Why Is My Bathroom Fan Causing Condensation on Windows

You flip on the bathroom fan, take a quick shower, and yet your windows are fogged up like you ran a steam room in there. What gives? A bathroom fan is supposed to clear out moisture, not make it worse. But if it’s causing condensation on your windows, chances are it’s not venting properly, pulling in outside air, or simply underpowered. In this guide, we’ll break down why this happens and how to fix it so your bathroom stays clear and mold-free.

The information in this article may not apply to your specific appliance model. We recommend consulting your manufacturer’s documentation or contact us with any questions.

Why Condensation Happens

Condensation forms when warm, humid air meets a cooler surface, like a bathroom mirror, tile wall, or glass window. The cooler the surface, the more likely water vapor will condense into visible droplets. In bathrooms, hot showers or baths dramatically raise the moisture level in the air, and without proper ventilation, that moisture has nowhere to go.

Bathroom windows are especially prone to condensation because glass is a poor insulator and quickly cools down the moist air in the room. Ideally, your bathroom fan should reduce humidity enough to prevent that from happening, but if it isn’t doing its job correctly, or if it’s set up in a way that disrupts airflow, the result can be foggy or dripping windows.

Common Reasons Your Fan Might Be Causing (or Failing to Prevent) Window Condensation

The Fan Is Too Weak for the Room Size

Every bathroom fan has a CFM (cubic feet per minute) rating, which measures how much air it can move. If your fan’s CFM is too low for the size of your bathroom, it simply won’t be able to remove moisture fast enough.

What you can do:

• Measure your bathroom (length × width × height) to get the volume.
• Divide the volume by 60 to find the minimum CFM rating needed.
• For bathrooms up to 100 sq. ft., use 1 CFM per square foot as a general rule.
• If your fan is undersized, consider upgrading to a more powerful model.

The Fan Is Not Running Long Enough

Even a properly sized fan won’t prevent condensation if it doesn’t run long enough after a shower. Moist air lingers even after the water is off and can settle on cool surfaces like windows unless it’s actively vented out.

What you can do:

• Run your fan for at least 15–30 minutes after showering.
• Install a timer switch to automate post-shower fan operation.
• Leave the bathroom door open slightly after use to improve airflow.

The Ductwork Is Poorly Installed or Insulated

If your bathroom fan duct travels through an unconditioned attic or crawl space and isn’t insulated, warm, moist air inside the duct can cool rapidly, causing condensation inside the duct itself. This can backtrack and make the fan less effective at removing moisture from the room.

What you can do:

• Check that your fan duct is properly insulated with R-6 or higher-rated duct wrap.
• Make sure the duct is sloped slightly downward toward the exterior vent (about 1/4 inch per foot).
• Replace sagging or crimped flexible ducting with smooth, rigid ducting where possible.

The Fan Is Venting Into the Attic or Crawlspace

If your bathroom fan isn’t venting all the way to the outside and instead dumps moist air into the attic or a soffit, it can create a humid backflow that reenters the bathroom, or causes moisture problems elsewhere in the house. This often results in ineffective moisture control and increased condensation.

What you can do:

• Go into the attic and follow the duct, make sure it exits to the outside through a roof cap or wall vent.
• If it’s just dumping into open attic space, you’ll need to install a proper termination cap.
• Seal all joints with foil tape to ensure no leaks in the duct path.

Cold Drafts or Negative Pressure in the Bathroom

A powerful exhaust fan can sometimes create negative pressure inside the bathroom if there’s not enough fresh air entering to balance the air leaving. This can cause cold air from elsewhere in the house to get pulled into the room, making windows even colder and increasing condensation risk.

What you can do:

• Keep the bathroom door cracked open slightly during and after showers to allow makeup air to flow in.
• Consider installing a door vent or louver to increase airflow.
• Don’t run multiple exhaust fans (like kitchen and bathroom) at the same time in tightly sealed homes without a fresh air system.

Excess Moisture from Other Sources

Sometimes the issue isn’t the fan at all, it’s that your bathroom is simply generating more moisture than your fan can reasonably handle. This often happens in homes with single-pane windows, poor insulation, or in cold climates where glass surfaces cool rapidly.

What you can do:

• Use a squeegee to wipe down shower walls after use.
• Keep the shower curtain or door open afterward to let moisture escape.
• Add a small portable dehumidifier if condensation continues despite good fan use.
• Upgrade to double-pane or storm windows for better insulation.

How to Test Your Bathroom Fan’s Performance

If you’re unsure whether your fan is doing its job, try this simple test.

The tissue test:

1. Turn on the fan and hold a piece of toilet paper or tissue up to the grille.
2. If the tissue is held tightly in place, the fan is pulling air effectively.
3. If the tissue falls or barely sticks, airflow is weak, possibly due to blockages, poor ducting, or an underpowered fan.

Also, step into the bathroom 15 minutes after a hot shower. If the mirror is clear and the air feels dry, your fan is working well. If not, it’s time to address one or more of the issues outlined above.

Upgrades for Better Moisture Control

If your current fan system is underperforming, consider some of these upgrade options.

• Install a humidity-sensing fan: These fans automatically turn on when moisture levels rise and shut off when the room dries out. Great for families where people forget to use the fan.
• Add a second fan: Large bathrooms or those with separate shower and toilet areas might benefit from dual fan setups.
• Use rigid ducting: Replacing flexible ducts with rigid ones can improve airflow and reduce condensation inside the duct itself.
• Upgrade your fan’s CFM rating: Fans rated between 80 and 110 CFM are ideal for most medium-sized bathrooms.
• Seal and insulate the fan housing: If your fan is older or poorly installed, seal any ceiling gaps and insulate above the housing in the attic to reduce heat transfer and improve performance.

Where To Find Us 

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