FreeHVACCalculators

Start here

Loading calculators…

Menu

Relative Humidity After Heating or Cooling Calculator

Relative humidity changes whenever air changes temperature, even though the amount of water in it stays the same. See what happens to outdoor air brought indoors, or to room air that touches something cold.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

New relative humidity
17.8%
Dew point (unchanged)
25.2°F

At 70 °F the same air has 17.8 % relative humidity.

  • Below 30 % RH is under EPA’s ideal indoor range of 30–50 %.
Humidity ratio (unchanged)
19.2gr/lb
Water condensed
0gr/lb
Show the working
  1. Moisture content stays the same: humidity ratio W
    19.25 gr/lb (dew point 25.2 °F)
  2. New RH = vapour pressure ÷ saturation pressure at the new temperature
    446 Pa ÷ 2,505 Pa = 17.8 %

Why heated air feels so dry

Warm air can hold much more water vapour than cold air. Outdoor air at 30 °F and 80 percent relative humidity, warmed to 70 °F, contains the same water but has less than 20 percent relative humidity. That is why winter indoor air is dry unless moisture is added.

The reverse also matters: cool humid air enough and it reaches 100 percent relative humidity at its dew point. Any further cooling condenses water — on a cold window, a duct or a coil.

Method

The humidity ratio and vapour pressure stay constant. The new relative humidity is the vapour pressure divided by the saturation pressure at the new temperature, using the Hyland–Wexler equations from the ASHRAE Handbook—Fundamentals (over ice below 32 °F). If the new temperature is below the dew point, the air is saturated and the excess moisture is reported in grains per pound of dry air.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Starting temperature
30 °F
Starting relative humidity
80 %
New temperature
70 °F
Site elevation
0 ft

Result

New relative humidity
17.8 %
Dew point (unchanged)
25.2 °F
Humidity ratio (unchanged)
19.2 gr/lb
Water condensed
0 gr/lb

At 70 °F the same air has 17.8 % relative humidity.

Assumptions

  • No moisture is added or removed while the temperature changes (sensible heating or cooling); pressure stays constant.

Limitations

  • Below 32 °F, saturation is taken over ice, so the dew point shown is the frost point.

Frequently asked questions

Does the dew point change when air is heated?

No. The dew point depends only on how much water vapour the air holds, so it stays the same as long as no moisture is added or removed.

What humidity should I aim for indoors?

EPA recommends below 60 percent, ideally 30 to 50 percent. In cold weather lower values help prevent condensation on windows.

Why does the tool mention frost?

Below 32 °F, water vapour deposits as frost rather than liquid, and saturation is taken over ice; the dew point shown is then the frost point.

Sources

  1. PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
  2. Water vapor pressure formulations — UCAR / NCAR Earth Observing Laboratory. Hyland–Wexler saturation vapour pressure coefficients over water and ice.
  3. A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency. If possible, keep indoor humidity below 60 % (ideally between 30 and 50 %) relative humidity; condensation can be a sign of high humidity; insulate cold surfaces such as cold-water pipes.

Spotted an error? Report a problem with this calculator — see our corrections policy.