WINDOWS · CONDENSATION
Your Windows Are Wet
Every Morning
Wiping the glass protects the sill, but it does not stop tomorrow’s water. The pattern tells you whether the room is too moist, the glass is unusually cold or water is entering from somewhere else.
Check which side of the glass is wet
Water on the room side usually forms when warm, moist indoor air meets cold glass. Moisture outside the outer pane can be a weather-related surface effect. Fog or droplets trapped between sealed panes suggest that the glazing seal may have failed and need professional assessment.
Separate condensation from rain or a leak
Condensation often appears broadly on cold glass after a cold night, showering, cooking, indoor laundry or sleeping with the room closed. A local patch around a frame, stain below the window, water after wind-driven rain, soft trim or damaged sealant points more toward water entry.
Measure the room—not only the window
Place a thermometer and hygrometer away from the glass, radiator and direct airflow. Record the reading before bed and after waking for several days. The U.S. EPA advises keeping indoor relative humidity below 60%, ideally around 30–50% where possible, but climate and outdoor temperature affect what a window can tolerate.
Remove the water before it reaches the frame
Wipe the glass, sill and frame dry each morning. Do not leave water sitting on timber, plaster, wallpaper or seals. Drying protects the material today; it does not explain why the same water returns tomorrow.
Reduce moisture at the source
Use bathroom and kitchen extraction that vents as designed, cover pans when practical, vent a clothes dryer outdoors where required, and avoid drying large amounts of laundry in an unventilated room. Turn down or stop a humidifier if windows begin collecting moisture.
Use the home’s ventilation correctly
Keep designed background vents open and clear, maintain extractor fans and follow the instructions for mechanical ventilation. Brief window ventilation can help when it is safe and the outdoor air is suitable; leaving a window open all day is not a universal solution.
Keep the inner surface warmer
Maintain reasonable, steady heating and allow room air to reach the glass. Heavy curtains, furniture or deep blinds can isolate the window from warm air. Persistent condensation on one unusually cold window may indicate a glazing, frame, installation or insulation issue rather than household humidity alone.
THE SEVEN-NIGHT TEST
Find what changes the morning result
- Photograph each affected window before wiping it.
- Record indoor temperature and humidity before bed and after waking.
- Note showers, cooking, laundry, humidifier use, closed doors, rain and outdoor temperature.
- Dry the window and use existing extraction and background ventilation correctly.
- Repeat for seven nights without making several major changes at once.
- Improvement means less water, faster drying and no new staining or musty odor.
What the pattern probably means
Several windows are wet: check whole-home humidity and ventilation first. Only one cold room is affected: check heating, airflow and that window’s performance. Water appears after rain: investigate seals, flashing, drainage or the surrounding wall. Moisture is between panes: consult a qualified window or glazing professional.
GET PROFESSIONAL HELP
Do not keep treating this as ordinary condensation when:
- water appears during or after rain, or enters around the frame;
- the wall, sill or timber is stained, swollen, soft, peeling or crumbling;
- droplets or fog remain trapped between sealed panes;
- mold repeatedly returns despite moisture and ventilation changes;
- the ventilation system is not working or the repair requires electrical, glazing, insulation or structural work.
Contact the appropriate qualified glazing, ventilation, building or damp professional. Renters should record dates, photos and weather conditions and report the problem to the landlord or property manager. Consult a healthcare professional for breathing difficulty, wheezing or worsening asthma.
SOURCES & SCOPE
Guidance is informed by the U.S. EPA guide to mold and moisture, the EPA guide to controlling home moisture, and the UK government guide to existing-home ventilation. Window construction, climate, ventilation systems and tenancy rules vary by location.