Window Condensation: What It Means and How to Stop It

Where the condensation forms tells you what is wrong. On the inside of the glass, it means indoor humidity is high or the glass is cold. On the outside, it usually means your windows insulate well. Between the panes, it means the glass seal has failed and the glass unit or window needs replacing. Inside condensation is fixed mostly by lowering humidity and improving ventilation. Outside condensation needs no fix. Fogging between the panes can't be wiped or dried out.

Condensation by location

Why condensation forms

Air holds water vapor. When air touches a surface colder than its dew point, some of that vapor turns to liquid. Pella describes it simply: condensation occurs when warm, humid air comes into contact with cool glass. Glass is usually the coldest surface in a room, so it shows up there first.

Daily life adds a lot of moisture to indoor air: showers, cooking, washing dishes, laundry, and even breathing. Anlin's care guide points out that new windows sometimes seem to cause condensation when they don't. Old, drafty windows let moist air leak out through gaps. Tight new windows keep it in, so humidity that used to escape now shows up on the glass.

Condensation on the inside of windows

Some room-side condensation on a cold morning is normal, especially in bedrooms and bathrooms. It becomes a problem when it runs down the glass every day, pools on sills, or leaves black spots on frames and caulk.

How to reduce it

• Run exhaust fans. Pella recommends running kitchen and bathroom fans for about 20 minutes after cooking or showering. Make sure the fans actually vent outdoors.
• Use a dehumidifier in rooms that stay damp, such as basements, bedrooms on the foggy side of the house, or rooms that dry laundry indoors.
• Keep air moving. Leave interior doors open, and don't block windows with heavy curtains pressed against the glass overnight.
• Open windows for a short time when the weather allows, especially after showers and cooking.
• Check moisture sources. A damp crawlspace, a dryer venting indoors, unvented gas appliances or many houseplants can all raise humidity.
• Upgrade single-pane glass. Insulated glass keeps the inner pane closer to room temperature, so it stays above the dew point far more often. Our guide to single, double and triple-pane windows explains the difference.

Condensation on aluminum frames

Older aluminum windows without a thermal break conduct cold straight through the frame, so the frame itself sweats and water collects in the tracks. Better humidity control helps, but the lasting fix is a vinyl, fiberglass, wood or thermally broken aluminum frame. Our frame materials guide compares them.

Condensation on the outside of windows

Exterior condensation surprises a lot of people after they install new windows. It is a sign the glass is doing its job. Anlin explains that well-insulated windows keep indoor heat from warming the outer pane, so the outer glass can drop below the dew point of the outside air, much like dew on grass or a car windshield. Pella lists the conditions that bring it on: high outdoor humidity, little or no wind and a clear night sky.

In the Bay Area that describes many mornings, especially in coastal towns like Pacifica, Daly City and Half Moon Bay, and in valleys where cool nights follow warm days. The moisture burns off once the sun hits the glass. It is not a defect.

Condensation between window panes

Double-pane windows are sealed at the factory with dry air or gas between the panes. When that edge seal fails, humid air gets in and condenses on the inside surfaces of the glass. You see fogging, streaks or a white film that you can't reach to wipe off. Pella states that a seal failure is the primary reason windows fog between the panes and that it can't be effectively repaired.

Your options:

• Replace the glass unit if the frame and sash are sound. This keeps the window and restores clear glass and insulation.
• Replace the window if several units are fogging, the frames are aging, or you want better glass anyway.
• Check the warranty first. Many manufacturers cover glass seal failure for a set period, and some include labor. Anlin's warranty, for example, includes the insulated glass unit, parts and labor.

You may see "defogging" services that drill and dry the glass. Manufacturers such as Pella point to replacement instead, since the original factory seal can't be effectively repaired. If you are deciding between repair and replacement, our guide on when to replace windows walks through it.

When condensation is causing damage

Repeated condensation runs down onto sills and into the corners of the frame. Over time it causes:

• Peeling paint and rot on wood sills, sashes and interior trim
• Mold on caulk, drywall returns and window coverings
• Stained or swollen drywall below the window

Be careful to separate condensation from leaks. If water appears after rain while the glass is dry, or stains spread from the corners of the window down the wall, water is getting in from outside. That points to failed flashing or sealant, and it can rot the sheathing and framing behind the siding without visible signs. Our guides to moisture behind siding and preventing dry rot explain what to look for.

Comparing windows for condensation resistance

NFRC labels can include an optional condensation resistance rating. NFRC states that the higher the number, the better a product resists condensation. Not every manufacturer lists it, and it doesn't predict whether your home will have condensation, since that depends mostly on indoor humidity. It is still useful for comparing two windows side by side. A lower U-factor, warm-edge spacers and non-metal frames all help keep room-side surfaces warmer.

When condensation means it's time to replace

• Fogging between panes on several windows
• Single-pane glass that runs with water every cold morning even after humidity is under control
• Rot in sills or sashes from years of condensation
• Old aluminum frames that sweat and drip into the wall

If those describe your house, our window replacement team can look at the windows and the wall around them and give you a free written estimate.

Frequently asked questions

Why is there condensation on the inside of my windows?

Warm, humid indoor air is touching glass colder than its dew point. Cooking, showers, laundry and breathing add moisture, and tight new windows hold it in. Lower humidity with exhaust fans, ventilation or a dehumidifier.

Is condensation on the outside of windows bad?

No. It usually means the windows insulate well, so the outer pane stays cool. It forms on humid, still, clear nights and evaporates as the day warms.

Can condensation between window panes be fixed?

Not effectively. Fogging between panes means the factory seal has failed. The usual fix is replacing the insulated glass unit or the whole window, and it may be covered by the manufacturer's warranty.

Do new windows stop condensation?

They greatly reduce room-side condensation when you are replacing single-pane glass or old aluminum frames. They don't fix high indoor humidity, and outside condensation can actually appear more often with efficient glass.

Is window condensation a sign of mold?

Not by itself, but daily condensation that wets sills and frames can lead to mold and rot. Dry the water, lower humidity, and check for black spots on caulk, drywall and trim around the window.

Sources

• Pella: How to deal with window condensation
• Anlin: Care and maintenance, interior and exterior condensation
• National Fenestration Rating Council: Understanding the window label
• Anlin: Limited Double Lifetime Warranty coverage summary