Short answer
Seal a drafty window by finding the leak points and using the right combo of materials: caulk or low-expansion spray foam for gaps around the frame, weatherstripping for moving sashes, and temporary options like rope caulk or indoor shrink film if you’re renting or need a quick fix.
Why windows get drafty
Drafts usually come from:
- Gaps between the window frame and the wall (interior trim or exterior siding)
- Worn or missing weatherstripping where sashes slide or meet the frame
- Cracked or missing caulk/glazing around the glass
- Misaligned locks that prevent the sash from sealing
A quick test: on a breezy day, close the window and hold a smoke pencil or a stick of incense around the frame and sash lines; smoke movement reveals leaks. An infrared thermometer can also show cold spots.
What you’ll need
- Caulk gun (dripless style)
- Painter’s tool or putty knife; utility knife; scraper
- Vacuum and brush; rags; denatured alcohol or mild cleaner
- Paintable acrylic-latex caulk with silicone (interior), 100% silicone or polyurethane sealant (exterior)
- Low-expansion “door & window” spray foam for larger hidden gaps
- Foam backer rod (for wide joints)
- Adhesive foam tape, V-strip (V-seal), or spring bronze weatherstripping
- Rope caulk or interior shrink-film kit (temporary)
- Tape measure; straightedge; caulk-smoothing tool
- PPE: safety glasses, gloves, and a mask if scraping old paint
Choosing the right method
Gap sizing guide:
- < 1/8 in: paintable caulk (interior) or exterior-grade caulk
- 1/8 – 1/2 in: backer rod + caulk
- 1/4 – 3/4 in (hidden cavity): low-expansion spray foam (door & window)
- Moving sash contact points: weatherstripping (V-strip, foam, or spring bronze)
Step-by-step: permanent fixes
1) Diagnose and prep
- Mark leak locations with painter’s tape. Remove loose, cracked caulk with a scraper. Vacuum dust; wipe surfaces clean and dry.
- Check sash locks: a loose lock prevents full compression of the weatherstrip.
2) Seal interior trim to wall
- Run a small bead of paintable acrylic-latex caulk where the interior casing meets the wall and where casing meets the window frame. Smooth the bead with a damp finger or tool.
- Tip: Tape both sides of the joint for crisp lines; pull tape immediately after tooling.
3) Seal exterior perimeter (if accessible)
- Inspect where the outer window frame meets siding. Remove failed caulk.
- Apply 100% silicone (non-paintable) or polyurethane (paintable, very durable) in a continuous bead. Do not seal the bottom weep paths of vinyl or aluminum windows; these drain water.
- Work in dry weather above ~40°F, and allow cure time per label.
4) Fill concealed gaps around the frame
- From the interior (with trim carefully pried back) or from accessible cavities, use low-expansion “door & window” spray foam. Light passes prevent bowing the frame.
- After curing, trim excess foam flush and reinstall trim, then caulk the trim joints.
5) Restore or add weatherstripping
- For sliding or double-hung sashes: use V-strip along the sides and top, adhesive foam at the meeting rail, or spring bronze on older wood windows. Clean surfaces before applying; cut square; avoid stretching foam.
- For casement windows: replace the bulb gasket if compressed or cracked; ensure the crank pulls the sash tight.
6) Address glass perimeter (older wood windows)
- If air leaks around individual panes, re-glaze: remove loose glazing putty, set new putty, tool smooth, and paint after skinning.
Step-by-step: quick temporary fixes
- Rope caulk: Press into gaps along sash tracks or between sash and frame. Removes cleanly later. $6–$12, 10–20 minutes.
- Interior shrink film: Apply double-sided tape to trim, stretch film with a hair dryer. Great for leaky old windows in winter. $10–$20 per window, 20–30 minutes.
Safety considerations
- Old paint may contain lead (pre-1978). Avoid dry sanding; use a lead-safe method, wear a respirator, and contain dust.
- Ventilate when using solvents, sealants, and spray foam. Wear eye protection and gloves.
- Ladder safety outdoors; don’t overreach.
- Keep spray foam away from moving sashes; it can lock them shut if overapplied.
Tips for best results
- Tooling matters: a smooth, slightly concave bead seals better and looks clean.
- Use backer rod so caulk adheres to two sides, not three; ideal joint depth is about half the joint width.
- Match the material to the job: paintable acrylic-latex inside, more UV/weather-tough sealants outside.
- Verify air-seal after curing with the smoke test again.
- Improve performance further with thermal curtains or cellular shades, but don’t block required egress windows.
Common mistakes to avoid
- Using high-expansion foam around frames—can bow the jamb and jam the sash.
- Caulking over dirt, dust, or damp surfaces—leads to early failure.
- Sealing drainage/weep holes at the bottom of the frame—causes water intrusion.
- Using pure silicone where you plan to paint—it’s not paintable.
- Ignoring the sash lock alignment—an unlocked or misaligned lock invites drafts.
Time and cost
- Simple interior caulking and weatherstripping: 30–60 minutes per window; $10–$25 in materials.
- Exterior re-caulking and low-expansion foam: 1–2 hours per window; $20–$50 in materials.
- Temporary fixes: 15–30 minutes; $6–$20 per window.
When to call a pro
- Rot in sills or frames, or soft/crumbly wood
- Fogged insulated glass (failed seals) or cracked panes
- Severely out-of-square frames or windows that won’t close smoothly
- Persistent condensation/mold pointing to bigger moisture or ventilation issues
- Historic windows needing sash cord replacement or full re-glazing at scale
With a careful inspection and the right materials—caulk, low-expansion foam, and quality weatherstripping—you can stop the draft, improve comfort, and lower energy bills. Most fixes are DIY-friendly, and temporary options can bridge the gap until you’re ready for a full upgrade.