Drafty Windows: Find the Leak Before You Seal It
Caulk fixes gaps in a fixed frame, but it does nothing for air leaking between two moving parts — using the wrong fix on the right spot means the draft comes right back the following winter.
A window leaks air from two completely different kinds of gaps, and each one needs a different fix — caulk works by hardening into a permanent seal, which makes it perfect for the fixed joint where the frame meets the wall but useless where the window sash actually slides open and closed, since hardened caulk there would prevent the window from opening at all. Weatherstripping solves that second problem specifically because it stays flexible and compresses when the sash closes against it, sealing the moving joint without locking it shut. Applying caulk to a moving gap or flimsy foam tape to a structural gap means treating the wrong type of leak with the wrong material, and the draft comes right back once that mismatch fails. That's why identifying exactly where a draft originates, using the candle or incense test, actually matters before choosing a fix rather than after — it tells you which of the two materials the specific spot actually needs. Get that diagnosis right, and the full sealing job across several windows costs about $22 and takes roughly 90 minutes.
What You'll Need
- Lit candle or incense stick, for draft detection
- Exterior caulk and a caulk gun, for frame-to-wall gaps — about $6
- Putty knife, for removing old cracked caulk
- Self-adhesive foam or V-seal weatherstripping, for sash-to-frame gaps — about $8
- Removable window insulation film kit (for older single-pane windows) — about $6-10
- Hair dryer, for shrinking insulation film tight
- Fabric draft snake, for sills — about $5, or make one from scrap fabric
- Cleaning cloth, for surface prep before applying anything adhesive
How to Do It
- Test each window on a breezy day by holding a lit candle or incense stick near the edges, watching for the flame or smoke to flicker, which pinpoints exactly where air is moving through.
- Note whether each detected draft comes from the fixed frame-to-wall seam or from the gap between the moving sash and frame — this distinction determines which material you actually need for that specific spot.
- Remove any old, cracked caulk from frame seams with a putty knife, and clean the surface thoroughly before applying fresh caulk.
- Apply a smooth, even bead of exterior caulk along the seam where the frame meets the wall or siding, using caulk only on these fixed, non-moving joints.
- Clean the contact edges of the sash and frame thoroughly before applying weatherstripping, since adhesive won't hold well over dust or grime.
- Apply self-adhesive foam or V-seal weatherstripping along the moving contact points between sash and frame, pressing firmly for full adhesion.
- Add a removable insulation film kit over older single-pane windows if needed, shrinking it tight with a hair dryer to create an insulating air pocket.
- Engage window locks on every window once weatherstripping is in place, since locking pulls the sashes tight together and meaningfully reduces remaining draft, and add a fabric draft snake along any sill that still feels cold.
Energy auditors who do professional home draft assessments often test windows on a windy day specifically rather than a calm one, since a five to ten mile-per-hour breeze creates enough pressure difference to reveal drafts that would be completely undetectable during still conditions, even at leak points that are genuinely significant contributors to heat loss over a full winter.
