How do I reduce energy loss through my foundation?

Insulate the rim joist area where the foundation meets the wood framing, as this is a major air leakage point. For crawl spaces, insulate the walls rather than the floor above and seal all vents in winter. Apply rigid foam insulation to basement walls and seal gaps around pipes and wires entering the foundation. Make sure your sill plate is sealed with foam or caulk. A vapor barrier in crawl spaces prevents moisture problems that reduce insulation effectiveness. These steps also help prevent cold floors in winter.