Insulating the basement or crawl space
Interior Alaska has cold soils. Even below the frost line, it may only be a few degrees above freezing. While that’s still warmer than the air temperature in the winter, you can lose a lot of heat through the basement or crawl space if it’s not properly insulated. Today’s building codes require a minimum R-value of 15 for below-grade walls, a good target for our climate. However, many homes have even less. The tricky part is how to fix it.
In older homes, it’s often more practical to insulate the basement from the inside. If there are any problems with water penetration, make sure to fix them first or any work you do on the inside will be compromised. If your problem can’t be solved by gutters and good site drainage, you may need to excavate the exterior and apply a coat of waterproofing.
Insulating from the inside
Rigid foam board or high-density spray-applied foam insulation are good choices for basement walls. Both are resistant to air flow and can tolerate occasional exposure to moisture. Depending on the type of foam, it will take 3–4 inches to produce the minimum R-value of 15. Remember that the concrete behind the insulation is cold and will attract condensation if exposed to moist inside air. If you use rigid foam board, the joints should be tight, taped, and also staggered if you are using several layers. Be aware that building codes are strict regarding exposed foam in living spaces, and almost all foam insulations will have to be protected with some type of fire-proofing. Once the foam is in place, wood framing or furring can be used to run wiring and plumbing and provide an attachment for Sheetrock (which is also considered a fire protection).
Using a plastic vapor retarder is not advised if you are insulating the basement walls with foam. If properly sealed, the foam provides a good air barrier, and a layer of plastic sheeting will only reduce the wall’s ability to dry out should moisture ever make its way in.
Basement walls that are well sealed and insulated can yield big energy savings and increased comfort. But because this area of your home will become much tighter, you may need to consider some form of mechanical ventilation to ensure good air quality and humidity control.