Condensation on windows inside your home

Why do windows get condensation on the inside?

Because the glass is colder than the air touching it. Air holds water vapor, and how much it can hold depends on temperature. When warm, humid room air meets a cold pane, it cools below its dew point and the extra moisture turns to water on the glass. The same thing happens to a cold drink on a warm day.

Two things drive it: how humid the indoor air is, and how cold the glass gets. The EPA lists condensation on windows, walls or pipes as something to act on quickly, and notes that condensation can be a sign of high humidity.¹

Is condensation on windows a problem?

It depends on how much and how often:

What you see	What it usually means
Light fog on cool mornings that clears within an hour or two	Normal, especially in bedrooms
Water running down the glass most mornings	Indoor humidity is high
Puddles on the sill, peeling paint, black spots on the frame	Ongoing moisture, and the start of mold
Stains or bubbling paint on the wall below the window	Water may be getting into the wall

Sills, wood frames, window tracks and the drywall underneath stay wet when condensation runs off every day. Mold will grow where there is moisture, and the CDC lists leaks around windows among the usual places.²

Why do some Los Angeles homes get so much condensation?

Mostly a combination of cold glass and moisture produced indoors. Many older LA homes still have single-pane windows, which get much colder than double-pane glass on a cool night. Nights near the coast are often cool and damp, so bedrooms on the Westside and in the South Bay can fog up even when daytime weather feels dry.

Then add what people do inside: closed windows overnight, several people sleeping in one room, showers without a running fan, cooking without a hood, and drying laundry indoors. Each adds water to the air. Our guide to why LA homes get mold without rain covers the indoor sources in more detail.

How do you reduce condensation on windows?

Lower the humidity, move more air and warm the glass. The EPA’s list of steps:¹

• Vent clothes dryers and other moisture-producing appliances to the outside
• Run the bathroom fan or open a window when showering
• Use exhaust fans or open windows when cooking or running the dishwasher
• Use air conditioning or a dehumidifier when needed
• Increase air movement by opening doors and windows when practical, and use fans
• Raise the air temperature

Small habits help too. Open curtains and blinds in the morning so air reaches the glass. Wipe the glass and sill dry when you see water. Crack a bedroom window at night if weather allows. A hygrometer will show whether the room is above the EPA’s recommended range of below 60%, ideally 30 to 50%.¹

What if there’s already mold on the sill or frame?

Clean it. On hard surfaces like painted wood, vinyl and aluminum, the EPA recommends scrubbing with detergent and water and drying completely.¹ Don’t paint over it; the EPA says paint applied over moldy surfaces is likely to peel.¹

Then look below the window. Drywall that has been wet repeatedly can grow mold on the back side where you can’t see it. Soft, stained or bubbling drywall under a window is worth a closer look. If you aren’t sure whether a spot is mold or a water stain, send us a photo.

When should you call an inspector?

Not for normal morning fog, and not for a small patch on a sill you can clean. Fix the humidity and see if it stops.

Consider a moisture and humidity assessment if condensation keeps coming back after you’ve improved ventilation, if it appears on walls or in closets as well as windows, or if the wall under a window is stained or soft. We take humidity and temperature readings in every room and moisture readings in the materials around windows, which shows whether the water is condensation or a leak. Assessments start at $495 for one dwelling up to 2,000 sq ft. The mold check can help you decide first.