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Indoor Humidity Level Checker

Enter your indoor temperature and a humidity reading. See how it compares with EPA’s guidance and whether your coldest surface will sweat.

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

Your inputs

Unit system
Humidity reading

A surface at 50 °F is at or below the indoor dew point (53.1 °F), so it will collect condensation. Lower the indoor humidity below 49 % or warm the surface (insulation, better glazing, air movement).

For example the inside of a window pane or an outside-wall corner in winter, measured with a surface thermometer.

Result

Condensation on cold surfaces

Indoor relative humidity
55%
Indoor dew point
53.1°F
Highest RH without condensation on the cold surface
49%

55 % RH is below EPA’s 60 % limit but above the ideal 30–50 %; surfaces at 50 °F will sweat.

Targets at this temperature

Dew point at 30 % RH
37.2°F
Dew point at 50 % RH
50.5°F
Dew point at 60 % RH
55.5°F
Show the working
  1. Dew point: the temperature where saturation pressure equals the air’s vapour pressure
    55 % at 70 °F → dew point 53.1 °F
  2. Highest RH before a surface sweats = saturation pressure at the surface ÷ saturation pressure of the room air
    49 %
  3. EPA guidance
    keep indoor humidity below 60 %, ideally 30–50 %

What EPA recommends

EPA advises keeping indoor relative humidity below 60 percent if possible, and ideally between 30 and 50 percent, to limit mould. Condensation on windows, walls or pipes is a sign that humidity is high for the surface temperatures in the home.

  • 30–50 %: the ideal range
  • 50–60 %: below the limit but above ideal
  • 60 % or more: too humid — ventilate, dehumidify or find the moisture source
  • Below 30 %: drier than the ideal range

The cold-surface check

A surface collects condensation when it is at or below the dew point of the air touching it. The tool compares your coldest surface — often a window pane or an outside-wall corner in winter — with the room dew point, and reports the highest indoor humidity that surface can tolerate. Measure the surface with an infrared or contact thermometer on a cold morning.

Worked example

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

Inputs

Indoor temperature
70 °F
Humidity reading
Relative humidity
Indoor relative humidity
55 %
Coldest surface temperature
50 °F

Result

Indoor relative humidity
55 %
Indoor dew point
53.1 °F
Highest RH without condensation on the cold surface
49 %

55 % RH is below EPA’s 60 % limit but above the ideal 30–50 %; surfaces at 50 °F will sweat.

Assumptions

  • EPA: keep indoor humidity below 60 % relative humidity, ideally between 30 and 50 %.
  • A surface condenses when its temperature is at or below the dew point of the room air next to it.

Limitations

  • Air right next to a cold window can be cooler and more humid than the room reading; treat a result within a few degrees of the dew point as a risk.
  • Hygrometer accuracy limits the result; check your meter’s specification.

Frequently asked questions

Why do my windows sweat even at 40 % humidity?

Because the glass is colder than the room dew point. At 70 °F and 40 % humidity the dew point is about 45 °F, so any pane or frame colder than that collects water.

Is a dew point reading better than relative humidity?

Dew point does not change with temperature, so it is easier to compare between rooms and with surface temperatures. Relative humidity is what most guidance is written in. The tool accepts either.

How accurate is my hygrometer?

Check the accuracy printed in its specifications; inexpensive meters can be several percent off. Treat results near a threshold as uncertain.

Sources

  1. 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.
  2. 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.
  3. Water vapor pressure formulations — UCAR / NCAR Earth Observing Laboratory. Hyland–Wexler saturation vapour pressure coefficients over water and ice.

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