Causes and remedies for window condensation and indoor humidity

Condensation on windows, particularly in the morning, is a thermodynamic phenomenon caused when warm, moist indoor air meets cold glass surfaces. This occurs when the temperature of the glass falls below the dew point, causing water vapor to transition into liquid droplets.

Key sources of indoor humidity include human respiration and perspiration—which can release between 300 and 500 milliliters of water per person during sleep—as well as cooking, showering, and drying laundry. In well-insulated modern homes, this moisture can become trapped, leading to persistent fogging, water pooling on sills, and potential mold growth on frames and walls.

To manage humidity, experts recommend maintaining a relative humidity level between 40% and 60%. Effective remedies include opening windows wide for several minutes several times a day rather than leaving them slightly ajar, and using a hygrometer to monitor levels. Simple habits, such as airing out a room for five to ten minutes before sleep, can also help mitigate the buildup of moisture overnight.