From above, along one of the outside walls, across the ends of the joists,
cut a two foot strip out of the floor, then you can drop underneath and put
fiberglass batts between each joist. You can then roll a sheet of plastic
out and staple it to the bottom off the joists.
Well, a few comments:
A vapor barrier in the floor system is to keep warm moist air from
migrating into the crawl space, condensing there, and causing the
usual problems of unwanted water. This should definitely be on the
warm side of the insulation; otherwise water vapor will travel through
the insulation, cooling as it does, and condense in the insulation,
where it would be trapped by the vapor barrier underneath.
A vapor barrier on the soil is to keep moisture in the earth from
rising into the crawl space. So it serves a different purpose. If
you have a vapor barrier in the floor system, I'm not clear on whether
having one on the soil would cause any problems. I'm just wondering
how any vapor in the crawl space would actually escape? I guess if
the crawl space walls have no vapor barrier, it can escape through
Lastly, I believe that a vapor barrier, unlike an air barrier, doesn't
need to be perfect to be effective. I believe that have a 95% vapor
barrier will give you 95% of the benefit. With an air barrier, any
wind would drive the air infiltration through the remaining 5%, giving
you less than 95% of the benefit. Hopefully someone else can verify
that my recollection in this regard is correct. :-)
All these unknowns! I have an idea that'll make this part of the kitchen
really unique. It'll be obscenely expensive. I'll redesign the floor so it
consists of four hinged 6x6 hatches, with hardware as shown below, for
lifting the hatches. If one insulation method doesn't work, I'll have easy
access for trying another, and another...endlessly. Maybe I'll use teak.
Furthermore, I'm pretty sure most code requires a minimum distance between the
ground and the house structure if it isn't concrete. Providing a medium that
moisture can follow or that critters can burrow up into the house doesn't strike
me as the best idea.
Given the limited area involved, boxing in the underside of the joists and then
filling them with insulation might be a better alternative...
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. 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. I
have seen a number of floors which were covered on the cold side with
poly and there were no apparent problems, though it would probably be
prudent to use housewrap on the cold side just in case.
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
That's true, but I think water vapor will still diffuse in all directions.
OK, so there's your vapor barrier on the warm side. :-)
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.
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.
One foam I was looking at said: ASTM E-84: FSI 15, SMK 450. That's
presumably flame spread index 15, and a smoke criterion of 450. Do
you know what the flame spread requirements are from the building
The spray foam I have seen applied is sprayed from a gun much like an
airless paint sprayer sprays paint. You aim the gun at a surface from
a fixed distance--about 12" is what I have seen--, be it a rim joist
or wall sheathing or whatever, press the trigger, and it adheres to
the surface and expands. I have never seen a foam application where
you could pump it into a hole with any kind of predictable results.
If you could get in there enough to install some sort of sheet
material (plywood, foamboard or the like) or even netting onto the
underside of the floor joists, you could blow dense pack cellulose
from the top through little holes in the subfloor.
The other alternative would be to figure out a way to insulate the
stem walls, and then pump heat in there.
closed cell foam is more expensive but unaffected by moisture, which
is why its used. wet fiberglass or cellouse has near zero insulation
value. plus foam fills all the little holes and voids. so it also
makes for quiet warm area........
theres a minimally expanding type that can go in walls floors etc.
i would probably excevate a trench access. eventually something else
may come up requiring access under there. such access would help home
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