Condensation on the Outside of Your Windows

• Why it happens
• When you’ll see it
• What to do
• Which side is it on?

Most of this site is about water on the room side of the glass. If the water is on the outside, the beads you can’t wipe from indoors, you’re usually looking at plain morning dew, and your windows are probably doing their job.

§Why it happens

• On clear, still nights, glass radiates heat to the sky. Pilkington’s technical bulletin says glass can end up about 2–3 °C cooler than the still outdoor air around it.
• Night air cools toward its dew point, so humidity near the glass climbs. Once the outer pane drops below the dew point, water condenses on it, just like dew on grass or a car windshield.
• Efficient glazing makes this more likely. Low-e coatings, gas fill, and good seals keep indoor heat from warming the outer pane, so it stays cold. Vitro and BBC Weather both describe exterior dew as evidence that the insulated unit is performing well.

§When you’ll see it

Andersen notes it’s most common in spring and fall, or when a cool night follows a warm day. It’s also more likely near lakes and rivers, on windows facing open sky, and on calm nights. Wind and dry air usually prevent it.

§What to do

• Usually nothing. It evaporates as sun, wind, or rising air temperature warms the glass, and it doesn’t affect the inside of the home.
• Don’t try to “fix” it with indoor humidity. Running a dehumidifier won’t change outdoor dew.
• If the view matters, some manufacturers offer hydrophilic exterior coatings (Pilkington mentions its self-cleaning glass) that make dew sheet off instead of beading. That’s an option to ask about at replacement time, not a retrofit.

§Which side is it on?

Wipe the inside face. If the water stays, it’s outside (dew) or between the panes. Fog between the panes that never fully clears means a failed seal, which is a different problem. Water on the inside face is the indoor-humidity problem the rest of this site covers.

Sources: Pilkington ATS-161, Condensation on Exterior Surface of High Performance Glazing; Vitro Glass Education Center; Andersen help center; BBC Weather.

Originally published at https://dry-glass-notes.surge.sh/articles/condensation-outside-of-windows.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.