Window Moisture: What Causes Moisture inside Your Windows?

Window moisture, or condensation, forms when moist air comes into contact with a surface where the temperature is lower than that of the air. Moisture is always present in air, and there is usually more of it found in warm air.

What Is Condensation?

As soon as the warm air meets a cold surface or even colder air, it can’t retain the same amount of moisture and has to release some. This is known as condensation.

The moisture collecting on the interior side of windows is the type of condensation you'll usually notice the most. Condensation is only really visible when the moisture forms on a surface that can’t absorb it such as your window. The problem is that the moisture can pool and eventually form mold and mildew if it’s not cleaned. Condensation is most prevalent in winter.

Older Houses

Many older houses might not have good water barriers. Water barriers stop moisture from the soil from coming into the house and rising into the living areas where they can cause window moisture.

Some windows might also not have adequate ventilation, especially in basement areas. The effect of this is to keep the moisture in the house. Kitchens and bathrooms, in particular, have plenty of warm air caused by cooking and the use of hot water, and these are more likely to contribute to window moisture. Moisture will also form on many other surfaces under these conditions since all of the surfaces present will usually be colder than the moisture itself.

Modern Houses

While older houses have less insulation, newer houses are much better insulated, and double-paned windows are now commonplace. All of this has combined to make houses much more sealed, which creates the ideal conditions for condensation, since the moisture is kept within the house.

The circulation of air in modern houses is not as good due to lower levels of ventilation. Additionally, since houses are empty and unheated for much of the day while people are at work or at school, the house cools off significantly during the day. The house is usually heated in the morning and evening, and this is when you’ll most frequently find water moisture inside the windows as the warmer air meets the colder glass.

Window moisture can also be the result of a bad structure, which can make for a high moisture content in the air. The problems could be with the construction itself or due to some form of structural failure.


Window moisture can also be caused by the people in the house. If there are five people in a family, they put around 20 pints, or 2.5 gallons, of moisture into the surrounding air every day without even considering sweat from heating. This level of moisture will be warmer than the glass in the windows. If all the people are in one room for an extended period of time, they will heat the air and window moisture can easily be the consequence.

Regular activities in the home will also produce condensation, which can ultimately become window moisture. Even breathing produces almost two pints of moisture a day, while running a washing and dryer can result in a further 10 pounds of moisture. All of this has to go somewhere, and it usually will collect on a cold surface, such as a window.