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Dew Point & Condensation Risk Calculator

Will a cold duct, pipe, or slab sweat in the crawl space? Dew point is the temperature where moisture in the air condenses. Enter air temperature and relative humidity to find the dew point, and optionally add a surface temperature to check condensation risk.

Educational estimate. Not engineering advice, a cut list, a bid, or a substitute for local code, manufacturer instructions, or a licensed professional.

Air conditions

Leave blank for dew point only. Add a surface reading to estimate condensation risk on that surface.

How to use it

  1. Enter air temperature in °F and relative humidity in percent.
  2. Optionally enter a surface temperature from a duct, water pipe, or slab.
  3. Read the dew point and, if you added a surface reading, how far that surface sits above or below it.

The formula

Temperatures are converted to Celsius, then the Magnus / August-Roche-Magnus approximation is applied:

γ = ln(RH/100) + (a × T) / (b + T)
Td = (b × γ) / (a − γ)

with a = 17.625 and b = 243.04 (Alduchov and Eskridge, 1996). The dew point is converted back to Fahrenheit. National Weather Service educational calculators use the same family of Magnus vapor-pressure approximations.

Worked example

At 70°F and 50% relative humidity, the dew point is about 50.5°F. A bare duct at 49°F sits below that dew point, so condensation is likely. A surface at 55°F sits several degrees above it, so condensation is unlikely under those readings. A surface within about 2°F of the dew point is treated as borderline — a labeled planning margin for sensor error, not a code threshold.

Limits and safety

  • Uses liquid-water Magnus constants; very cold frost-point cases need different coefficients.
  • Does not measure wood moisture content or diagnose mold.
  • Sensor error and drafts can move real conditions.

Also useful: Crawl space humidity and moisture control, the dehumidifier guide, and musty crawl space smell.

Sources

The Magnus coefficients a = 17.625 and b = 243.04°C follow Alduchov and Eskridge (1996), a widely used August-Roche-Magnus revision for saturation vapor pressure over liquid water. The National Weather Service publishes educational dew-point worksheets in the same Magnus family (for example the El Paso Weather Forecast Office wet-bulb and dew-point from relative humidity calculator notes). Condensation occurs when a surface is at or below the dew point of the surrounding air; the 2°F borderline band is a labeled planning assumption.