What Window Condensation Really Is

• Two levers, three kinds of fix
• Why the bottom edge gets wet first
• Why mornings and deep cold make it worse
• Why tighter homes can struggle more

You do not need a physics degree to fix wet windows. One idea explains almost every tip that works.

Indoor air always holds some water as invisible vapour. Warm air can hold more vapour than cold air. When room air touches a surface cold enough, it cannot hold all of that vapour anymore, and the extra turns into liquid on the surface. That temperature is the dew point. Wetter air means a higher dew point, so glass does not have to be as cold before water appears.

In winter, window glass is usually the coldest surface in the room. Outside cold travels through to the inside face. When moist indoor air hits that cold face, water condenses. If the glass is cold enough, you get frost or ice on the inside.

§Two levers, three kinds of fix

Indoor condensation always comes down to:

1. How much moisture is in the indoor air
2. How cold the inside surface of the glass is

Every useful fix does one of three things: remove moisture already in the air, stop moisture from entering the air, or warm the inside of the glass. Keep that frame in mind when you judge advice from anywhere else.

§Why the bottom edge gets wet first

Water often shows along the bottom edge and lower corners while the middle of the pane stays dry. Edges meet the frame and spacer, which usually let more cold through than the centre of the glass. Air next to cold glass also cools, gets heavier, and slides down the pane, so the bottom sits in the coldest air in the room.

That bottom edge is an early-warning line. A thin damp line means you are close to dry. A fully wet pane means you are far from it.

§Why mornings and deep cold make it worse

Overnight the outdoor temperature often hits its lowest point, many homes turn heat down, bedroom doors stay closed, and people breathe moisture into a small room for hours. By morning the bedroom air is wetter and the glass is colder than at any other time of day.

The colder it is outside, the colder the inside face of the glass gets. Humidity that caused no trouble in October can soak windows in January. The right indoor humidity for your home is not one fixed number for the whole winter — it moves with the weather.

§Why tighter homes can struggle more

Older homes often leak air. That leaking lets moist indoor air escape and pulls in dry outdoor air. When a home is air-sealed or renovated, those leaks shrink and the moisture you produce stays inside. The answer is not to make the house leaky again. It is to remove moisture on purpose with fans and ventilation instead of by accident.

Originally published at https://dry-glass-notes.surge.sh/articles/what-window-condensation-really-is.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.