Finding drafts around doors and windows before winter

In most homes, cold air tends to enter low and warm air tends to leave high, so a draft check works best when it starts at the floor and moves upward. Focus on the places where two materials or parts meet, where something moves, or where a pipe, cable or vent passes through an outer wall. Door thresholds, window sashes, the trim around frames and the gaps around outdoor outlets are the most common culprits. Check on a breezy day, when air moves through gaps most easily, and before the heating system has to run for long stretches.

Use simple tests to find the leaks

Several low-cost tests show where air is getting in. The paper test is the simplest: close a door or window on a sheet of ordinary paper and try to pull the paper out. If it slides free with little resistance, that spot has a weak seal. Repeat it at several points around the frame, including the top corners and the strike plate where the latch meets the frame, since gaps often cluster there.

The hand test works well on a windy day. Move the back of your hand slowly along the edges of the frame, the meeting rail of a double-hung window and the bottom of the door. A cold spot or a moving draft tells you where to look more closely. A stick of incense or a smoke pencil held near a suspected gap shows the direction of the air clearly. Keep it away from curtains and paper, hold it over a dish, never leave it unattended and keep water nearby. Avoid candle flames for this job, because a flame near fabric is a fire risk.

The light test works at night. With the room lit, step outside and look at the window frame and the door edges; any light showing through a gap is a leak you can see. On a sunny day, do the reverse: stand inside with the lights off and look for daylight around the frame. Mark each leak with painter's tape so you can find it again when you start sealing.

Check the less obvious openings

Many drafts come from places that are not windows or doors. Electrical outlets and switches on exterior walls often have gaps behind the cover plates, which can be sealed with inexpensive foam gaskets placed behind the plate. Recessed ceiling lights below an unheated attic can leak a surprising amount of air, and an attic hatch or pull-down stairs are often left unsealed. Look at the exhaust fan dampers and dryer vent flaps, which should close when the fan is off, and check the mail slot, the pipes and cables that enter under sinks and behind appliances, and the gap where a dropped ceiling meets a wall.

Fireplace dampers need special attention. A damper that does not close tightly lets warm air escape straight up the chimney. Close the damper when the fireplace is not in use, but never close it while a fire is burning or embers remain. If the damper will not seal, have a technician check it.

Seal the largest gaps first

Start with the largest and highest-leakage openings, which usually means the attic hatch, the fireplace damper, the exterior door threshold and the bottom edge of the door. Then move on to the gaps around window frames and the outlets on exterior walls. Small openings are worth sealing too, but the bigger gaps move the most air, so they give the most return for the time spent.

Use the right material for each gap. Caulk suits fixed joints, such as the seam between a window frame and the wall, while weatherstripping suits anything that moves, such as a window sash or a door edge. A door sweep or threshold seal handles the gap under the door. Choose caulk and weatherstripping labeled for exterior use if the gap is on the outside, and check the temperature range on the caulk tube, because many products need milder conditions to apply and cure properly. Clean and dry the surface first, since caulk will not bond to dust or moisture, and avoid overfilling a gap.

Weatherstripping loses its effect over time. If a foam or rubber strip stays flattened when you release it, or it tears away from the frame, replace it. Tighten the screws on a loose strike plate so the door pulls into its seal rather than resting against it. Adjusting a sticking window lock or a door latch can close a gap that no sealant could reach.

Sealing has limits. Do not block the air openings that a gas furnace, water heater or other fuel-burning appliance uses for combustion or venting, and keep the ventilation that bathroom fans and kitchen exhausts require. An appliance that does not get enough air can produce carbon monoxide, which is why a working carbon monoxide alarm belongs on every level of the home.

When a professional check is worth it

Some drafts are beyond a weekend's work. If they persist after the obvious gaps are sealed, if one room stays colder than the rest of the house in a way the leaks do not explain, or if you cannot find the source, a home energy auditor can run a blower door test. That test uses a fan in an exterior doorway to depressurize the house so that leaks can be found with smoke or a thermal camera. A licensed insulation contractor can also check whether the attic and wall insulation is complete and evenly placed, since gaps in insulation can feel just like drafts.

Consider a professional too if windows fog between the panes or the frames and sashes have rot and no longer stay in position. Those are signs the unit may need repair or replacement, a decision a licensed window contractor can help with. Sealing still helps in the meantime, but it cannot fix a window that has failed.

Repeat the paper and hand tests once the sealing is finished and the weatherstripping has settled. Note which openings still feel cold, and deal with those next. Sealing is most comfortable in mild weather, so a steady afternoon of work in early autumn is worth more than a rushed weekend when the first hard frost arrives.