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.

Inspection Overview
Why Get a Home Inspection?

Recent Posts

Whole House Fan

A whole house fan can be a great way to keep cool in the summer while saving money and energy.  Here are some important things to know about them. A whole house fan is found

AC insulation

Here’s a quick schematic diagram of a central air conditioner system.  There’s the outside unit, known as the condensing unit.  There’s everything inside the house, consisting mostly of the evaporator coil, a metering device, and

Water temperature

According to an article published in 2013 in the academic publication Journal of Burn Care and Research “Burns from tap water result in an estimated 1500 hospital admissions and approximately 100 deaths per year.”  J

Don’t let your pipes freeze

I’ve heard the question many times: How low should I set my thermostat if I leave town in the winter, to save money but prevent my pipes from freezing? 65 °F?  60 °F?  50?  I

Request Inspection

More Posts You May Find Interesting

Whole House Fan

June 17th, 2024|Comments Off on Whole House Fan

A whole house fan can be a great way to keep cool in the summer while saving money and energy.  Here are some important things to know about them. A whole house fan is found

AC insulation

June 10th, 2024|Comments Off on AC insulation

Here’s a quick schematic diagram of a central air conditioner system.  There’s the outside unit, known as the condensing unit.  There’s everything inside the house, consisting mostly of the evaporator coil, a metering device, and

Water temperature

May 31st, 2024|Comments Off on Water temperature

According to an article published in 2013 in the academic publication Journal of Burn Care and Research “Burns from tap water result in an estimated 1500 hospital admissions and approximately 100 deaths per year.”  J

Don’t let your pipes freeze

February 7th, 2024|Comments Off on Don’t let your pipes freeze

I’ve heard the question many times: How low should I set my thermostat if I leave town in the winter, to save money but prevent my pipes from freezing? 65 °F?  60 °F?  50?  I

Wood shrinkage

January 16th, 2024|Comments Off on Wood shrinkage

Let’s talk about wood shrinkage. Your hardwood floors are the place where you’re mostly likely to see the problems associated with wood shrinkage, but old panel doors can also experience problems.  In floors, as the

Window condensation

January 10th, 2024|Comments Off on Window condensation

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