What Wet Windows Are Trying To Tell You

Condensation on a window during a rain shower.

What Wet Windows Are Trying to Tell You: It’s easy to know that wet, sweaty windows are an unsightly, unhealthy problem. But with difficult-to-diagnose causes and tricky-to-install solutions, it’s harder to know what to do about them. What causes window sweat?

If you’ve ever walked through dew-covered grass, you’ve felt the physics behind window sweat. When warm, moist air collides with a cold surface (your front lawn or your windows), it quickly cools below the dew point and deposits moisture. That’s no problem outside, but as interior “dew” builds up, the excessive moisture clouds your windows and sills with a sweaty mess. Even worse, that moisture can foster mold and mildew and create serious air quality problems.

What can we do about window sweat? Physics tells us that cold windows + warm, moist air = window sweat. To eliminate window condensation, we need to change the equation. Let’s look at the possibilities:

Increase the surface temperature of window glass. If you’re replacing windows anyway, look for a high condensation resistance number on the window’s energy rating. But don’t assume you need new windows. Custom thermal inserts from Evergreen Home Performance are a simple, affordable way to prevent window condensation.

Decrease the moisture level in the air. There are many ways to manage interior moisture, from dehumidification and ventilation improvements to full basement encapsulation. The best solution will be specific to your home and your priorities. Whatever the cause, window condensation is a clue that something is amiss.

Water vapor condenses into a liquid after making contact with the surface of a cold bottle.

Condensation is the change of the physical state of matter from gas phase into liquid phase, and is the reverse of evaporation. It can also be defined as the change in the state of water vapor to liquid water when in contact with any surface. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition.

Condensation in building construction is an unwanted phenomenon as it may cause dampness, mold health issues, wood rot, corrosion and energy loss due to increased heat transfer. To alleviate these issues the air ventilation in the building needs to be improved. This can be done in a number of ways; opening windows, turning on extractor fans, drying clothes outside and covering pots and pans whilst cooking to name a few. Air ventilation systems can be installed that help move air throughout a building. The amount of water vapour that can be stored in the air can be increased simply by increasing the temperature. Interstructure condensation may be caused by thermal bridges, insufficient or lacking damp proofing or insulated glazing.

These windows are too wet and indicate poor indoor air quality. Increasing ventilation solves both these problems.

If your windows sweat, you will get mould:  Are your windows getting wet from condensation again? Now’s the time to do something about it. Every year at this time, outdoor temperatures begin their slow decline into winter, and every year windows across Canada start to get wet from condensation on interior surfaces. The degree of wetness ranges from insignificant to downright debilitating, and understanding how to keep windows dry is a must-have skill for every Canadian homeowner.

Windows “sweat” during winter as indoor air cools against cold window glass and loses its ability to hold moisture. This excess water has got to come out somewhere, and glass and window frames are excellent places for droplets of condensation to form.

Besides being a powerful trigger of mould growth, window condensation is also a signal that your home is suffering from inadequate ventilation. If enough excess moisture is being held inside your home to cause running window condensation, then it’s almost certain that your household air has more contaminants than it should, too.

That’s why many Canadians need to consider intentionally ventilating their homes in some way when window condensation begins to appear. There are two ways to do it.

Drafty homes that naturally leak lots of cold air never need intentional ventilation because enough fresh air comes inside on its own. That’s why you’ll never see window condensation in an old, cold farmhouse. But a leaky home is an expensive home to heat, and that’s why we’ve been building houses tighter and tighter for more than 40 years.

The easiest way to boost indoor air quality and reduce window condensation in a tight home is by opening windows a little and running exhaust fans more often in the bathroom and kitchen. For every cubic foot of stale air pushed outside by fans, another cubic foot of fresh air is drawn in through windows opened a little here and there.

Do this enough to reduce window condensation and you’ll be healthier for it. Healthier, but poorer, since you’ll also be losing quite a bit of heat in your quest to breathe easier.

Adding fresh air into your home from outside while also retaining most of the heat invested in the stale air before it gets sent outside is what a heat recovery ventilator (HRV) is for. I write about these things every year because poor indoor air quality and window condensation is the most common home situation people ask for help about during winter.

HRV technology was invented in Canada more than 30 years ago, but many Canadians still don’t know how it works or the difference HRVs make. If you’ve had excess window condensation in past winters, chances are good that you’ve got at least a little mould around window edges right now. Maybe a lot.

Killing this mould is a good way to start the heating season because dormant mould can start growing again at lower moisture levels than would normally be required to trigger mould growth from a fresh start. Just don’t use bleach solutions for the job.

Bleach is the traditional way to kill mould, but according to the Occupational Health and Safety Administration (OHSA) in the U.S., “the use of a biocide, such as chlorine bleach, is not recommended as a routine practice during mould remediation.”

Instead, stop the source of mould-promoting moisture, treat the area with a registered, non-bleach fungicide to kill the mould, then wipe the area down with a damp rag or sponge. As long as your windows stay dry, mould won’t come back.