Frost forms inside your windows when warm, moist indoor air meets very cold glass. NDSU Extension suggests keeping winter humidity near 40 percent. Even then, double-pane windows can show condensation once it's below zero outside. Shades closed tight can trap moist air against the glass. On the coldest nights, raise them a few inches or leave them open.

Why does frost show up inside Rochester windows?

Because Rochester gets cold. Really cold.

The city's normal January low is 7 degrees, per the National Weather Service. Plenty of nights go well below that. In January 2024 the city hit 14 below on the 15th. A University of Michigan climate profile says January averages fall below 16 degrees.

Inside, you keep the house near 70. Cooking and showers add moisture to the air. So do laundry and plain old breathing. That moist air drifts to the coldest surface in the room. In winter, that's the window glass.

When the glass is cold enough, the moisture turns to water. Colder still, and it turns to frost. It shows up first at the bottom corners and edges. That's where the glass is coldest.

So frost means two things met: cold glass and moist air. Change either one and the frost changes too.

Does frost mean your windows are failing?

Myth: Frost on the glass means your windows are shot.

Fact: Some frost on a below-zero night is normal, even on good windows. NDSU Extension looked at this. At 40 percent indoor humidity, double-pane windows start to show condensation below zero outside. Triple-pane windows hold out until it's closer to 40 below. That's from an NDSU Extension release on window condensation.

What to watch for instead:

Frost or fog between the panes, inside the sealed unit. That points to a failed seal.
Water pooling on the sill day after day.
Frost on milder nights, well above zero. That usually means the air inside is too humid.

Will closing your shades tight fix it?

Myth: Close the shades tight and the frost goes away.

Fact: It often makes condensation worse. A tightly closed covering keeps the room's warm air away from the glass. That sounds good. But the glass gets even colder. Any moist air that slips behind the shade gets trapped there. NDSU Extension's advice is simple. If condensation forms behind drapes, try leaving them open at night. You trade a little heat for less moisture.

Iowa State University Extension's answer line on winter condensation says it another way. Close drapes at night to hold heat in. Just don't make it a tight seal. Leave room for air to move.

Is more humidity always better in winter?

Myth: Dry winter air is the enemy, so run the humidifier hard.

Fact: Too much humidity ends up on the glass. NDSU suggests about 40 percent in winter. Iowa State calls 30 to 40 percent the best range. And the University of Minnesota Extension lists window condensation in winter as a sign of too much moisture in the home.

Where does the extra moisture come from? UMN Extension points to bathing and cooking. Dishwashing and breathing add to it too. A few fixes:

Kitchens. Use a range fan that vents outside while you cook.
Bathrooms. Run the fan during showers and for a while after.
Laundry. Make sure the dryer vents outside.

A simple humidity gauge takes the guessing out of it.

Can window coverings help at all?

Myth: Shades are the problem, so go without.

Fact: Coverings help when you use them well. They hold heat in on cold evenings. They block drafts off the glass. They keep rooms comfortable. The trick is how you set them on the coldest nights.

Three common choices, compared on a frosty window:

Covering	Warmth on cold nights	Air at the glass	Easy to adjust overnight
Cellular shade	High, thanks to trapped air in the cells	Can trap moisture if closed tight	Easy to raise a few inches
Roller shade	Moderate	Easier for air to move behind it	Very easy
Lined drapery	High when closed	Can hold moisture if sealed to the wall	Easy to open partway

For more on warmth, see our guide to single, double and blackout cellular shades for a Rochester winter. Simple roller shades are another good fit for windows that frost a lot. They're easy to adjust.

Which windows frost first, and why?

Not every window frosts the same way. The pattern tells you a lot:

Bedroom windows. Doors closed overnight. People breathing for eight hours. Often a cooler thermostat, too. Bedrooms are frequent frost spots.
Bathroom windows. Shower steam lands on the nearest cold surface. A fan that runs long enough is the fix.
Kitchen windows. Pots on the stove add moisture right next to the glass over the sink.
Patio doors. Lots of glass and metal. They often frost along the bottom track.
Lower-level windows. Basements tend to run more humid. The glass sits close to cold ground and snow.
Windows on the windward side. Wind across the glass makes it colder still. The same University of Michigan profile says Rochester has been reported as the country's second windiest city.

If one room frosts far more than the rest, look at that room's moisture first. If every window frosts on a mild night, look at whole-house humidity.

What should you try first?

A short list for the next cold snap:

Check your humidity. Aim for about 30 to 40 percent.
Run the fans. Kitchen while you cook. Bathroom during and after showers.
Open coverings on sunny days. Let the sun warm the glass and dry it out.
Close them at dusk, but not as a tight seal.
On below-zero nights, raise the shade a few inches on windows that frost. Or leave the worst ones open.
Wipe the sill in the morning if water collects. Keep it dry.
Note which windows frost first. That's where your glass is coldest.

In a newer townhome or condo? Tight construction can hold moisture in. Our post on planning shades for a Rochester condo or townhome walks through smaller, shared-wall spaces step by step.

When should you call for help?

Call a window or heating pro if you see fog between the panes. Same goes for frost on mild nights or water damage on sills or walls. Those point to the window or the house, not the shade.

Call us when you want coverings that keep the house warm and still play nice with cold glass. We'll look at your windows and talk through how you use each room. We'll show you fabrics right at the glass. We're locally owned and serve the whole Rochester area. Book your free consultation and we'll set a time that works.

Kevin Sill is CFO and co-owner of the family-owned Budget Blinds franchises serving Rochester and southeast Minnesota.