Why is there condensation on the inside of my windows, and do I need new ones?

At 20°F outside, your indoor air can hold about 35 to 40% relative humidity before the room side of your window glass starts to bead. At -20°F, that ceiling falls to 15%. That's the whole problem in two numbers, and it's why a house that was fine in October can drip in January without anything having broken.

Before anything else, do the wipe test. If a dry cloth clears the water, it's on the room side and this article is for you. If the haze stays put no matter which side you wipe, the seal between the panes has failed, and that's a different repair covered in our foggy window guide.

The humidity ceiling, by outdoor temperature

Glass is the coldest surface in most rooms in winter. When warm indoor air touches it and cools below its dew point, the water vapor in that air condenses. Cardinal, which makes the coated glass inside many major window brands, publishes the indoor humidity ceilings that window and humidifier makers have used for decades:

You can't use that table without a number for your house, so put a digital hygrometer in the room with the worst windows. And the ceiling moves: a setting that's safe at 25°F will fog the glass during a cold snap.

EPA's general guidance is to keep indoor humidity below 60%, ideally 30 to 50%, and to treat condensation on windows, walls, or pipes as a sign that humidity is too high. In a cold climate, the window chart is the stricter of the two limits, so go by the chart.

Flat rule: if your hygrometer reads above the chart, fix the humidity first. A window company can't sell you a cure for that.

Where the water shows up tells you where it came from

Every window, every room. House-wide humidity. Check the furnace humidifier first, then whatever is adding moisture faster than the house exhausts it.

One or two rooms. Bathrooms, kitchens, and bedrooms with the door closed overnight are the usual suspects. People breathe and showers steam. A closed bedroom with two sleepers can fog its own windows on a night when the rest of the house stays dry.

Only along the bottom edge of the glass. The bottom few inches sit next to the spacer and frame, the coldest part of the glass, and Andersen points to poor airflow at the window as a common cause. Heavy drapes, cellular shades pulled down all night, or a deep sill blocking the heat register under the window all keep warm room air off the glass. Open the coverings during the day and see if it clears.

Frost or ice on the inside. This is the same physics with glass below 32°F. It usually means the humidity is well above the chart for that night, or the window has a cold aluminum frame or single pane.

What's putting the moisture in the air

Work down this list in order. It's sorted roughly from free to expensive.

Bath fans are the most common miss. A fan's cfm rating is measured at a set static pressure, and IRC Section M1505.4.4 says the bathroom rate has to be met at 0.25 inch of water column. A cheap fan pushing through a long, kinked flex duct to a roof cap may move a fraction of what its box says. Broan's own sizing guide uses 1 cfm per square foot of floor for bathrooms up to 100 square feet. So an 80-square-foot bath with a 50 cfm fan is undersized on the manufacturer's math, even though it meets the code minimum. If the mirror is still fogged ten minutes after a shower with the fan running, the fan or its duct is the problem. Check that the duct ends outdoors, not in the attic. Running new duct through an attic or replacing a ceiling fan means working near wiring, so hire an electrician or HVAC contractor for it.

The humidifier is the most common overshoot. A furnace-mounted humidifier keeps adding moisture to hit whatever number you set. Its control has no idea it's -5°F outside unless you turn it down or it has an outdoor temperature sensor. Many have neither. If you set it to 45% in November and left it there, that's your condensation.

Unvented gas heaters are the one source that's also a safety issue. Burning gas produces water vapor along with other combustion gases. EPA advises against installing vent-free space or water heating appliances, and ASHRAE has a position document on the indoor air problems they create. If one is running in a house with dripping windows, don't try to work around it with fans. Shut it off and have a licensed gas contractor look at replacing it with vented, sealed-combustion equipment. Make sure there's a working CO alarm in the house.

When condensation becomes a repair

The damage comes from water running off the glass. Wood sills and the bottom rail of a wood sash absorb it, paint peels, and the drywall below the stool can grow mold. EPA's advice is to dry the wet surface quickly and reduce the moisture source. Wipe the sill every morning while you work on the humidity. A soft, dark, or crumbling sill is no longer a condensation problem. It's rot, and a carpenter should probe how far it goes.

Flat rule: if mold keeps coming back on the same sill after the humidity reads within the chart, have the window opening inspected. At that point, an air leak or a flashing defect around the frame may be keeping that spot colder or wetter than the glass alone explains.

When the window really is the problem

Sometimes the humidity is fine and the glass still runs cold. Single-pane glass, metal frames with no thermal break, and old double-pane units that lost their gas fill all fall in this group. Cardinal says double- or triple-pane glass with low-E coatings and argon fill keeps the room-side glass warmer, and that warmer glass is what resists condensation.

If you're shopping, read the NFRC label. Every ENERGY STAR certified window carries one. The label can include a Condensation Resistance rating, and a higher number means the room-side surface stays drier at a given humidity. Not every label lists it, so ask for it if yours doesn't. Cardinal also cites the Condensation Resistance Factor, a separate rating that runs from 30 to 80, so compare numbers only on the same scale.

Flat rule: don't buy new windows to fix condensation until your hygrometer shows the humidity is within the chart and the glass still sweats. If it does, our window replacement cost guide covers the budget, and the line-by-line quote guide covers what belongs on the bid. Budget without a federal credit: the Section 25C Energy Efficient Home Improvement Credit doesn't apply to windows placed in service after December 31, 2025.

Research sources

• Cardinal Glass Industries: Indoor Condensation
• Cardinal Glass Industries: Condensation Resistance and Ratings
• Andersen: Window Condensation 101
• EPA: A Brief Guide to Mold, Moisture and Your Home
• EPA: Moisture, Mold and Mildew (Appendix C, PDF)
• ASHRAE: Position Document on Unvented Combustion Devices and Indoor Air Quality (PDF)
• ICC: 2018 IRC Section M1505.4.4, Local Exhaust Rates
• Broan-NuTone: Bathroom Exhaust Fan Buying Guide
• Carrier: Whole House Humidifier Costs
• Carrier: How Much Is a Dehumidifier?
• ENERGY STAR: Independently Tested and Certified Energy Performance
• IRS: Energy Efficient Home Improvement Credit