“Why is our heating bill four hundred dollars?” my wife asked, holding the envelope like it was evidence of a crime. It was January. The house was supposedly sealed. Clearly it was not.

That was the winter I learned that the gaps around doors and windows add up to the equivalent of leaving a window wide open all season. The Department of Energy says air leaks account for twenty-five to forty percent of heating and cooling costs in the average home. I walked around with a stick of incense and found seven drafts I had never noticed.

The Incense Test

On a windy day, light a stick of incense and hold it near window frames, door frames, electrical outlets on exterior walls, baseboards, and anywhere pipes enter the house. If the smoke wavers or blows sideways, you have found a leak. Mark each spot with a piece of painter’s tape.

I found leaks around:

• All four exterior doors
• Three windows in the living room
• The bathroom exhaust fan housing
• The gap where the dryer vent goes through the wall
• Two electrical outlets on the north wall

That is a lot of heated air escaping into the backyard.

Fixing the Leaks, Ranked by Cost

Weatherstripping (under ten dollars per door): The foam tape with adhesive backing works for windows. For doors, get the V-shaped tension-seal weatherstripping — it compresses when the door closes and springs back when it opens. I replaced the weatherstripping on all four exterior doors in one afternoon. Immediate difference. No more cold draft at ankle level.

Door sweeps (ten to fifteen dollars each): The gap at the bottom of an exterior door can be half an inch. A door sweep is a strip of rubber or bristles that screws onto the bottom of the door. Measure before you buy — they come in different lengths and you will need to cut them to fit.

Outlet gaskets (under a dollar each): These are thin foam pads that go behind the outlet cover plate. You unscrew the cover, place the gasket, screw the cover back on. It takes thirty seconds per outlet. I did every outlet on exterior walls for about six dollars total.

Caulk (under five dollars per tube): For gaps around window frames and baseboards where the original caulk has cracked. A tube of paintable latex caulk and a caulking gun cost less than ten dollars combined.

When to Do This

Do it in September or early October. You want a windy day so the incense test actually reveals leaks. You also want to finish before you need the heat on, because some fixes — like caulk — need time to cure. Waiting until you are already cold in November means scrambling.

That four-hundred-dollar heating bill dropped to about two-eighty the following month after I sealed everything. The fixes paid for themselves in six weeks. And the house felt warmer even at the same thermostat setting, because the warm air was staying inside where it belonged.