"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 believe one of the varieties of spray foam can be used as a vapor
barrier--"closed cell" comes to mind. I am sure someone will the correct
knowledge will chime in.
tim
On Wed, Feb 22, 2012 at 9:15 AM, Karl Vacek wrote:
> 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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