The most effective way to reduce window condensation and prevent winter mold is to lower indoor humidity, improve ventilation, seal air leaks around windows, and raise the interior glass temperature with insulation measures like window film or storm panels. Monitor humidity with a hygrometer and aim for winter targets adjusted to your outdoor temperature.
Why windows sweat in winter
Condensation forms when warm, humid indoor air hits cold glass and the surface temperature drops below the air’s dew point. You can reduce condensation by tackling both sides of the equation:
- Lower indoor moisture (humidity)
- Raise the window surface temperature (insulation and air sealing)
Recommended indoor RH in winter (approx.):
- Outside 40°F (4°C): 40–45% RH
- Outside 20°F (-6°C): 30–35% RH
- Outside 0°F (-18°C): 25–30% RH
- Outside -20°F (-29°C): 15–20% RH
Step-by-step plan
1) Measure and manage humidity (30–45% typical winter range)
- Place a digital hygrometer near problem windows. Cost: $10–$25. Time: 5 minutes.
- If RH is high, reduce sources: cover aquariums, run a clothes dryer vented outdoors, avoid drying laundry indoors, and limit long, hot showers.
2) Ventilate moisture at the source
- Bathrooms: Use an exhaust fan rated at least 50 CFM (or 1 CFM per sq ft). Run it during showers and 20–30 minutes after. Add a timer or humidity-sensing switch. Cost: $100–$200 for a quiet 80–110 CFM fan; $20–$50 for a timer switch.
- Kitchen: Use a range hood vented outdoors (recirculating filters don’t remove moisture). Target 150–250 CFM for most homes. Run during cooking and 10–15 minutes after.
- Basements/crawlspaces: Fix bulk water issues, cover soil in crawlspaces with a continuous vapor barrier, and ensure vents/fans are functional.
3) Dehumidify as needed
- For persistent RH >40% in cold climates, add a 50–60 pint ENERGY STAR dehumidifier. Place it in the dampest area and drain continuously to a sink or condensate pump. Cost: $200–$350. Time: 15–30 minutes setup.
4) Seal air leaks and fix glazing
Cold drafts chill glass and frames, creating more condensation.
- Use acrylic-latex or silicone window/door caulk to seal gaps around interior trim and exterior siding-to-frame joints. For larger gaps, insert foam backer rod first.
- Weatherstrip operable sashes with V-seal, adhesive foam, or silicone bulb seals.
- Older single-pane windows: Re-bed loose glass with glazing points and glazing putty.
- Check and clear weep holes in storm windows to allow moisture escape.
Cost: $5–$8 per caulk tube; $10–$25 weatherstripping; $10–$20 glazing putty.
5) Warm the glass (temporary and permanent upgrades)
- Apply interior heat-shrink window insulation film. It creates a still-air layer that raises glass temperature and cuts drafts. Cost: $10–$20 per window; Time: 20–30 minutes per window.
- Add interior or exterior storm panels (magnetic acrylic or aluminum storms). These are reusable and very effective. DIY interior storms: $100–$200 per opening.
- Use insulated curtains/thermal shades, but keep a 1–2 inch gap at the bottom/top or open them daily so moisture doesn’t get trapped on the cold glass.
- If replacing windows, choose double- or triple-pane, low-E units with warm-edge spacers.
6) Daily habits for less condensation
- Keep blinds slightly raised or open for airflow, especially overnight.
- Pull furniture a few inches away from exterior walls.
- Run ceiling fans on low, winter setting (clockwise) to mix air without drafts.
- Don’t block supply registers or cold-air returns near windows.
7) Treat existing mold safely
- For small areas (<10 sq ft) on non-porous surfaces: Wear gloves, eye protection, and an N95. Clean with detergent and water; dry thoroughly. You can use a diluted bleach solution on non-porous surfaces if desired (ensure ventilation and never mix with ammonia).
- Replace moldy, porous trim/caulk that can’t be fully cleaned.
Tools and materials
- Hygrometer
- Caulk gun; acrylic-latex or window/door silicone caulk; foam backer rod
- Weatherstripping (V-seal, foam, or silicone bulb)
- Window insulation film kit (shrink film), painter’s tape, utility knife, hair dryer/heat gun
- Glazing putty and points (for wood, single-pane sashes)
- Dehumidifier (50–60 pint); optional condensate pump
- Exhaust fan upgrades; timer or humidity-sensing switch
- PPE: gloves, safety glasses, N95 mask
- Optional: infrared thermometer or thermal camera to find cold spots
Browse these categories in the ToolStash catalog: humidity meters, weatherization (caulk/weatherstripping), dehumidifiers, bath/kitchen ventilation, and window insulation kits.
Safety notes
- Use ladders properly and cut power at the breaker when swapping fan switches.
- Old homes (pre-1978): Disturbing paint may release lead. Follow EPA RRP practices or hire a certified pro.
- Ventilate well when using caulks, cleaners, or bleach. Never mix bleach with ammonia or vinegar.
Tips for best results
- Balance house pressure: Make sure makeup air (cracked window or dedicated vent) is available if you run strong exhaust fans.
- Program bathroom fan timers: 20–30 minutes post-shower is the sweet spot.
- Direct heat to cold windows: Baseboard or radiator beneath windows helps keep the interior pane warmer.
- Track results: Note RH and outdoor temperature for a week to dial in your targets.
Common mistakes
- Setting humidifiers too high in winter. Aim for the temperature-adjusted RH above, not a constant 45–50%.
- Relying on recirculating range hoods—they don’t remove moisture.
- Closing heavy drapes tight to the glass overnight; moisture gets trapped and condenses.
- Skipping air sealing before adding window film. Drafts reduce film performance.
- Ignoring bulk water leaks around frames; condensation fixes won’t overcome leaks.
When to call a pro
- Condensation between panes in double- or triple-pane windows: the seal has failed; consider sash/IGU replacement.
- Widespread mold (>10 sq ft), musty odors, or respiratory symptoms: call a mold remediation specialist.
- Rotten sills/frames or ongoing water intrusion: a window/door contractor should assess flashing and replacement.
- Whole-home ventilation solutions (HRV/ERV) for tight homes or persistent humidity: HVAC contractor. Typical installed cost: $2,500–$5,000.
By combining humidity control, targeted ventilation, tight air sealing, and a warmer interior glass surface, you’ll dramatically cut window condensation and stop mold before it starts—often in a weekend with modest materials and a clear plan.