Why Is My Window Sweating or Fogging Up?

A window that “sweats” on a cold January morning can look like a window that’s failing. Usually, it isn’t. In most cases the window is doing exactly what glass does when warm, humid air touches a cold surface, the same thing that happens to a cold glass of water on a summer day.

The key is figuring out where the moisture is. Inside the glass, outside the glass, and between the panes are three different problems with three different fixes—and only one of them usually involves the window itself.

First, Find Where the Moisture Is

The fastest way to diagnose a foggy window is to wipe it. Run your finger or a cloth across the glass from inside the room, then from outside if you can reach it safely.

Most of this guide covers the first row, fog on the inside of the glass. The other two are covered at the end.

Why the Inside of the Glass Fogs Up

Condensation forms when a surface is colder than the dew point of the air around it. The dew point is the temperature at which moisture in the air turns into liquid water or frost. The more humid your indoor air is, the higher its dew point, and the warmer a surface has to stay to remain dry.

In winter, the inside of your window glass is often one of the coldest surfaces in the room. When outdoor temperatures drop, the glass cools, and humid indoor air that touches it can reach its dew point and leave water or frost behind.

The Fenestration and Glazing Industry Alliance (FGIA), the trade association that sets window performance standards, names three conditions that drive indoor condensation:

1. Window surfaces that are colder than the dew point of the room air
2. High indoor humidity
3. Cold outdoor temperatures

Everyday activities add moisture to indoor air. FGIA points to showering, cooking, and even breathing as sources, along with houseplants, aquariums, and fresh paint or masonry from a renovation. It also notes that kitchens, bathrooms, and basements tend to have higher condensation potential, and that today’s tighter, more efficient homes can trap more moisture inside than older, draftier ones did.

How Much Indoor Humidity Is Too Much in Winter?

For general indoor air quality, the U.S. Environmental Protection Agency (EPA) recommends keeping indoor humidity below 60%, and ideally between 30% and 50%. The EPA also notes that condensation on windows can be a sign of high humidity.

In winter, though, the right number depends on how cold it is outside. The colder the glass gets, the less humidity it takes to fog it. FGIA publishes these maximum indoor humidity levels for avoiding condensation, assuming your home is heated to 70°F:

What that means in Cleveland and Akron

According to the National Weather Service, Cleveland’s normal January high is about 36°F and its normal low is about 22°F. On a typical January day, that puts the target at roughly 35% to 40% indoor humidity. On colder nights and during cold snaps, it drops further. A humidifier set to 45% or 50% for comfort can be enough to fog the glass when the temperature falls.

An inexpensive hygrometer, a small humidity meter sold at most hardware stores, will tell you exactly where your home stands.

How to Reduce Condensation on the Inside of Your Windows

Because interior condensation is usually a humidity problem, the fixes are mostly about moving moisture out of the house or keeping it from building up. These steps come from FGIA’s homeowner guidance:

• Turn down the humidifier. If you run one, lower its setting as outdoor temperatures drop, using the table above.
• Run exhaust fans. Use bathroom fans during and after showers and the range hood while cooking.
• Run ceiling fans. Moving air across the glass works the same way a car’s defroster does.
• Open curtains and blinds during the day. FGIA notes that keeping them closed makes condensation more likely.
• Watch for added moisture sources. Houseplants, aquariums, and recent projects like fresh paint or new masonry all add moisture.
• Use a dehumidifier if needed. This is especially useful in basements and other damp areas.

If you see water on the glass, wipe it up. The EPA recommends drying condensation quickly and reducing the moisture source, since standing moisture can lead to mold on sills and trim. If humidity stays high no matter what you do, FGIA suggests talking to an HVAC professional about options like whole-house ventilation.

When Inside Condensation Points to the Window

Humidity is the usual cause, but the window still plays a role. A window with colder interior glass will fog at lower humidity than a better-insulated one.

Research from the National Research Council of Canada shows how big that difference can be. In a 1960 building digest, NRC calculated that on a windy 20°F day, a window with a single layer of glass could fog at about 24% indoor humidity, while a window with two layers could handle about 53% before fogging. The principle hasn’t changed: more insulating glass keeps the room-side surface warmer, so it can tolerate more humidity.

That means it’s worth looking at the window itself if:

• You have single-pane glass or older aluminum frames. These run cold in winter and will fog at humidity levels that wouldn’t affect a modern insulated window.
• Condensation collects mainly around the edges of the glass. The spacer that separates the panes affects how warm the edge of the glass stays, which is why many modern windows use “warm-edge” spacers.
• You’ve lowered your humidity and one or two windows still fog while the rest of the house stays clear. That points to something specific about those windows, such as worn weatherstripping or a cold draft around the frame. Our guide to cold air coming through closed windows covers those causes.

If you’re comparing replacement windows, FGIA notes that three rating systems measure condensation resistance: AAMA’s Condensation Resistance Factor (CRF), NFRC’s Condensation Rating (CR), and the CSA Temperature Index. Higher numbers are better in all three, but only compare products rated on the same system. FGIA also cautions that these ratings are for comparing products, not for predicting exactly what will happen in your home.

Empire’s ClimaTech^® glass packages use warm-edge spacer systems designed to keep the edge of the glass warmer and reduce condensation. See our energy-efficient glass page for the options.

Fog on the Outside of the Glass

If the moisture wipes off from the outside, it’s dew, the same thing that forms on your car or the grass on a cool morning. It isn’t a problem with the window, and it doesn’t affect your home’s interior.

It’s also more common on newer windows, and that’s a good sign. Researchers at Tampere University of Technology, publishing in the Journal of Building Engineering in 2018, explained why. Better-insulated windows keep more heat inside, so the inner glass stays warmer and the outer glass stays colder. That reduces condensation on the inside but makes dew more likely on the outside. Their study found that windows with better insulating values had more hours of exterior condensation per year. A few patterns to expect:

• It tends to show up on cool, clear, still mornings—most often early mornings in the fall.
• It usually clears on its own as the sun and air warm the glass.
• It may appear on some windows and not others, depending on shade and exposure.

If your windows used to stay dry outside and now fog after a replacement, that’s usually a sign the new glass is keeping more heat in your home!

Fog Between the Panes

If you can’t wipe the moisture away from either side, it’s trapped inside the insulated glass unit. That means the seal around the edge of the glass has failed and moist air has gotten in.

This is the one type of window fog that does point to the window itself, and no amount of humidity control will clear it. This Old House general contractor Tom Silva notes that a broken seal is the cause, and that replacing the sash is the only way to get rid of the fog.

Whether that means replacing one sash, the glass unit, or the whole window depends on the window’s age and condition. Our Guide to Condensation Between Window Panes walks through that decision.