The first winter in our house, I could stand next to the living room window and feel cold air leaking in like the window was half open. The furnace was working overtime and the bill showed it. Sealing those drafts myself was the cheapest money I ever saved.

Find the leaks first

On a cold day, hold a lit candle or a stick of incense near the window frame and move it slowly around the edges. If the flame flickers or the smoke bends, you found a draft. Do this on a windy day and the leaks basically announce themselves.

Seal them up, in order of effort

• Weatherstripping tape: stick foam or rubber tape along the sash where the window closes. Five dollars and fifteen minutes per window.
• Caulk: run a bead around the outside frame where it meets the wall to close small gaps.
• Window film: a clear plastic sheet you shrink tight with a hair dryer. It adds an insulating air pocket and works wonders on old single-pane windows.

I started with the weatherstripping and the film on the two worst windows and felt the difference the same night. The room was warmer and the furnace ran noticeably less.

The best part is the math: the materials cost less than a single month of the savings. Everything after that is money back in my pocket.