Cold weather is coming! There are four basic options for insulating beneath the floor.
The most traditional and least expensive is to run fiberglass insulation between the joists. This method, however, does not stop airflow completely –– nor does it stop moisture. The closer your house hovers to the ground, the more moisture problems you’ll face. If your house is high you might be able to get away with it!
Another approach is to encapsulate the entire subfloor with a rigid foam board, attached at the bottom of the joists and sealed tight. The materials are more expensive than fiberglass insulation, and in a small crawlspace it’s difficult to execute properly. It depends heavily on the quality of the installation. It must be sealed tight as a drum and secured in such a fashion that the seal will hold up over time.
The other two approaches deal with spray-foam insulation squirted up against the subfloor. This is more flexible than foam board and “more goof-proof.
There are two kinds of spray foam: open cell (low density) and closed cell (high density).
Closed-cell spray foam gives the better performance at a lower risk of failure. It is nonabsorbent and will stand up to flooding. But it’s also an expensive product.
Open cell is less expensive but can let some water vapor through. It may be a good compromise between cost and performance –– “promising but not yet proven.” To reduce moisture penetration, it’s recommended coating the underside with a vapor-barrier paint.
But no matter what approach you take start with the basics. Deal with drainage issues underneath the house, making sure the ground is higher than around the house and covering it with plastic
