"Floor" in this case refers to the plywood upon which my furniture rests.
Beneath the "floor" is a crawl space with a dirt surface which is damp but
not wet. The crawl space is well ventilated and essentially the same
temperature and humidity as outside. Pipes are insulated but there is no
insulation between the joists. Therefore, when it is zero degrees outside,
the kitchen floor inside is around 45 degrees. At one end of the house, the
crawlspace is barely a foot high and at the other end it is four feet high.
I've seen a "profile" of the topsoil at the nearby quarry. The soil is clay
approx two feet deep on top of solid limestone so drainage beneath the clay
is poor, although with it being on the side of a mountain, there is no
surface accumulation of water. It helps that I'm 10 vertical feet from the
top of the 300 foot mountain.
The question is, what type of insulation to use between the joists? I
don't care if moisture from the house goes into the crawl space because the
crawl space is essentially open. But, it could be bad if moisture from the
house went halfway through the insulation, then condensed and turned the
insulation into a soggy, icy mess. I don't know how moisture acts within the
insulation fibers when it's 70 degrees in the house and zero degrees
outside. I know that in the walls, the purpose of the vapor barrier is so
moisture doesn't condense on the inside of the exterior surface, but what
happens when the other side of the insulation in unfinished and open to
Bob, you have a bad situation with the depth of the crawl space. The
reason you have moisture down there is that the dirt base is not
covered, so moisture is constantly traveling through the dirt to the
surface and keeping a moist atmosphere under the house. So as Meehan
says, no matter what you do, you first need to put down a plastic
cover (4-6 mill) over the dirt--that will mostly change the crawl
space to a dry area. Then you need to think about limiting moisture
flow from the house to the crawl space insulation; insulation must
stay dry. Your largest consideration is that insulating the floor
will be impossible where the depth to the dirt is only 1 foot.
You still have two choices. Insulate the walls or insulate the
floor. If you insulate the walls, you essentially change the
crawlspace into a heated area. There could be some debate about how
the facing goes, but if you don't have seepage I would put the facing
(moisture barrier) away from the concrete wall (unfaced side against
the concrete) as for any heated space. You close the vents in the
winter, and if you don't have heat leakage (you probably do if you
have a hot air system with pipes below the house) you can provide
some. You don't heat this area to the same temperature as the living
room; in fact, with good insulation and no heat this are should remain
between 50 and 60 degrees.
If you insulate the floor, I would staple, 2-3 mill plastic directly
to the underside of the floor and then put unfaced bats between the
joists. I think you would have to leave the floor uninsulated in
those very shallow areas that you can't reach. If you are so lucky as
to have vinyl flooring above the shallow area, which act as a moisture
barrier, you could just push unfaced batts up in between the joists
using long poles or something. You should provide some ventilation in
the winter but not as much as you would have in the summer. Good
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