Spray Foam

A dollop sprayed through a thin nozzle expands, like hair mousse, to close cracks 1/2 to 3 inches wide in walls and floors. Foam’s main ingredient, sticky polyurethane, makes it ideal for sealing around pipes (not hot ones, though) and filling odd-shaped gaps. It takes seconds to spray (several hours to dry) and costs about $6 a can.

Caulk

For gaps 1/2 inch wide or smaller in walls and floors, use silicone or acrylic-latex indoor-outdoor caulk. If you’re caulking near a heat source (chimney, furnace, hot pipe), use high-temperature caulk. Applying it in a nice, straight line takes minutes (about two hours to dry); a tube of caulk and the gun you apply it with cost about $5 each.

Window-Insulating Film

Short of installing more energy-efficient windows, the best thing you can do to cut down on heat loss through panes is to cover them with clear insulating film. To install it, you lay a sheet across the glass and tape it to the frame, then use a blow-dryer to shrink it, which makes it adhere to the glass. The process takes 10 to 30 minutes; the film comes in many sizes and costs about $6 for three big windows’ worth.