How do I stop condensation on windows?

Short answer

Stop window condensation by lowering indoor humidity, boosting ventilation at moisture sources, improving air circulation, and making the window surface warmer. In practice: run bath/kitchen fans, use a dehumidifier, fix leaks and vent the dryer outside, weatherstrip and caulk around windows, add interior storm/insulation film in winter, and replace failed double-pane seals.

Why condensation happens

Condensation forms when the glass temperature drops below the air’s dew point. You can attack it from two sides:
- Reduce indoor moisture so the dew point is lower.
- Raise the glass temperature with better sealing/insulation.

Types matter:
- Inside surface wet: too much indoor humidity and/or cold glass.
- Between panes cloudy/wet: failed insulated glass unit (IGU) seal. The fix is repair/replacement, not dehumidification.
- Outside surface dew: common on high-performance low‑E glass overnight; not a problem.

Quick checklist to diagnose

• Measure indoor humidity with a hygrometer near problem windows.
• Note where moisture appears (inside, between panes, outside).
• Check if blinds/curtains are tight to glass (traps cold air).
• Inspect caulk and weatherstripping for gaps and drafts.
• Confirm bath and kitchen fans actually exhaust outside.

Target indoor RH in cold weather:
- Above 32°F (0°C) outdoors: 35–45% RH
- 10–32°F (−12 to 0°C): 30–40% RH
- 0–10°F (−18 to −12°C): 25–35% RH
- Below 0°F (−18°C): 20–30% RH
Room temp assumed ~68–72°F (20–22°C)

Step-by-step fixes

1) Reduce moisture at the source (low cost, high impact)

• Use bath fans during showers and for 20–30 minutes after. A timer or humidity‑sensing switch keeps them running.
• Run a vented range hood while cooking/boiling; use pot lids. Avoid recirculating-only hoods if condensation is a problem.
• Vent the dryer outdoors; clean lint and check the duct flap opens freely.
• Avoid drying clothes indoors. Moderate houseplants, aquariums, and humidifier settings.
• Fix plumbing leaks; cover crawlspace soil with 6‑mil poly; address wet basements with drainage/dehumidification.

Tools/materials:
- Hygrometer
- Timer or humidity‑sensing switch for bath fan
- Foil tape and exterior vent hood (for dryer/hood)

Cost/time: $10–$40 hygrometer; $25–$60 timer switch; 30–60 minutes to install a switch.

2) Increase ventilation and air movement

• Upgrade underperforming fans. Size bath fans at about 1 CFM per square foot (minimum 50 CFM). Choose quiet models (<1.5 sones) so they get used.
• Duct fans with smooth, short runs to the exterior (4–6 in rigid duct preferred). Seal joints with foil tape and mastic, not fabric duct tape.
• Crack windows briefly after moisture‑heavy activities if fans are lacking.
• Keep blinds 1–2 inches off the glass and open louvers slightly to allow airflow. Run ceiling fans on low.

Tools/materials:
- Bath fan (80–110 CFM typical), 4–6 in rigid duct, foil tape, mastic, hole saw

Cost/time: $100–$250 fan; 2–4 hours DIY install if replacing like-for-like; electrician recommended if adding a new circuit.

3) Dehumidify

• Use a portable dehumidifier near problem areas. For most homes, a 35–50 pint/day unit works well.
• Set to 35–45% RH in winter. Drain via hose to a sink/floor drain if possible.

Cost: $150–$300 for quality portable units; whole‑home options via HVAC cost more.

Safety: Keep cords and water away from tripping hazards. Use GFCI‑protected outlets in basements.

4) Warm up the window (seal, insulate, or upgrade)

• Weatherstrip operable sashes (adhesive foam, V‑seal) to cut drafts that cool the glass.
• Caulk gaps where frames meet walls (interior acrylic latex; exterior window/door silicone). Use backer rod for wide joints.
• Install interior window insulation film in winter (heat‑shrink kits). This creates an air gap that raises interior surface temperature.
• Add interior acrylic storm panels or magnetic storms for reusable, clearer performance.
• Long-term: Upgrade to double/triple-pane low‑E windows or add quality storm windows to existing single-pane units.

Tools/materials:
- Caulk gun, acrylic latex caulk (interior), exterior silicone (window/door), backer rod
- Weatherstripping (V‑seal or foam), utility knife, putty knife
- Window insulation film kit, hair dryer/heat gun

Cost/time: $5–$10 caulk; $5–$15 weatherstripping per window; $10–$20 film kit per window; 15–30 minutes per window.

Tip: Don’t caulk over weep holes on storm windows; they must drain.

5) Address failed double-pane seals

• Persistent fogging or moisture between panes indicates a failed IGU. The fix is replacing the IGU or sash; defogging services are usually temporary.

Cost: $200–$600 per window for IGU replacement; full window replacement varies widely.

Best practices and small habits

• Keep interior doors open to balance humidity.
• After showers, squeegee tile and leave the curtain/door open for drying.
• Leave a small gap at the bottom of curtains to promote airflow across glass.
• New construction or recent painting can emit moisture for months—expect higher RH the first season; ventilate more.

Common mistakes

• Running humidifiers too high in winter.
• Using unvented gas/propane heaters indoors (they release water vapor and CO—avoid).
• Relying on recirculating range hoods only.
• Closing blinds tight against glass all day.
• Sealing a home aggressively without adding controlled ventilation (consider HRV/ERV for tight homes).

Safety considerations

• Mold cleanup: wear gloves and an N95 if wiping minor mildew on frames; use a mild detergent. Large or recurring growth may indicate hidden leaks.
• Ladders and cutting tools: use eye protection; keep a stable footing.
• Lead paint: homes pre‑1978 may have lead—follow RRP practices if disturbing paint.
• Electrical: bath fan or new outlet work may require an electrician; use GFCI where required.

When to call a pro

• Persistent condensation despite following the steps and achieving target RH.
• Fogging between panes (window contractor for IGU replacement).
• Installing new bath fans, exterior venting, or whole‑home ventilation (HRV/ERV) or if makeup air is needed for high‑CFM range hoods (>400 CFM).
• Chronic basement moisture or structural leaks (waterproofing specialist).

By targeting humidity, ventilation, and window temperature together, you can usually eliminate window condensation and protect your home from mold and trim damage while improving comfort.