Margin of Error: 1/4"
Before adding insulation to the basement walls or the crawl space foundation, and prior to refinishing a basement room, it is essential that you repair any leaks and solve any problems with dampness. Wait until it is thoroughly dry before you install the insulation, to be sure all problems have been eliminated. If you are unsure whether or not you have such a problem, tape a square-foot piece of plastic to the basement wall or floor. Leave it in place for a week. If condensation builds up under the plastic, you have a problem that needs to be solved.
The causes of such condensation can be extensive. Some of the most common are leaks and cracks in the concrete, and seepage, condensation, or drainage problems around the foundation. That may sound like a lot of work but a systematic going over of each potential trouble spot will save you a great deal of time, trouble, and money later on.
Cracks can be caulked. Seepage can be lessened by painting waterproofing sealant on the interior walls.
I recommend installing closable vents for the crawl space. Open them in warm weather to air out the crawl space and close them in winter to prevent heat loss.
In low-ground areas where drainage is a problem, install drain tiles or pipes around the perimeter of the foundation. In some cases you may even need a sump pump to pump out excess water. Be sure the clothes dryer is properly vented outside.
For more information on caulking and sealing, see the section on Weatherization. Weatherization goes hand in hand with insulation, and many of the solutions overlap.
Insulating basement and crawl space sometimes calls for a different type of insulation than does insulating attic and walls of a home. In the first place, the basement and crawl space are more susceptible to moisture seepage, which can lead to problems like wet or damp surfaces, stained finishes, mildew, and so on. Water vapor moves easily through most materials used in construction, including brick and concrete block. A basement wall that is not adequately insulated with a moisture-resistant material will conduct warm moist air from the living space through to the cooler outer wall where it is likely to condense. If you are not heating the basement or crawl space, you will want to insulate underneath the first floor. This is best done with fiberglass insulation installed between the joists with the vapor barrier up toward the heated area.
You may want to use a closed-cell, rigid foam panel or the reflective layered type of insulation in these areas if you plan to heat the space below the floor. In this case, you will be insulating the basement walls, not the floor.
Rigid foam panels can be used to insulate both interior and exterior walls. The closed-cell type is not as susceptible to moisture as are other types of insulation. Use only closed-cell types in below-grade application. It usually comes in 2' x 8' sheets and should be covered with a fire-resistant material, such as drywall, when exposed to the inside.