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Finding drafts with a candle

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candle
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1The short version

The flame gives up the house's secret in about a second, and it's rarely the secret people expect. A window with insulating glass already blocks most heat loss through the glass itself; nearly all of what a candle test finds is air leaking around the frame, sash and sill, not through the pane. Insulated glass units use two or more panes with a gas-filled gap between them, air, or more often argon, to cut heat transfer across the glass.

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2What it takes, and what goes wrong

Testing a house takes twenty minutes: every window sash and sill, every exterior door edge and threshold, on a breezy day for the clearest result. The common mistake is testing on a still day and concluding nothing leaks; wait for wind, or test near an exhaust fan running to create a pressure difference.

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3The two or three ways people actually fix it

Self-adhesive foam or V-strip weatherstripping is the fastest and cheapest fix for movable parts like sashes and door edges. Caulk is for the parts that never move, the frame-to-wall joint. A door sweep, a thin strip screwed to the door bottom, handles the threshold gap that weatherstripping tape cannot reach.

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