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?

It’s the same thing that happens to a glass of iced tea on the porch. Air holds moisture, and when it touches a surface colder than its dew point, some of that 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 isn’t a sign of anything wrong with your house.

Is condensation on the inside a problem?

It depends on how much and how often. A little fog along the bottom corners on the coldest January morning is common, especially on single-pane glass or aluminum frames. It becomes a problem when:

• 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.
• It happens on many windows, 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. It’s a window issue, not a humidity issue.

How do you stop windows sweating on the inside?

Lower indoor humidity or warm up the glass. Practical steps:

• Keep winter indoor humidity around 30 to 40 percent.
• 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 the inside pane stays warmer.

When should you look further?

Look further if the sweating is regular, damages trim or comes with a musty smell. Those point to humidity that’s high all 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.

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.