Condensation on windows is a problem that I see a lot in the winter, and I see the damage from window condensation year round.  Here’s some information about the issue.

Condensation will occur on a surface anytime its temperature gets low enough to cause the moisture in the air to reach its saturation point.  As the air temperature goes down the air’s relative humidity goes up, until it hits 100 per cent relative humidity and water condenses.  So the air just next to a cold surface will get cold enough for this to happen, and water condenses on the surface.  In nature we see this as dew on the grass on a spring morning, and we see it as condensation on a cold glass on a warm summer day.

This can happen on windows when it’s very cold outside.  The glass surface becomes very cold and so moisture in the air condenses onto the glass surface.  Then that water might run down the surface of the glass and cause problems with the window sash.

A wood sash might be stained or even rotted by this water.  Even vinyl windows can be damaged by this water, and mold can also be a big problem.

Of course the question is how to prevent this condensation, and here are some options.  Keep in mind that our basic problem is too much humidity in the air and the glass surface being too cold.

You can install better windows, with a lower U-factor.  A window’s U-factor is a measure of how well it allows heat to move through it, so a lower U-factor means a more energy efficient window, and it means that the glass surface would be warmer.  But replacing windows is extremely expensive and this really isn’t a viable solution.  Plus, it might not solve the problem, depending on other factors.

You can reduce the relative humidity in the house.  This is usually achieved by lowering the setting on your humidifier, but if there’s some other reason for high humidity during the winter then you’ll need to figure out that.  Lowering the humidity might make the air uncomfortably dry, so it isn’t always going to work out well.  But it’s a great place to start.

You can raise the temperature of the glass surface by opening the blinds and curtains, letting warm air from the house wash over the window glass.  It’s common for people to close their blinds at night, for several reasons – to block out light and to get the extra insulation value of the curtains over the cold glass.  But this just means that the glass will stay extra cold, and so condensation is more likely to happen.  And sometimes I see screens on the windows with condensation.  Again, the screens are preventing warmer room air from washing over the window glass.  And you don’t really need the screens in the winter since you won’t be opening the windows, so remove the screens.

Here’s a chart showing roughly when window condensation will start based on the outside temperature, the indoor relative humidity, and the quality of the windows.

There are four curves representing four different windows, with U-factors ranging from 1 to 0.25.  As a reminder, the U-factor is a measure of how well the window allows heat to move through it, so a lower U-factor is a more energy-efficient window and its interior surface will be warmer.  Double pane windows generally have a U-factor of anywhere from 0.3 to 0.4 or maybe a little higher if they’re fairly old.  Old single-pane windows have a U-factor of somewhere around 0.7 or higher, but that can vary based on the presence of storm windows.  A U-factor of 0.25 is a very high quality window, almost certainly triple glazed.  Also, keep in mind that this chart doesn’t apply if you put blinds, curtains, or screens over the window.

If you want to know what your home’s humidity level is, many thermostats have a humidity display built in.  Otherwise you can buy a hygrometer to measure relative humidity.

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