Condensation Inside Windows: What It Means and What to Change

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.

Run a five-minute dew-point check

Measure room temperature and relative humidity with a hygrometer, then measure the coldest part of the glass with an infrared thermometer. At 70°F and 45% RH, the calculated dew point is about 47.7°F. A lower glass edge at 43°F is therefore cold enough for condensation.

Repeat after reducing indoor RH. At 70°F and 35% RH, the dew point is about 41.1°F. If glass remains at or below that temperature—or the same edge keeps getting wet—investigate blocked airflow, air leakage, insulation, or the window assembly instead of only lowering humidity. Use the dew-point calculator for your own readings.

• Measure before sunrise, when glass is often coldest.
• Keep the hygrometer away from supply air and the wet glass.
• Compare the center of the pane with the lower edge or corner.

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.