Frost, Ice and Condensation on the Inside of Your Windows: A Minnesota Winter Guide

Wake up on a January morning to frost creeping up the inside of your windows? Or condensation running down the glass and pooling on the sill? You're not alone. Here in St. Cloud and across Central Minnesota, it's a common sight on cold winter mornings.

The good news: most of the time, the fix doesn't start with new windows. It starts with the air inside your house. This guide explains what causes condensation on the inside of your windows in a Minnesota winter. You'll find the indoor humidity limit for each outdoor temperature, the quick fixes to try first, and how to tell when the window itself is part of the problem.

What Causes Condensation, Frost and Ice on the Inside of Windows

Warm indoor air carries moisture. When that air touches a surface that's cold enough, it cools down and can't hold as much water, so the extra moisture turns into droplets on that surface. The temperature where this starts is called the dew point. The more humid the air, the higher the dew point, and the sooner your glass starts to sweat. In winter, the coldest surface in most rooms is the window glass.

The colder it gets outside, the colder your glass gets on the inside. That's why a window that stays clear at 30°F can start dripping at 0°F with the same air inside the house. When the glass drops below freezing, the moisture shows up as frost or ice instead of water. It often starts at the bottom of the glass and along the edges, where a window runs coldest.

The moisture comes from everyday life: showers, cooking, breathing, drying clothes, houseplants, firewood stored indoors and, of course, humidifiers. A damp basement or crawl space adds more, and so does any gas heater or fireplace that isn't vented outside, because burning gas gives off water vapor. The University of Minnesota Extension puts "condensation on windows during the winter" at the top of its list of signs that a home has too much moisture.

One safety note: if heavy condensation shows up suddenly and nothing else in the house has changed, have a heating contractor check that your furnace and water heater are venting properly, and make sure your carbon monoxide alarms work.

Winter Indoor Humidity Limits by Outdoor Temperature

Relative humidity is how much moisture the air is holding compared with the most it could hold at that temperature. In a Minnesota winter, the right number isn't one number. It drops as the weather gets colder.

The Center for Energy and Environment (CEE), a Minnesota energy nonprofit, recommends keeping indoor humidity below these levels in winter:

CEE calls this a general guide, and it warns that when it's extremely cold out, even 40% humidity can still cause condensation on windows and walls. UMN Extension agrees that in cold winter weather, indoor humidity needs to stay low to prevent condensation. It calls 25% generally healthy and comfortable for winter, and on the coldest days, CEE's table calls for even less.

To know where you stand, pick up an inexpensive indoor humidity gauge (called a hygrometer) at a hardware store. Check it in the room with the problem, away from the humidifier, kitchen and bathroom. Cheap gauges can be off by a few points, so treat the reading as a ballpark. If it's above the limit for that day's outdoor temperature, humidity is the likely cause.

How to Stop Condensation on Windows: Quick Fixes to Try First

Start with the cheapest, easiest changes. Give each one a few days and keep an eye on the glass.

• Turn the humidifier down or off. If you use one, CEE suggests running it only for short periods at a low setting. Never set it higher than the limit in the table above for that day's outdoor temperature. CEE's general range of 30% to 40% only fits milder winter days, above about 10°F. If you see moisture building up on your windows or walls, CEE says to turn the humidifier off and lower the setting before you turn it back on. CEE's energy auditors typically recommend room humidifiers over whole-home units, because a whole-home setting is easy to forget about.
• Use your bathroom and kitchen fans. Run the bath fan when you shower and for a while after, run the range hood when you cook, and cover your pots. Those fans need to vent outdoors. A microwave or range hood that blows air back into the kitchen won't remove any moisture. A bath fan that dumps moist air into the attic can cause its own winter problems; see why bathroom vents leak in winter.
• Vent the dryer outside. If you're fighting condensation, skip the indoor drying rack for the winter.
• Cut back on moisture sources. Keep firewood outside or in the garage, and don't crowd a lot of houseplants into one room.
• Watch the bedrooms. UMN Extension notes that we release a lot of moisture into the air just by breathing, so a bedroom with the door shut all night can fog up by morning. Leaving the door open lets that air mix with the rest of the house.
• Let warm air reach the glass. Heavy curtains and closed blinds keep room heat off the glass, so it runs colder, while moist air still sneaks in behind them. That's why the worst frost is often behind closed blinds in the morning. On a window that keeps frosting, leave the blinds tilted open or raised a few inches at night, open curtains during the day, and don't block or close the heat register under the window.
• Run your air exchanger. Many newer Minnesota homes have an air exchanger or other whole-house ventilation system. If yours is off or set low, turn it up and keep its filters clean. UMN Extension recommends checking that exhaust ventilation systems are working properly and suggests considering a whole-house ventilation system.
• Air the house out briefly. UMN Extension suggests opening a window just a little for a short time in winter, which lets warm, moist air out and cold, dry air in.

Is the Moisture Really on the Inside?

Everything above is about moisture on the inside surface of the glass. If fog is trapped between the panes of sealed double- or triple-pane glass, the seal has failed, and turning down the humidity won't clear it. Dew on the outside of the glass on a cool morning usually just means your windows are keeping the heat in. Our window and door replacement page explains all three.

Not every wet window is condensation, either. Water at the top of the window or on the wall above it, especially on a mild day after a cold snap, may be coming from an ice dam on the roof or frost melting in the attic. That's a roof and attic problem, not a humidity one.

When the Window Itself Is the Problem

If your humidity is in the right range and you still see steady condensation, something else is going on. CEE puts it plainly: if you're not using a humidifier in the winter and still have consistent condensation, "it may be because of inefficient windows or under-insulated walls." CEE's first fix is to insulate and air seal those windows or walls. When the window itself is worn out, that's when replacement comes into the picture.

Signs to watch for:

• One or two windows frost up while similar windows in similar rooms stay clear. Rule out the easy causes first: a bathroom, a bedroom slept in with the door shut, a closed heat register or heavy drapes.
• Frost or ice builds up on the frame or sash (the part that holds the glass), especially at the lock, where the two sashes meet or along the weatherstrip. That usually points to an air leak or a frame that lets the cold through.
• Frost builds up between an older window and its storm window. That's not a failed seal. It means warm, moist air is leaking past the inside window.
• Old single-pane glass or metal frames that feel ice-cold to the touch.
• Water has sat on the sill long enough that the wood is stained, soft or peeling. Whatever the cause, that sill needs repair.

Before you shop, know this: new windows won't fix a humidity problem. Even brand-new, energy-efficient windows can fog up when the air inside is too humid for the weather outside. New windows also stop drafts, and a tighter house holds onto more moisture, so your indoor humidity may go up the first winter after replacement. Keep checking your humidity gauge and running your fans. Get your humidity under control first. If the condensation still won't quit, that's when it's worth looking at the windows themselves.

At Zablocki Roofing & Exteriors, we install windows from Andersen, Vector (Envision Series) and Sierra Pacific. If your humidity is in range and the condensation keeps coming back, our project specialists can walk you through replacement options and give you a free estimate. You can compare the Andersen, Vector Envision and Sierra Pacific windows we install or read about our window replacement service in Central Minnesota.

Mold on Window Sills: Cleanup and Prevention

When water sits on a sill day after day, mold can follow. UMN Extension notes that mold grows quickly on damp surfaces like the trim around windows. In winter, moist air can turn to water on the cool surfaces around windows and doors, and mold can start to grow there.

For small spots, UMN Extension recommends:

1. Protect yourself. Wear rubber gloves, eye goggles and an N-95 respirator.
2. Scrub. Use a stiff brush, hot water and a soap or detergent that does not contain ammonia. Rinse with clean water and dry the area thoroughly.
3. Disinfect. Wipe the area again with a mix of 5 cups of water and 1/2 cup of household bleach. Follow the directions on the bleach label, open a window for fresh air, and never mix bleach with ammonia. Many glass cleaners contain ammonia, so don't spray one nearby while the bleach is wet. Bleach can also lighten stained wood and harm some window finishes, so test a hidden spot first and check your window maker's care instructions.
4. Let it air dry.

Then deal with the moisture, or the mold will come back. Wipe water off the glass and sill on cold mornings, and work through the humidity fixes above. If mold covers a large area or keeps coming back, call a mold cleanup professional. If the wood under it has gone soft, the sill or frame may be rotting and need repair or replacement.

How Warmer Glass Helps

Condensation forms on cold surfaces, so warmer glass means less of it. CEE explains that more insulation "raises surface temperatures inside your home so that moisture is unable to cool and condense." Windows work the same way: a better-insulating window keeps the inside pane warmer, so it takes more humidity before the glass fogs or frosts.

A window's U-factor measures how fast heat escapes through the whole window, frame included, and lower is better. Because condensation starts at the edges of the glass, some window labels also list an NFRC Condensation Resistance rating on a scale of 1 to 100, where higher is better. Warm-edge spacers between the panes help keep those edges warmer. When you shop, look for windows that are ENERGY STAR certified for the Northern zone, which covers all of Minnesota. Vector, which builds its windows in Fergus Falls, says its Envision Series windows are ENERGY STAR certified for the Northern zone when ordered with triple-pane glass. For what U-factor means and the numbers to look for, read Can New Windows Really Lower My Bills?

Frequently Asked Questions

Is some condensation on my windows normal in winter?

A little fog at the bottom corners on the coldest mornings is common in Minnesota, especially on older windows. Moisture that keeps building up, runs down the glass or pools on the sill is a sign there's too much moisture in the air.

What humidity should I keep my house at in a Minnesota winter?

It depends on the temperature outside. CEE's guide: under 40% when it's 20°F to 40°F, under 30% at 0°F to 10°F, and under 15% at -20°F or colder. Check the table above and adjust as the weather changes.

Why is there frost or ice on the inside of my windows?

When the inside of the glass drops below freezing, moisture in the air freezes on it instead of turning to water. Lower your indoor humidity and let the ice melt on its own. Don't chip at it or aim a heat gun at cold glass, which can crack it. Once it melts, wipe the glass and sill dry so water doesn't soak into the wood.

Will new windows stop condensation?

They help, because better-insulated glass stays warmer. But new windows can still fog if your indoor humidity is too high for the outdoor temperature, and a tighter house can raise your humidity. Fix the humidity first, then decide on windows.

Does condensation mean my windows are bad?

Not usually. Moisture on the inside of the glass most often points to high indoor humidity. Fog between the panes of sealed glass, frost on the frame or rotting sills are better signs that the window itself needs attention.

Still Fighting Frost? Let's Talk

If you've brought your humidity down and your windows are still sweating, frosting or icing up, it may be time to look at new windows. Call us at (320) 203-1297 or request a free estimate on replacement windows. Our project specialists serve St. Cloud and the surrounding Central Minnesota communities.