Why Is There Condensation on My Windows? Causes and How to Stop It

Condensation forms on your windows because warm, moist air inside your home touches the cold glass and turns back into water. It is the same thing that happens when a cold drink "sweats" on a summer day. A little on a very cold morning is normal. But if your windows are wet every day, your home has too much moisture in the air.

The fix is to lower your indoor humidity and help the glass stay a bit warmer. Most of the steps below are free or cheap, and you can start today.

This guide explains why windows sweat, where the extra moisture comes from, and how to stop it before it turns into mold.

Where is the condensation?

Where you see the water tells you what is going on.

Most of this guide covers condensation on the inside of the glass, since that is the one you can fix.

Why windows sweat in cold weather

Air always holds some water as an invisible gas called water vapor. Warm air can hold more water than cold air. When warm air touches something cold, it cools down fast and has to let go of some of that water. The water collects as tiny drops on the cold surface.

In winter, your windows are usually the coldest surface in the room. The colder it is outside, the colder the glass gets, and the more water you see. This is why the problem often gets worse on the coldest nights of the year.

7 common sources of extra moisture

Your family adds a lot of water to the air every day without knowing it. Here are the biggest sources.

1. Showers and baths

A hot shower fills the bathroom with steam. Without a fan, that moisture spreads through the house.

2. Cooking

Boiling water, making soup, and running the dishwasher all add steam to the air, especially with the lids off.

3. Drying clothes indoors

Wet laundry hung on racks gives off a lot of water as it dries. A dryer that is not vented outside does the same thing. If your dryer is also slow, our guide on fixing a dryer that takes forever covers vent problems.

4. Breathing and sleeping

People and pets breathe out moisture all night. Bedrooms with closed doors often have the wettest windows in the morning.

5. Houseplants and fish tanks

Plants give off water through their leaves, and open fish tanks let water into the air. A few are fine, but a lot of them in one room can make windows fog up.

6. Humidifiers set too high

A humidifier helps dry winter air, but running it too high is a very common cause of wet windows.

7. A tight, well-sealed home

Newer and updated homes keep air in better. That saves energy, but it also traps moisture unless you use fans and fresh air.

How to stop condensation on the inside of windows

Check your humidity first

Buy a small humidity meter, also called a hygrometer. The EPA notes they cost about $10 to $50 at most hardware stores. Place it in the room with the worst windows.

The EPA suggests keeping indoor humidity between 30 and 50 percent. In very cold weather, you may need to stay close to 30 to 40 percent to keep the glass dry.

Run exhaust fans

• Turn on the bathroom fan during showers and for about 20 minutes after.
• Use the kitchen range hood when cooking, and make sure it vents outside if possible.

Use a dehumidifier

If the humidity stays above 50 percent, a dehumidifier pulls water out of the air. Put it in the room with the most condensation, or in a damp basement. Empty the bucket often, or connect it to a drain.

Turn down the humidifier

If you run a humidifier, lower it or turn it off until the windows stay dry.

Let air reach the glass

Heavy curtains and closed blinds trap cold air against the window. Open them during the day. A ceiling fan on low can also keep air moving across the glass.

Dry clothes the right way

Use a dryer that vents outside, and check that the vent hose is connected and not crushed.

Crack a window for a few minutes

Opening a window for 5 to 10 minutes a day lets moist air out and drier outside air in. It sounds strange in winter, but it works.

Move plants away from windows

Group plants in a room with good air flow, and do not overwater them.

Add window film or storm windows

Plastic window insulation kits and storm windows add a layer of air that keeps the inside glass warmer. Warmer glass means less condensation. This is a good choice for older single-pane windows.

Wipe it up to stop mold

Water that sits on windows and sills for days can lead to mold. The EPA says that if wet areas are dried within 24 to 48 hours, mold usually will not grow.

• Wipe windows and sills with a dry towel each morning.
• If you see black spots, clean them with soap and water, then dry the area well.
• Check the corners of the window frames and the wall below the window.

If a room smells damp even when the windows are dry, there may be hidden moisture. Our guide on how to get rid of a musty room smell can help you find it.

What about condensation between the panes?

Double-pane and triple-pane windows have a sealed space between the glass layers. When the seal fails, moist air gets in and fogs up the inside of the window. You cannot wipe it off because it is sealed inside.

Your options are:

• Replace just the glass unit. A window company can often swap only the glass, which costs less than a new window.
• Replace the whole window if the frame is also damaged or very old.
• Check the warranty. Many windows have a warranty on the seals.

A failed seal makes the window a little less energy efficient, but it is usually not an emergency.

Why it gets worse when the heat comes on

When you turn on your heating in the fall, the inside of your home warms up while the outside gets colder. That big difference makes windows sweat more. Also, keeping the house closed up traps more moisture inside.

If your heater is struggling to keep your home warm, it can also mean colder rooms and more condensation. See our guide on a furnace that blows cool air to check your system.

Common mistakes to avoid

• Ignoring it. Daily condensation can lead to rotten wood and mold.
• Running a humidifier all day without checking the humidity level.
• Closing all the vents in unused rooms. Cold rooms get more condensation.
• Blocking air vents with furniture or curtains.
• Turning off bathroom fans too soon after a shower.

When to call a pro

Call a window or HVAC professional if:

• Your humidity stays above 50 percent even with fans and a dehumidifier
• You see mold that keeps coming back or covers a large area
• Your window frames feel soft or look rotten
• You see fog between the panes of several windows

The bottom line

Condensation on the inside of your windows means your indoor air has too much moisture. Keep your humidity between 30 and 50 percent, run bathroom and kitchen fans, use a dehumidifier if needed, and let air reach the glass. Wipe up any water each morning to stop mold before it starts. With a few small changes, your windows can stay clear all winter.