Drafty windows: locate the gap before choosing weatherstripping

Map the draft before buying a seal

Bring a flashlight, a strip of tissue, a soft cloth, a tape measure and your phone for a photo. Work from inside or from the ground. The sash is the framed part that holds the glass; the surrounding window frame stays in place.

Cold glass can cool nearby room air, which then sinks toward the sill. That convection can feel like outside air coming in. A cold pane, a cold sill or tissue movement in front of the glass alone does not locate a leak. Wet glass is another separate clue: identify which surface is wet before treating condensation as a sealing problem.

1. Open and close the window through its normal controls. On a double-hung unit, check that the upper sash is fully up and the lower sash fully down. Engage the locks without forcing them. If a sash will not stay up or a lock will not engage, stop using it and arrange repair.
2. With the window closed and locked, slowly check the sash edges, meeting rail, lower sash-to-sill contact and trim-to-wall seam with your hand. Hold tissue near each joint, then hold it still long enough to distinguish a draft from your own movement.
3. Compare a suspect seam with the middle of the glass and nearby intact seams. Record the exact location where movement repeats, plus whether wind gusts change it. Photograph damaged or missing seals. A quiet check on a calm day cannot rule out leakage.

Match the location to the repair

Use this table only after locating a suspect joint. A window can have both cold-glass discomfort and an air leak; one repair may not eliminate both.

Scroll across to read every column.

Choose weatherstripping that fits the moving joint

First identify the brand, series and opening type from a label, etching or purchase record. Photograph the seal in place and its attachment groove. A replacement bulb, pile strip or molded corner piece must match the window; a roll labeled “window weatherstrip” is not automatically equivalent. Keep the original seal until the replacement is identified.

For a suitable older window without a proprietary seal, V-strip can suit a sliding contact; compressible tape can suit a sash that closes down onto a stop. Tape is a poor substitute for a sliding seal that rubs on every opening. Felt wears readily and is unsuitable where it stays wet or rubs. Use the product’s stated surface and gap range, not color or the thickest available strip.

1. Measure each intended sealing run separately and note where the gap changes. Use the window or weatherstrip maker’s measuring method to choose the profile and compression range. Do not pack an uneven, misaligned sash with layers of foam.
2. Where the matching instructions allow homeowner replacement, clean the receiving surface with the approved cleaner and let it dry. Address the paint hazard below before scraping anything. Cut and attach the new seal without stretching it, following its corner and joint instructions.
3. Open, close, lock and unlock the window before doing another one. If the latch now takes force or the sash drags, correct the size or placement; a tighter-feeling window is not a successful repair if it cannot operate normally.

Caulk a fixed seam, not the window shut

For a small, accessible interior trim-to-wall leak in sound material, choose sealant whose label covers both surfaces and the indoor application. Painted wood, bare wood, vinyl, fiberglass and metal do not all accept the same products or preparation. Check joint width, application temperature, ventilation, cure time and paintability on the actual cartridge.

After any required lead-safe preparation, remove failed material as the product instructions direct; clean and dry the joint. Apply a continuous bead that contacts both sides, tool it as directed, and leave it undisturbed for the stated cure. Do not smear caulk over dirt, a loose old bead or moving hardware.

Air emerging behind trim can come from an unsealed rough opening. That may need a properly designed interior air seal behind the casing, rather than more surface caulk. Do not blindly inject expanding foam through holes. The window installation instructions determine whether backer rod and sealant or a specified low-expansion window-and-door foam is appropriate. Sash-weight pockets and drainage spaces need their own treatment.

Leave the drain path and escape route usable

Lower-frame drainage slots, sometimes covered by small flaps, let water escape outdoors. They are different from the contact where a closed sash seals against the sill. Locate the window’s actual weep system before working along the bottom. Do not caulk over its openings or spread adhesive across the track. Exterior perimeter sealant patterns vary by installation; a continuous bead around all four sides is not a universal repair.

Clear only accessible loose debris using the window’s maintenance directions. Do not drill new holes, remove unfamiliar covers or flood a track to test a window whose drainage layout you have not identified. After rain, check for new pooling and interior wetting. Continuing water intrusion needs assessment of the water path, not another bead over the nearest gap.

Keep any intentional trickle vent or other required ventilation opening working. Do not tape, caulk or fit a seasonal panel across a required emergency escape window in a way that prevents ready opening. Any storm panel must preserve the required escape function. Renters should get the owner’s permission before applying adhesive or changing seals.

Check whether the repair actually reduced the draft

After the seal is installed and any sealant has cured, repeat the same seam-by-seam check with the window closed and locked. Compare under similar wind and outdoor temperature, with the room’s fans and heating in the same operating state. Less movement at the previously identified joint is useful evidence of improvement. Less cold sensation on a milder day is not.

Use a hand or tissue for these checks, never a candle, lighter or burning incense. Do not turn on multiple exhaust fans to create a test draft or interfere with combustion equipment or its air supply. If natural conditions leave the result uncertain, ask a qualified home-energy assessor about controlled air-leak testing.

Check operation again, including the normal lock and any tilt feature you ordinarily use. Look for displaced weatherstrip, squeezed-out adhesive, new drainage obstruction and wet finishes after the next rain. If the seam draft is gone but the glass still feels cold, reassess glass comfort separately rather than continuing to fill joints.

Stop before paint, rot or glass work becomes the project

In a pre-1978 home, or one of unknown age, do not sand, scrape, cut painted seams or pull trim until possible lead paint has been addressed. EPA recommends lead-safe certified contractors for renovation work in pre-1978 homes. Routine seal replacement can become paint-disturbing work when old adhesive or painted-in parts resist removal.

Soft wood, recurring rain leaks, cracked glass, loose glazing, a sagging sash or a lock that will not engage calls for window service. Send the provider your joint photo, product identification and the result of the closed-and-locked check. Ask whether the needed work is a seal, hardware adjustment, glazing repair or rough-opening repair before agreeing to whole-window replacement.