Why Do Your Windows Sweat, and Is It a Problem?

Quick answer: Windows sweat when the glass is colder than the dew point of the air touching it, so moisture condenses on the surface. In Austin, outside fogging on muggy mornings is usually harmless. Inside fogging during winter cold snaps means indoor humidity is high for the weather, and it’s worth fixing if water runs onto the sills.

Why does condensation form on windows?

Condensation forms when moist air touches a surface colder than its dew point, the temperature at which air can’t hold its water vapor any longer. It’s the same thing that happens to a glass of iced tea on the porch: some of the moisture turns back into water. Glass is usually the coldest surface in a room, so it fogs first. The bucket analogy for relative humidity makes dew point easy to picture.

Is condensation on the outside of the window a problem?

Usually not. Outside fogging shows up on humid summer mornings, when dew points are in the 70s and the glass has been cooled by the AC inside. Newer insulated windows can also fog on the outside after clear, still nights, because the outer pane cools off overnight. It clears once the sun hits it, and it doesn’t mean anything is wrong with your house.

Is condensation on the inside of the window a problem?

Inside condensation is a problem when it’s heavy or frequent. A little fog along the bottom corners on the coldest January morning is common, especially on single-pane glass or aluminum frames. Watch for these signs:

• Water runs down and pools on the sill.
• Paint peels, or wood sills and trim darken or soften.
• Black spots show up on the caulk, the frame or the sill.
• Many windows fog on ordinary days, not just during a hard freeze.

Fog trapped between the two panes of a double-pane window is different. That means the seal has failed, so it’s a job for a window company, not a humidity problem.

How do you stop windows sweating on the inside?

Lower the indoor humidity or warm up the glass. These steps do most of the work:

• Keep winter indoor humidity around 30 to 40 percent (a basic hygrometer shows you the number).
• Run bath fans during showers and the range hood while cooking.
• Make sure the clothes dryer vents outdoors.
• Open blinds and curtains during the day so room air can reach the glass.
• Wipe up standing water so it doesn’t soak into the sill.

Over the long run, better windows help, because a double-pane or low-e window keeps the inside pane warmer.

When does window condensation point to a bigger humidity problem?

It points to a bigger problem when the sweating is regular, damages trim or comes with a musty smell. Those signs mean humidity is high most of the time, which can also show up as sweating vents and ducts or damp closets. Our guide on how indoor moisture affects your home’s air covers where that moisture comes from and how to bring it down.

If the fog shows up week after week, Gold Eagle’s indoor air quality team can check your home’s humidity and the HVAC side of the problem.

What should you do next?

• On the next cold morning, note which windows fog on the inside and whether water reaches the sill.
• Put a hygrometer in the main living area and aim for 30 to 40 percent indoor humidity in winter.
• Make fans a habit: bath fans during showers, the range hood while cooking, and a dryer that vents outdoors.
• Wipe up standing water and watch the sills for black spots, peeling paint or soft wood; if the sweating keeps coming back, schedule a visit with Gold Eagle or call 512-489-4653.

Related guides: Why Your Vents and Ducts Sweat: Dew Point and Condensation Explained, What Relative Humidity Really Means: The Bucket Analogy and How Indoor Moisture Affects Your Home’s Air.

About Gold Eagle Services: This guide comes from the team at Gold Eagle Services, a family-run Austin HVAC, indoor air quality and mold remediation company founded in 2017. We’re licensed (TACLA119235 and RCO1783), insured and bonded, and we serve homes across the greater Austin area.

Need help with this in your home? Learn about our humidity control and indoor air quality solutions, or call 512-489-4653 to schedule a visit.