How to Seal Air Leaks Around Windows

Light an incense stick and slowly move it around the perimeter of each window — along the frame, where the sash meets the frame, and at the meeting rail where upper and lower sashes overlap. Watch the smoke stream for deflection. Mark every leak point with painter's tape. Check both inside and outside. Do this on a windy day for the most obvious results.

Outside, inspect where the window frame meets the siding. Old caulk may be cracked or missing. Use a putty knife to remove old caulk and debris. For gaps wider than 1/4 inch, press backer rod (foam rope) into the gap first, leaving 1/4 inch depth for caulk. Apply a continuous bead of exterior-grade paintable caulk along all frame-to-siding joints. Smooth with a wet finger. Let cure 24 hours.

Inside, check where the window casing (trim) meets the wall. Gaps here allow air from the wall cavity to enter the room. Apply interior paintable caulk along the top and sides of the trim. Also caulk the joint between the sill and the wall. Use a smooth, thin bead — this caulk will be visible, so neatness matters.

For double-hung windows, apply V-strip (self-adhesive bronze or plastic) to the channels where the sash slides. The V-strip compresses when the sash closes, creating a seal. For the meeting rail (where upper and lower sashes overlap), apply foam compression tape. For casement windows, replace the perimeter weatherstrip with a matching profile. Test by closing the window — it should seal snugly without requiring excessive force.

For single-pane or drafty windows, apply a window insulation shrink film kit. Clean the window frame with rubbing alcohol. Apply the double-sided tape to the frame perimeter. Press the film onto the tape, leaving it slightly loose. Use a hair dryer on medium heat to shrink the film taut and wrinkle-free. The trapped air layer acts as insulation, reducing heat loss by up to 50%.

The lock mechanism on double-hung windows often has gaps around it. Apply a thin bead of clear silicone caulk around the lock housing on the interior side. For sliding windows, check the latch and weep holes. Don't seal weep holes — they're designed to drain water from the sill track.

Repeat the incense smoke test around every sealed window. The smoke should now drift straight up with no deflection at the sealed points. Close all windows firmly and verify the weatherstripping compresses. A properly sealed window should feel noticeably warmer to the touch on cold days compared to before the sealing work.