Interior window condensation forms when glass temperature falls below the dew point of indoor air. Lowering indoor moisture, improving air movement, and correcting window or insulation defects can all help; the right fix depends on where the water appears.
What matters most
- Water on the room side points to indoor air meeting cold glass.
- Water between panes usually indicates an insulated-glass seal problem.
- Recurring puddling can damage trim and should not be ignored.
Locate the water first
Wipe the glass and determine whether moisture is on the indoor surface, outdoor surface, or trapped between panes. Each location has a different cause.
Exterior morning condensation can occur on efficient glass and is not the same as indoor condensation. Moisture between panes is a window assembly issue, not a room dehumidifier problem.
Reduce indoor vapor at the source
Run bath and kitchen exhaust fans to the outdoors, vent the dryer correctly, cover cooking pots when practical, and avoid drying laundry indoors during cold weather.
If a whole-home humidifier is operating, lower its setting during cold spells and watch the result over several days.
Warm the surface and move air
Open blinds enough for room air to reach the glass. Keep furniture from blocking supply registers and inspect weatherstripping, trim gaps, and obvious insulation defects.
Use the dew-point calculator to compare indoor conditions with the coldest part of the glass. The result shows whether lowering RH, warming the surface, or both would create the most margin.
A measured action plan
- 1Identify which side of the glass is wet.
- 2Measure room RH and glass temperature before sunrise.
- 3Reduce indoor vapor sources and open window coverings.
- 4Request window service if moisture remains between panes.
Useful tools to compare
These product categories match the checks in this guide. Compare specifications and current instructions; they are not hands-on test recommendations.
Track RH away from vents and wet surfaces.
View current options on Amazon ↗02Infrared thermometersLocate cold surfaces while accounting for reflective-material errors.
View current options on Amazon ↗03Window insulation filmA seasonal air-sealing aid, not a repair for failed glazing or leakage.
View current options on Amazon ↗Sources & further reading
- U.S. EPA — A Brief Guide to Mold, Moisture and Your Home ↗
- U.S. EPA — Controlling Moisture in Your Home ↗
Reviewed against the linked sources in August 2026. Product instructions and local codes take priority for a specific installation.