Condensation on the Inside of Your Windows in Winter

Condensation on the inside of your windows in winter happens when warm, humid indoor air touches glass that's colder than the air's dew point. The moisture turns to droplets, and on the coldest Minnesota nights it can freeze into frost or ice. A little on the bottom edge of the glass on a subzero morning is normal. Heavy condensation day after day usually means your indoor humidity is too high for the outdoor temperature.

The good news is that you can usually fix it without replacing anything. Here's why it happens, how to bring it under control, and how to keep it from damaging your sills, frames, and glass.

Why Windows Sweat in a Minnesota Winter

Glass is the coldest surface in most rooms. When it's 10 below outside, even a good double-pane window has an inside surface noticeably colder than your walls. Indoor air always carries some moisture, and when that air hits a cold enough surface, the moisture condenses. That's the same reason a glass of ice water sweats in July.

Several things push moisture levels up in a typical Twin Cities home during winter:

• Humidifiers set too high, whether a whole-house unit on the furnace or a portable one in a bedroom
• Showers and baths without the exhaust fan running long enough
• Cooking, especially boiling water, without the range hood on
• Lots of houseplants, an aquarium, or firewood stored indoors
• Clothes drying on racks inside
• Newer, tightly built homes that hold moisture in, and new construction where drywall and framing are still drying out
• Closed blinds or heavy curtains that trap cold air against the glass overnight

Where Condensation Shows Up First

Condensation rarely shows up evenly across the house. You'll usually notice it first in bathrooms and kitchens, where moisture is produced, and in bedrooms, where several hours of breathing behind a closed door adds humidity overnight. North-facing windows that never see winter sun run colder, so they sweat sooner. Basement windows, windows behind heavy drapes, and big picture windows on lake homes and open great rooms are other common trouble spots. If only one or two windows sweat while the rest stay dry, check whether those rooms have more moisture or less air movement than the others.

How Much Humidity Is Too Much?

The colder it gets outside, the lower your indoor humidity needs to be to keep windows dry. Window manufacturers publish guidelines based on engineering studies done at the University of Minnesota. One widely used chart, printed in Simonton's condensation brochure, recommends keeping indoor relative humidity no higher than these levels:

• 20°F to 40°F outside: not over 40%
• 10°F to 20°F outside: not over 35%
• 0°F to 10°F outside: not over 30%
• -10°F to 0°F outside: not over 25%
• -20°F to -10°F outside: not over 20%
• -20°F or below: not over 15%

An inexpensive digital hygrometer from any hardware store will tell you where you stand. If the reading is above the chart for today's temperature, that's almost certainly your answer.

Simple Ways to Reduce Window Condensation

• Turn your humidifier down as temperatures drop, and keep adjusting through the season.
• Run bathroom fans during and after showers, and use the range hood when cooking.
• Open blinds and curtains during the day so warm room air can reach the glass.
• Keep furniture and heavy drapes from blocking heat registers under windows.
• Run ceiling fans on low to keep air moving.
• Use a dehumidifier in damp areas like basements.
• Have your furnace and any air exchanger checked so ventilation is working as designed.

Inside the Room, or Between the Panes?

Where the moisture shows up matters. If you can wipe it off the room side of the glass, it's humidity and the steps above will help. If the fog or droplets are trapped between the panes and you can't reach them, that's a failed seal, and no amount of cleaning or dehumidifying will clear it. We explain how to tell the difference in our post on why windows stay foggy after cleaning.

What Condensation Does to Your Windows

A bit of morning moisture won't hurt anything. Weeks of it can. Water that runs down the glass and sits on the sill can lead to:

• Mildew and black spots on sills, frames, and in window tracks
• Peeling paint and swollen or stained wood trim
• A cloudy film and water spots on the inside of the glass
• Damp, musty-smelling window coverings

When you see moisture, wipe the glass and sill with a dry microfiber towel. For mildew spots, a mild dish soap and warm water solution works well on most finished sills. Dry everything completely afterward. Avoid harsh chemicals on wood trim or vinyl frames, and never scrape ice off the inside of the glass with anything metal.

Start the Season With Clean Glass and Sills

Clean windows don't stop condensation by themselves, but they make it much easier to manage. Dust and grime on the glass give moisture something to cling to, and dirty tracks hold water and grow mildew faster. A thorough interior cleaning before or during winter clears the film, cleans the sills and tracks, and lets you spot problem windows early.

Frequently Asked Questions

Is it normal to get condensation on the inside of windows in winter?

A small amount along the bottom of the glass on very cold mornings is normal in Minnesota. Condensation that covers much of the glass, drips onto the sill, or lasts all day means humidity is too high.

Does condensation mean my windows need to be replaced?

Usually not. Condensation on the room side is mostly about indoor humidity. Moisture trapped between the panes is a different issue and points to a failed seal.

Why is there ice on the inside of my windows?

When the glass surface drops below freezing, condensation turns to frost or ice. Lowering indoor humidity on the coldest days and keeping air moving across the glass are the most effective fixes.

Get Your Windows Ready for Winter

Starbrite Window Cleaning has been family-owned, minority-owned, and locally owned in Blaine since 1988. We're fully insured and licensed, BBB A+ accredited since 2012, and we've cleaned more than 12,000 homes across the Twin Cities. Our crews clean glass, sills, tracks, and screens inside and out. Click Request a Quote or call (952) 922-6860 for a free quote.