Short answer
Find leaks by feeling for drafts on a windy day, using a smoke pencil/incense or a flashlight test, then seal with the right combo of caulk, low-expansion spray foam, and weatherstripping. Replace or add a door sweep and adjust the threshold. Use paintable acrylic-latex caulk indoors, polyurethane or 100% silicone outside, and low-expansion “window & door” foam around frames. Prep carefully, apply in the right temperature range, and retest after curing.
How to identify leaks
- Hand test: On a windy day or with bathroom fans running, run your hand around window/door perimeters, locks, and thresholds for cool drafts.
- Smoke/Incense test: Slowly move a smoke pencil or incense stick around suspect areas; smoke wavers where air moves. Keep away from curtains and combustibles.
- Flashlight test (night): One person shines a bright flashlight from outside while another looks from inside for light bleeding through door sweeps or jamb gaps.
- Dollar-bill test (doors): Close the door on a dollar bill at several points; if it slides out easily, compression is weak and needs weatherstripping/adjustment.
- Visual inspection: Look for cracked or missing caulk, gaps where trim meets walls, dried glazing putty (older windows), and daylight at thresholds.
Tools and materials
- Dripless caulk gun (see ToolStash catalog)
- Caulks/sealants:
- Interior: paintable acrylic-latex or siliconized acrylic
- Exterior: polyurethane or 100% silicone (UV/weather-resistant)
- Low-expansion “Window & Door” spray foam
- Backer rod (closed-cell foam)
- Weatherstripping: adhesive foam, V-strip (metal or vinyl), bulb gasket
- Door sweep (adhesive or screw-on); adjustable threshold if needed
- Painter’s tape, utility knife, caulk removal tool/scraper, putty knife
- Rags, isopropyl alcohol or mineral spirits (match to sealant), vacuum/brush
- Optional: infrared thermometer or thermal camera, smoke pencil
- PPE: safety glasses, nitrile gloves, respirator if sensitive to solvents
Typical gaps and materials
- Hairline to 1/4" → caulk
- 1/4" to 3/4" → backer rod + caulk
- 3/8" to 1" around frames → low-expansion "window & door" foam
- Ideal caulk bead: 1/8"–1/4"
- Application temp: 40–90°F (5–32°C)
Step-by-step: sealing windows and doors
1) Inspect and mark
- Use smoke/hand test to find leaks; mark with painter’s tape.
- Check both inside and outside. Don’t block window weep holes on the exterior.
2) Prep the surfaces
- Remove loose/failed caulk with a caulk removal tool or sharp utility knife.
- Clean dust and oils; let fully dry. Repair any soft/rotted wood first (wood epoxy consolidant/filler).
- Mask edges with painter’s tape for clean caulk lines.
3) Choose the right product
- Paintable areas: acrylic-latex or siliconized acrylic.
- Exterior/weather-exposed: polyurethane or 100% silicone.
- Large gaps around frames (behind trim): low-expansion foam only. Avoid general-purpose/high-expansion foam that can bow frames.
4) Caulk trim-to-wall/frame joints (small gaps)
- Cut caulk nozzle small at a 45° angle; puncture inner seal.
- Apply steady pressure with a dripless gun and pull the bead along the joint.
- Tool the bead with a caulk tool or damp finger within the open time; remove tape while wet.
- For 1/4"–3/4" gaps, press in backer rod flush to about 1/8" below the surface, then caulk over it.
5) Foam the rough opening (bigger hidden leaks)
- If accessible, gently pry off interior casing. You’ll see the gap between the window/door frame and the framing.
- Lightly mist with water, then apply low-expansion window & door foam, filling about 50% of the cavity. Let it expand to fill.
- After curing (per can, usually 1–8 hours), trim flush with a knife and reinstall trim.
6) Weatherstrip doors and operable sashes
- Jambs: Install V-strip or bulb weatherstrip so the door just compresses it when closed.
- Latch side: Ensure the strike plate aligns; adjust hinges or strike to improve compression before adding extra-thick strip.
- Door sweep: Replace or add a sweep so it kisses the threshold without dragging.
- Adjustable threshold: Raise/level to meet the door evenly.
- Windows: Add V-strip along meeting rails on older double-hungs; replace brittle sash seals on sliders.
7) Exterior perimeter sealing
- Run a high-quality exterior sealant where the window/door frame meets siding or brickmould meets siding. Do not seal the bottom edge of window frames that rely on weeps.
8) Retest
- After curing, repeat the smoke/hand test. Tweak sweep height and weatherstrip as needed.
Safety
- Ventilate when using solvents, oil-based sealants, or spray foam; wear gloves and eye protection.
- Blade safety: cut away from your body; retract blades when not in use.
- Ladder safety outdoors; avoid working during rain or sub-freezing temps for most sealants.
- Homes built pre-1978 may contain lead paint—avoid sanding without proper precautions.
Time and cost
- Materials: $50–$150 for a typical entry door plus a few windows (caulk, foam, weatherstrip, sweep).
- Time: 30–60 minutes per window; 1–2 hours per door. Whole-home weekend for many DIYers.
Tips for best results
- Warm caulk to room temp; store cans upright. Keep a damp rag and a trash bag handy.
- Use painter’s tape for crisp lines and remove it immediately after tooling.
- Label tubes with the open date; don’t use expired caulk.
- Paint acrylic-latex caulk after it skins over (check label); do not paint silicone.
- For clean foam application, consider a pro-style foam gun with adjustable flow.
Common mistakes to avoid
- Using high-expansion foam around windows/doors—can bow frames and jam sashes.
- Sealing window weep holes or the bottom drainage paths.
- Applying caulk to wet/dirty surfaces or in temps below spec—poor adhesion.
- Choosing silicone where you plan to paint, or thick weatherstrip that prevents latching.
When to call a professional
- Persistent drafts after sealing, misaligned or warped frames, or visible rot/structural issues.
- Fogged double-pane glass (failed IGU seal) or complex historic windows you want to preserve.
- If you remove trim and discover water intrusion or mold.
Tightening up doors and windows is a high-impact weekend project. Start with diagnosis, choose the right materials for each gap, and work methodically—your comfort and energy bills will thank you.