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 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 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 people 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 ventilator 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.