Dew Point & Condensation Risk

Dew point and surface condensation risk for pipes, ducts, and building surfaces.

Air Conditions
Psychrometric Results
Dew Point
—
°F
Wet Bulb
—
°F
Humidity Ratio
—
gr/lb
Enthalpy
—
BTU/lb
Condensation Risk — Surface Temperature Check
—
—

Common Surface Temperatures
Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This dew point calculator takes a dry-bulb temperature, relative humidity, and site elevation and returns the dew point, wet bulb, humidity ratio, and enthalpy of the air, then checks the result against common HVAC surface temperatures to flag where condensation is likely. It is built for quickly screening pipes, ducts, glazing, and building surfaces for sweating and moisture problems.

Dew point is the single most useful number for condensation control: any surface colder than the dew point will collect water. The tool shows how many degrees each common surface sits above or below the dew point, grades the risk on those surfaces, and tags chilled-water pipe and cold supply duct so you can confirm insulation or dehumidification before moisture causes damage or mold.

Formula & Method
Vapor pressurePw = RH · Pws(T), Pws from the Hyland–Wexler equations
Dew pointPws(Tdp) = Pw, solved numerically (frost point below 32 °F)
Humidity ratioW = 0.621945 · Pw ÷ (P − Pw), P = 14.696 · (1 − 6.8754×10−6 · Z)5.2559 psia
Enthalpyh = 0.240 · T + W · (1061 + 0.444 · T)
Wet bulbThermodynamic wet bulb from the ASHRAE adiabatic-saturation energy balance, solved numerically

All properties use the ASHRAE Handbook—Fundamentals (2021) Chapter 1 moist-air equations in IP units — the same set as the AIM Works psychrometric calculator, so the same air state reads the same on every page. Dew point depends only on the vapor pressure, so elevation does not change it; humidity ratio, wet bulb, and enthalpy are computed at the barometric pressure for your elevation (standard atmosphere). Condensation is flagged whenever a surface temperature falls at or below the calculated dew point; the surface temperatures listed are typical values, not a heat-transfer calculation for your assembly.

Frequently Asked Questions
What is dew point and why does it matter in HVAC?
Dew point is the temperature at which air becomes saturated and water vapor begins to condense. Any surface at or below the dew point will collect moisture. In HVAC it sets where you will get condensation on chilled-water pipes, supply ducts, cold glazing, and uninsulated equipment, so it drives insulation thickness and dehumidification targets.
How is dew point calculated from temperature and relative humidity?
Vapor pressure is RH times the saturation pressure at the dry bulb, using the Hyland–Wexler saturation-pressure equations from ASHRAE Handbook—Fundamentals (2021) Chapter 1. The dew point is the temperature whose saturation pressure equals that vapor pressure, found by solving the same equation exactly rather than with a Magnus-type shortcut. Below 32 °F the result is the frost point (saturation over ice). Dew point does not depend on elevation; humidity ratio, wet bulb, and enthalpy do, so enter your site elevation.
When is there a condensation risk on a surface?
Condensation occurs whenever a surface temperature is at or below the air dew point. The calculator compares the dew point with six common surfaces: 44 and 50 °F chilled-water pipe, 55 °F cold glass and supply duct, a 60 °F slab, and a 65 °F exterior wall. It shows red when the 55 °F surfaces are at or below the dew point, amber when only the chilled-water piping is, and green when every listed surface stays above it, along with how many degrees each surface sits above or below the dew point.
How do I prevent condensation on chilled water pipes?
Keep the pipe surface above the room dew point with continuous closed-cell or fiberglass insulation sized for the design dew point, and seal the vapor barrier so humid air cannot reach the cold surface. Where dew points are high, lower the space humidity with dehumidification rather than relying on insulation alone.
Related Tools
Psychrometric Calculator Wet Bulb / RH Humidity Ratio Enthalpy Calculator Coil Leaving Air
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