latex foam 6-7 years ago and it
was pretty useless (DAP or some known brand as I recall). This is a very
small project, and I'd only use about 1/3 of one of the real home-foaming
kits, so that's not really an option.
In any event, what I'm doing is very small volumes, open all along one side,
and I won't be going into closed areas except on top, where it can expand up
and all I intend to do is shoot enough to seal it off (balloon frame house).
Most of this is spaces a couple of inches wide and a couple of feet long,
between real 2" x 4" rough-sawn timbers with actual 1" board sheathing on
the outside, and open to the interior (from where I'll spray it), so I don't
expect lots of problems with expansion other than having to cut off the
excess on the inside.
More to my main concern (which I never really stated), is whether it's at
all advisable to first try to line the areas with Tyvek house wrap or
similar, or just foam directly into the little spaces. Or at least on the
several wider spaces where I can use fiberglass batts, should I try to line
those areas with Tyvek in the cavity first, before installing the
fiberglass? Seems like it'd cut the wind a bit and give the fiberglass a
chance to work. There's building paper on the outside under the wood
siding, but I found large voids around the windows where I've pulled off the
trim. And pulling the siding on the whole front of the (freshly-painted)
house to wrap it properly is just not going to happen.
One of my worries with directly foaming was whether I should leave some sort
of ventilation space between the foam and the exterior sheathing to avoid
moisture issues. I'll put vapor barrier inside after it's all insulated,
but I was worried about sealing the cold surfaces too well and causing
moisture problems.
Finally, is there any reason to put foil on the inside with the vapor
barrier, over the areas that don't have foil-backed fiberglass? I've always
wondered if the foil really did much, or if it was more of a theoretical
thing.
Thanks!
Karl
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