On Feb 29, 9:26 pm, Wayne Whitney wrote:
On 2008-03-01, marson wrote:

Two reasons. First, houses tend to act as chimneys--warm moist air
rises, finds a way out near the top, and thus floors tend to be
under negative pressure--outside air is being pulled in down low.

That's true, but I think water vapor will still diffuse in all directions.

Even if this isn't the case, most floors are sheathed with plywood
which which has a high enough perm rating to function as an
air/vapor barrier.

OK, so there's your vapor barrier on the warm side. :-)

I meant to add to this discussion that most foams cannot be exposed
in a crawlspace. they must be covered with a thermal barrier like
drywall.

Actually, I've wondered about that. Isn't the point of the thermal
barrier to separate the foam from the living space? In that case the
floor plywood would do the job and there wouldn't need to be a thermal
barrier on the underneath.

Cheers, Wayne

Actually, while there are definitely some gray areas in my mind
concerning vapor barriers and floors, there is no gray area with foam
in crawl spaces and rim joists (including the rim joist where the
ceiling below is drywalled); the foam must be covered by a thermal
barrier. At least according to how the code is interpreted where I
live. I've spent some joyous time grovelling around in a crawl space
screwing OSB onto ICF's per the BI instructions. There are some foams
that meet the flame spread requirements, but not many AFAIK.