How much insulation should I have under my floor?

Floors over unheated crawlspaces or basements should be insulated with no less than R-30. Cantilevered floors (overhangs over open air) should have the space between the sub-floor and the soffits fully insulated to at least R-31 and have rigorous air sealing. If your home has a basement containing your heating system or other sources of heat, you should insulate the basement walls to at least R-18, rather than insulating the floor above.