Want to insulate crawlspace

[rck]

"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 outside air?

Bob

[George E. Cawthon]

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.

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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 Luck