Information current as of 09/13/2026.
First locate the leak with a paper or smoke-based test while the window or door is closed. Use caulk for small cracks around stationary frames and trim, low-expansion foam labeled for windows and doors or backer rod plus caulk for larger gaps, and weatherstripping for moving parts; use a door sweep at the bottom of an exterior door. Do not rely on fibrous insulation alone as an air seal, and seek professional advice if extensive sealing could affect combustion appliances or ventilation.
1. Find the draft before sealing
Close the window or exterior door and check the perimeter with a paper test or a smoke-based test. Mark the locations where air movement is detected so you can match each leak to the right repair rather than sealing unrelated joints.
2. Match the material to the leak
For a crack around a stationary window or door frame, jamb, or trim, use compatible caulk or sealant. For a larger gap between the unit and the rough opening, use low-expansion spray foam specifically labeled for windows and doors, or use foam backer rod with caulk. For a gap at a moving door or operable window component, use replaceable weatherstripping; for a gap beneath an exterior door, use or replace a door sweep while preserving normal operation. Fibrous insulation by itself is not a reliable air seal.
3. Apply the seal without affecting operation
Remove failed sealant and prepare the surfaces so the new seal can adhere. Keep the seal at the frame perimeter and do not fill a movement gap in a way that prevents the frame from accommodating movement. Do not force backer rod into place or use foam in a way that bends or deflects the frame, because this can interfere with window or door operation.
4. Test again and decide when to escalate
Repeat the paper or smoke-based check after sealing. If the draft persists, investigate the rough opening, frame condition, adjacent wall penetrations, or possible water damage rather than repeatedly adding surface sealant. Damaged frames, water intrusion, inaccessible rough-opening leaks, or persistent drafts warrant professional assessment. Extensive air sealing may affect combustion-appliance backdrafting, ventilation, or indoor pollutant conditions, so obtain professional advice when those concerns apply.