Condensation forms when humid indoor air reaches a surface colder than the air’s dew point. Windows are often among the coldest interior surfaces, so they are where the first visible droplets appear.
Two variables matter: humidity and glass temperature
The more moisture the air contains, the less cold the glass needs to be before condensation begins. Poorer insulating performance and colder outdoor weather both lower the interior glass temperature.
Why it is often worse in the morning
Overnight, a closed bedroom gains moisture from breathing and may cool slightly. Heavy curtains close to the glass can also reduce warm room-air circulation at the window.
Showers, cooking, and indoor drying contribute
Moisture produced in one room moves through the house. A poorly exhausted shower can show up later as droplets on a bedroom window.
Condensation is not the same as a leak
Uniform droplets across the room-side glass point toward condensation. Water entering at the frame during rain, localized wall staining, or moisture unrelated to indoor humidity and temperature deserves investigation for leakage.
Fog between panes is a different problem
If an insulated double-pane unit fogs inside the sealed space where you cannot wipe it from either side, the window seal may have failed.
Where the droplets form adds information
Condensation concentrated near glass edges may simply mark the coldest parts of the assembly. If it appears only behind a closed blind or heavy curtain, improve air circulation there before assuming the entire window needs replacement.
Colder outdoor weather changes the threshold
The same indoor humidity that caused no condensation in fall may create droplets during a much colder night. A home can therefore start showing wet windows even when indoor moisture production has not increased.
The window is showing the relationship between indoor humidity and surface temperature. Lowering moisture, warming the surface, or both is the basis of prevention.
Sources
- U.S. Environmental Protection Agency (EPA) — Indoor Air Quality — Indoor humidity, ventilation, particles, and indoor air quality.
- U.S. Environmental Protection Agency (EPA) — A Brief Guide to Mold, Moisture and Your Home — Moisture control, drying, and general household mold guidance.