How to Seal Drafty Windows

On a cold or windy day, hold a lit candle or stick of incense near the edges of each window — where the sash meets the frame, where the glass meets the frame, and where the frame meets the wall. Watch the flame or smoke: if it flickers, bends, or gets pulled sideways, you've found a draft. Mark each leak location with a small piece of painter's tape. Check the window lock too — many drafts are simply caused by a window that doesn't close or latch tightly.

Before applying new sealant, you need to remove any cracked, peeling, or brittle old caulk and worn-out weatherstripping. Use a utility knife to score along the edges of old caulk, then peel and scrape it away with a putty knife. Pull off any old foam tape or felt weatherstripping — it usually comes off in strips. Scrape any adhesive residue with the putty knife and wipe the surface clean with rubbing alcohol on a rag. New sealant won't stick properly over old, degraded material.

Caulk is for joints that don't move — the seam where the window frame meets the wall (both interior and exterior), and where the glass pane meets the frame (the glazing line). Cut the caulk tube nozzle at a 45-degree angle to create a 1/8-inch opening. Hold the gun at a 45-degree angle to the joint and squeeze a steady, continuous bead along the entire seam. Immediately smooth the bead with a wet finger or a damp rag, pressing it into the gap. Use paintable latex caulk on interior surfaces and silicone caulk on exterior surfaces for better weather resistance.

Closed-cell foam tape is the easiest weatherstripping for the gap between the window sash and the frame. Measure the length of each side of the window where the sash meets the frame when closed — top rail, bottom rail, and both sides. Cut strips to length with scissors. Peel the adhesive backing and press the foam firmly along the frame face where the sash makes contact when closed. Close the window to compress the foam and check for a snug seal. If the foam is too thick and the window won't close, switch to a thinner strip (1/4-inch instead of 3/8-inch).

V-strip (also called tension seal) is a thin, folded plastic or metal strip that creates a spring-loaded seal in the channels where a double-hung window sash slides up and down. Measure the height of the sash channel and cut the V-strip to length. Open the window, slide the V-strip into the channel between the sash and the frame with the open end of the V facing outward (toward the weather). Peel and press the adhesive backing against the channel, or tack metal V-strip in place with the small nails provided. Close the window — the V-strip should flex and press against the sash, sealing the gap without restricting movement.

For windows you won't open until spring, a clear shrink-film kit adds a dead-air insulating layer that mimics the effect of a second pane of glass. Clean the window frame with rubbing alcohol and let it dry. Apply the double-sided tape from the kit around the entire perimeter of the frame (not the glass). Cut the plastic film slightly larger than the window and press it onto the tape, starting at the top and working down. Use a hair dryer on medium heat, holding it 2-3 inches from the film, and slowly sweep across the surface — the film will shrink taut and become nearly invisible.