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Glass Condensation Calculator

Will the window sweat? Give it the room temperature and humidity, the outdoor temperature, and the window's U-value, and it works out the indoor dew point, the temperature of the room-side glass surface, and whether condensation will form, plus the highest indoor humidity you can hold before it does.

Conditions

Typical winter design conditions are a good starting point. Adjust to your situation.

°C
%
°C
W/m²K
W/m²K
The interior film coefficient (hi) sets how well room air warms the glass surface. NFRC/ISO winter still-air is about 8.3 W/m²K. Lower it (to ~6) for a sheltered spot behind a blind or heavy curtain, where surfaces run colder and condensation is more likely.

Result

Room-side, centre-of-glass, at steady state.

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Indoor dew point
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Glass surface temp
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Margin to dew point
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Max safe indoor RH
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How it is worked out, and its limits

Two standard, published formulas, no fabricated tables:

  • Indoor dew point from temperature and humidity uses the Magnus formula (a = 17.62, b = 243.12 °C). Air that touches a surface below this temperature gives up water as condensation.
  • Interior glass surface temperature from a steady-state balance: T_surface = T_in − (U ÷ h_i) × (T_in − T_out), where hi is the interior film coefficient. A lower U-value keeps the glass warmer and drier.
  • Condensation forms when the surface temperature drops to or below the dew point. The max safe indoor RH is the humidity at which the dew point just reaches the surface temperature.

Honest limits. This is centre-of-glass at steady state. Real windows condense first at the edge of glass and the frame, which run several degrees colder, so treat a small positive margin as "still at risk near the edges." It assumes still indoor air; a warm-air supply across the glass, or a blind trapping cold air against it, changes hi. Use it to compare specifications and set humidity targets, not as a guarantee. For sealed-unit (in-cavity) dew point to ASTM E2190, use the QC Calculators.