Frost on the inside is a message.
If frost or water is building up on the inside of your windows, the air in your house is holding more moisture than the glass can handle at that outdoor temperature.
Why it happens
Warm air holds more water than cold air. When the warm, moist air in your house touches cold window glass, it cools and drops its water on the glass, which freezes if the glass is cold enough. The colder it is outside, the less humidity your windows can tolerate.
A rough guide
- Around 20°F outside, indoor humidity of 35% or so is usually comfortable without frost.
- Around 0°F, closer to 30%.
- At twenty below, 20 to 25% may be all the windows can take.
That’s why the humidifiers we install use an outdoor sensor that lowers the target automatically as it gets colder.
If you don’t have a humidifier and still get frost
Then moisture from cooking, showers and people isn’t getting out. Run bath and kitchen fans longer, and in a tight newer house, consider a ventilator. Persistent heavy frost can lead to mold on sills, so it’s worth fixing.
Windows icing up? We’ll measure it.
The cold-climate heating site, built in seven days.
Sixteen pages: a live outdoor-temperature ladder, an honest cold-climate heat pump page that says when to keep the furnace, and a no-heat line that leads every page from November to March.