Doors and windows are the most likely sources of air leaks. To find them, wait for a windy day, then slowly move a lighted candle, smoke pencil, or incense stick around window and door frames.

Wherever the flame or smoke flutters or drifts, an air leak is the cause. It needs to be closed, caulked, or weather-stripped.

Less-obvious leaks can occur where plumbing, ducting, and electrical wiring penetrate exterior walls. Using the flame or hanging plastic technique, check around your electrical service panel (circuit-breaker box), plumbing where it passes through exterior walls, electrical outlets, ductwork, exhaust vents, window-mounted air conditioners, and other places where outside air might be sneaking into the house.

When you find a leak, seal it with caulking, insulation, or a gasket.