Condensation on glass is not the kind of mystery Scotland Yard would ask Sherlock Holmes to solve. The door and window professional should be able to evaluate conditions and help the property owner understand the cause of water condensation on windows.

Water inside a home can do a lot of harm. It can damage paint, rot wood and cause mildew. Mildew can ruin the air quality, become a health hazard and remediation can be an expensive repair. Helping solve water problems at windows is part of our responsibility.

Let’s look at what causes water to collect on the inside of a door or a window glass before we try to solve this mystery.

The science behind condensation is pretty simple. Room-side condensation can only occur on door and window glass when water-saturated air comes into contact with a surface colder than the dew point.

The dew point is the temperature at which water vapor in air evaporates at the same rate it condenses. When the temperature of any impermeable surface (such as glass) is lower than the dew point, water leaves the air and condenses into a liquid. The condensed water is called dew when it forms on a solid surface.

In fact, water can also condense on the outside of a window during hot weather, once again because the glass temperature is below the dew point as the outside temperature is hot and humid.

Most of us have practical experience with dew. The condensation that forms on our soda cans is a common example of a surface reaching a temperature below the dew point and causing dew to form.

We expect this and put coasters under the glass to prevent the condensation from damaging our furniture. We wipe the dew off of glass table tops on humid summer mornings. And we avoid walking on lawns first thing in the morning in spring so we don’t get our shoes wet. These are all examples of how we try to avoid dew.

So why would we expect the glass in the windows of homes to act differently from any other impermeable object when it reaches the dew point?

1 Comment

  1. Anyone who has spent more than one heating season in the window industry knows, with the first frost comes the first calls of condensation problems… mostly blaming the windows for causing the condensation. It seems, the more technical an explanation you provide, the less believable it is.

    Hygrometers and sling psychrometers may look cool, but do little to convince a good number of consumers. All they know is that before their windows were replaced, they didn’t have the problem (go figure!), and now they do. It MUST be the windows.

    I know of a couple manufacturers who won’t even respond to a condensation complaint for three weeks, figuring (quite accurately) that within that time the building will complete it cyclic adjustment and all will be resolved. For us, a couple years ago we produced a consumer-friendly video which explains the phenomenon. When we get a complaint, we direct them to that, and that takes care of the problem 99% of the time. You can see it here:

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