Window condensation calculator
Windows sweat when the inside of the glass drops below your air's dew point. Enter your indoor and outdoor temperature and your window type to find the highest indoor humidity you can run before the glass fogs up.
Evidence-checked. Facts and figures on this page are verified against published standards and government sources (Magnus dew-point formula; ASHRAE interior surface film coefficient; NFRC/ENERGY STAR window U-factors). Last verified July 2026 — our methodology.
Keep indoor humidity below
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- Inside glass temperature
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- Dew point at that RH
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- Verdict
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The inside face of the glass is estimated from the indoor–outdoor temperature gap and the window's insulating value (interior air-film coefficient ≈ 1.46 BTU/h·ft²·°F, U-factors from NFRC/ENERGY STAR typicals). Condensation begins when your indoor dew point rises above that glass temperature — so the number above is the RH that keeps the two equal. Real windows vary with wind, frames, and coverings; treat it as a close guide.
Why windows sweat — and how to stop it
Condensation on the inside of a window is not a window fault; it means the indoor air holds more moisture than the cold glass can tolerate. The inside surface of a single-pane window on a freezing night can sit 30°F below room temperature, so even modest humidity condenses on it. Better glazing keeps the inside face warmer, which lets you run higher humidity without fogging. This tool uses the samedew-point math as our other tools, then compares your dew point with the estimated glass temperature. If your rooms read above this ceiling, lower the humidity (see winter indoor humidity targets), warm the glass, or improve airflow — the full playbook is incondensation on windows.