Water droplets forming on windows during cold months seem minor—an aesthetic nuisance at most. But persistent window condensation signals broader moisture and air quality problems that can lead to mold growth, structural damage, and health issues if left unaddressed.
Understanding why condensation occurs, what it indicates about indoor conditions, and how to eliminate it prevents both immediate discomfort and long-term damage to homes and health.
The Science of Condensation
Condensation occurs when water vapor in air contacts a surface cooler than the air’s dew point—the temperature at which air becomes saturated and water vapor condenses into liquid.
The process:
Warm indoor air contains water vapor. When this air contacts cold window glass, the air temperature at the glass surface drops. If the glass temperature is below the dew point, water vapor condenses directly on the glass.
Key variables:
Indoor air temperature: Higher temperature means air can hold more moisture before reaching saturation.
Indoor humidity: More water vapor in air means higher dew point. Condensation occurs at warmer temperatures when humidity is high.
Window surface temperature: Determined by outdoor temperature and window insulation properties. Single-pane windows get much colder than double or triple-pane.
The equation: Condensation occurs when window surface temperature < dew point temperature of indoor air.
What Condensation Reveals About Indoor Conditions
Condensation isn’t the problem—it’s the symptom. It reveals indoor moisture levels that may cause other problems even where condensation isn’t visible.
High Indoor Humidity
If condensation appears on windows, indoor humidity is elevated relative to outdoor temperature.
Ideal winter indoor humidity: 30-50% RH depending on outdoor temperature
When condensation occurs:
- Outdoor temperature 20°F or below: Condensation at >35-40% indoor RH
- Outdoor temperature 0°F or below: Condensation at >25-30% indoor RH
- Outdoor temperature -10°F or below: Condensation at >20-25% indoor RH
These are approximations for typical double-pane windows. Single-pane windows condense at lower humidity levels; triple-pane or low-E windows resist condensation at higher humidity.
Inadequate Ventilation
Condensation often indicates insufficient air exchange. Without ventilation, moisture-generating activities (cooking, bathing, breathing) accumulate humidity faster than natural infiltration removes it.
Poor Window Performance
Condensation concentrates on coldest windows. If some windows develop condensation while others don’t, the affected windows have worse insulation properties.
Common Moisture Sources in Homes
Understanding where indoor moisture comes from helps control humidity levels.
Human Occupancy
People generate moisture through respiration and perspiration. A family of four produces 2-4 gallons of water vapor daily just from breathing and normal activity.
More occupants = more moisture = higher humidity = increased condensation risk.
Cooking and Boiling
Stovetop cooking releases substantial moisture:
- Boiling pot: 0.5-1 pound water vapor per hour
- Simmering: 0.25-0.5 pounds per hour
- Oven use: Minimal moisture
Extended cooking without ventilation raises indoor humidity significantly.
Bathing and Showering
Showers and baths release large moisture amounts rapidly:
- 10-minute shower: 0.5 pounds of water vapor
- Bath: 0.25-0.5 pounds
Without exhaust fans or open windows, bathroom humidity spikes to 70-80% and spreads to other rooms.
Laundry
Washing machines: Minimal moisture release (sealed systems)
Dryers: Should vent outside. If vented indoors (against code in most jurisdictions), releases enormous moisture load—2-4 pounds per load.
Air-drying clothes indoors: Intentionally releases moisture to increase humidity. One load of wet laundry releases 2-5 pounds of water vapor.
Humidifiers
Intentional moisture addition to combat dry winter air. If over-used or not monitored with hygrometers, humidifiers can raise humidity excessively, causing condensation.
Houseplants
Plants release moisture through transpiration. Large plants or many plants contribute measurable humidity.
Effect is usually modest (beneficial in dry winter air) but can contribute to excessive humidity in homes with many plants and poor ventilation.
Basements and Crawlspaces
Ground moisture migrates into basements year-round. Unsealed basements in humid climates can add substantial moisture to homes through floor penetrations and air movement.
Unvented Combustion Appliances
Gas stoves, unvented gas logs, and kerosene heaters produce water vapor as a combustion byproduct. These should be used with adequate ventilation.
Where Condensation Appears and What It Means
Condensation patterns reveal specific problems:
Lower Window Corners
Most common location. Cold air settles, making lower corners coldest part of window. Indicates general humidity or window performance issue.
Entire Window Surface
Suggests either very high indoor humidity or very poor window insulation. Occurs more often with single-pane windows in cold climates.
Between Panes (Double/Triple-Pane Windows)
Indicates seal failure. Inert gas leaked out, moisture entered sealed space. This condensation can’t be wiped away from inside and permanently impairs window insulation.
Solution required: Window replacement or sealed unit replacement.
Specific Windows Only
If only certain windows condense while others don’t, those windows are colder due to:
- Single-pane where others are double-pane
- Older/degraded windows
- North-facing exposure with no solar gain
- Shaded locations never receiving direct sun
Around Window Frames
Condensation at edges between glass and frame indicates air leakage allowing warm moist air to contact cold surfaces. Also may indicate inadequate frame insulation.
Consequences of Persistent Condensation
Ignoring condensation causes progressive damage:
Mold Growth
Water on windows runs down to sills and frames. Persistent moisture creates ideal mold conditions. Black mold commonly appears on window sills and adjacent walls.
Mold releases spores that become airborne, degrading indoor air quality and potentially causing respiratory symptoms and allergic reactions.
Wood Rot
Water damages wood window frames, sills, and surrounding framing. Rot compromises structural integrity and requires expensive repair.
Paint failure, peeling, and staining indicate ongoing water damage.
Wall Damage
Water running off windows onto walls damages drywall, plaster, and paint. Staining appears below windows. In severe cases, wall materials deteriorate.
Ice Formation
In extreme cold, condensation freezes on windows. Ice buildup can reach substantial thickness, blocking views and light. Melting cycles create even more water that runs onto sills and walls.
Reduced Window Lifespan
Persistent moisture accelerates window degradation. Metal frames corrode. Wood swells and deteriorates. Seals fail faster. Window operation becomes difficult.
Solutions: Controlling Indoor Humidity
The primary solution is reducing excessive indoor humidity to appropriate winter levels.
Measure Current Humidity
Use hygrometers ($10-30) to measure actual humidity levels rather than guessing. Place meters in multiple rooms for complete picture.
Target humidity based on outdoor temperature:
- Above 40°F outdoor: 40-50% RH acceptable
- 20-40°F outdoor: 35-40% RH maximum
- 0-20°F outdoor: 30-35% RH maximum
- Below 0°F outdoor: 25-30% RH maximum
These targets balance comfort with condensation prevention.
Reduce Moisture Generation
Cooking: Use range hood vented outdoors. Cover pots to reduce evaporation. Avoid extended simmering without ventilation.
Bathing: Run bathroom exhaust fan during and 20-30 minutes after showers. Open window if no exhaust fan exists.
Laundry: Ensure dryer vents outside properly. Avoid indoor clothes drying if possible, or dry in well-ventilated areas with dehumidifier.
Humidifiers: Turn off or reduce output during coldest weather when windows condense. Monitor humidity with hygrometer and adjust accordingly.
Houseplants: Reduce number of plants or relocate to areas where higher humidity is acceptable.
Increase Ventilation
Adding fresh outdoor air dilutes indoor humidity.
Natural ventilation: Open windows 10-15 minutes daily, even in winter. This exchanges air and removes excess moisture with minimal heat loss.
Mechanical ventilation:
- Run HVAC fan continuously (raises energy cost but improves air circulation and distribution)
- Install bathroom and kitchen exhaust fans if absent
- Use exhaust fans whenever moisture-generating activities occur
- Consider ERV/HRV system for controlled ventilation with heat recovery
Whole-house ventilation: In tight homes, mechanical ventilation becomes essential. Without it, moisture and other pollutants accumulate.
Dehumidification
In homes with persistent high humidity despite ventilation and source control, dehumidifiers provide direct solution.
Portable dehumidifiers: Place in problem areas (often basements). Size appropriately for space.
Whole-house dehumidifiers: Integrate with HVAC system. Expensive but effective for homes with chronic humidity problems.
Note: Dehumidifiers are rarely necessary in heating season in most climates. If needed during heating season, investigate unusual moisture sources (basement moisture, plumbing leaks, etc.).
Solutions: Improving Window Performance
If humidity levels are appropriate but condensation persists, window improvement may be necessary.
Immediate Low-Cost Interventions
Insulated window treatments: Cellular shades, insulated curtains, or heavy drapes reduce heat loss from windows and keep interior glass surface warmer. Close at night, open during sunny days for passive solar gain.
Window insulation film: Plastic film applied with double-stick tape creates dead air space, improving insulation. Inexpensive ($10-30 per window) and reasonably effective. Appearance is poor, making this temporary solution.
Draft stoppers: Address air leaks around window frames that bring cold air to glass surfaces.
Medium-Cost Solutions
Interior storm windows: Removable or permanent interior windows create insulating air space. More aesthetically acceptable than plastic film. Cost $50-300 per window depending on size and style.
Weatherstripping and air sealing: Properly seal air leaks around window frames. Caulk on exterior, weatherstrip on operable components.
Long-Term Solutions
Window replacement: Installing energy-efficient windows (double or triple-pane, low-E coatings, gas-filled) raises interior glass surface temperature, preventing condensation.
Cost: $300-1000+ per window depending on size, style, and quality.
Benefit: Eliminates condensation, reduces heating costs, improves comfort. Should be considered for failed windows or in homes with many old single-pane windows.
Replacement priority: Start with coldest windows (north-facing, shaded, bedrooms where condensation is most problematic).
Special Considerations for Different Window Types
Single-Pane Windows
Most prone to condensation. Interior glass surface temperature approaches outdoor temperature.
Short-term: Window treatments and plastic film help but don’t eliminate problem.
Long-term: Replacement with double or triple-pane windows is most effective solution.
Double-Pane Windows with Condensation Between Panes
Seal failure allows moisture into space between panes. Cannot be fixed by homeowner.
Options:
- Replace sealed glass unit (if window frame is good)
- Replace entire window
Condensation between panes permanently impairs insulation value and appearance.
Casement and Awning Windows
Often seal tighter than double-hung windows, but hardware and seals can fail, allowing air infiltration to glass.
Ensure latches engage fully and weatherstripping is intact.
Sliding Windows
Often have more air leakage than other styles due to sliding track design.
Ensure tracks are clean and weatherstripping is present and functional. Consider interior storm windows if condensation persists.
Basement Window Condensation
Basement windows present unique challenges:
Colder microclimate: Basements are naturally cooler, and windows are often partially below grade, making them colder.
Higher humidity: Basements tend toward higher humidity from ground moisture.
Window wells: Trap moisture and cold air around windows.
Solutions:
- Dehumidify basement to 40-50% RH maximum
- Improve window well drainage
- Consider window replacement with better-insulated units
- Ensure proper grade and waterproofing to prevent moisture infiltration
When Condensation Indicates Larger Problems
Sometimes window condensation is the visible symptom of serious issues:
Active Water Intrusion
If condensation is severe or appears in unusual patterns, check for:
- Plumbing leaks
- Roof leaks
- Foundation water infiltration
- Improperly vented bathrooms allowing moisture into walls
These create moisture loads far exceeding normal occupancy.
HVAC Issues
Oversized or short-cycling HVAC systems may not adequately dehumidify. Systems that run briefly, shut off, run briefly again don’t remove humidity effectively.
Undersized return air prevents adequate circulation and moisture removal.
Ventilation System Failures
Bathroom or kitchen exhaust fans that aren’t ducted outside deposit moisture in attics or wall cavities. This moisture can:
- Migrate back into living spaces
- Cause hidden mold growth
- Damage building materials
Check that all exhaust fans vent to building exterior, not attic or crawlspace.
Prevention Maintenance
Preventing condensation long-term requires ongoing attention:
Monitor humidity: Check hygrometers weekly during heating season. Adjust moisture sources and ventilation as needed.
Maintain exhaust systems: Clean range hood filters. Verify exhaust fans work properly. Clear dryer vent annually.
Seasonal adjustments: Reduce humidifier output as outdoor temperatures drop. What’s comfortable at 40°F outdoor creates condensation at 0°F.
Window inspection: Check weatherstripping and caulk annually. Replace deteriorated materials before heating season.
Clean condensation promptly: If condensation occurs, wipe dry immediately to prevent mold growth and material damage.
The Bottom Line
Window condensation indicates indoor moisture levels too high for window insulation capacity under current conditions. Solutions involve either reducing moisture or improving windows—usually both.
Appropriate winter humidity (30-40% RH during cold weather) balances comfort needs with condensation prevention. Maintaining this requires moisture source control and adequate ventilation.
Persistent condensation shouldn’t be ignored. The visible water on glass indicates conditions throughout the home that can cause hidden mold growth, respiratory issues, and structural damage even where condensation isn’t visible.
Addressing condensation improves indoor air quality, protects home structure, and increases comfort—making it worth the attention and effort required.