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Q.Can I use rigid foam panels sandwiched between subfloor and finish floor to insulate over a crawlspace?

A. This can be done, but I would not recommend it. Movement of anything against polystyrene foam creates a terrible squeaking noise. Remember the last time a foam cooler rubbed against something in the back of your truck?

Except in the most extreme northern climates, insulating a floor over a crawlspace is questionable because you lose some of the cooling advantage. In the summer, there is considerable cooling by heat radiating from the floor to the soil.

The best way to insulate a crawlspace is to use foam on the stem walls. However, if you opt to insulate the floor cavity, all piping and ductwork must be insulated also. An inexpensive alternative, if there are no ducts or pipes in the crawlspace, is to use a sheet of double-faced reflective insulation on the bottoms of the floor joists. Reflective insulation works well against downward heat flow, yielding up to R-11 if the air space above is tightly sealed.