Enrichment · Passion Projects · Home science

Why the bathroom
window fogs and
the bedroom’s does not.

Type the temperature and humidity in each room and the temperature of your coldest window. The calculator gives every room’s dew point and shows exactly where water will condense.

Check the roomsWhat you get

The dew point calculator

The worked example is a made-up house on a January evening with 50°F window glass. Nothing you type leaves this browser.

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What you get

Enrichment · Passion ProjectsFree

What you finish with
Each room’s dew point and water content, which rooms will wet the cold surface, and which are outside the healthy humidity range, with what to do. It prints as a summary and downloads as a spreadsheet.
Who it is for
Families and renters checking for damp, mould or dry air, and students studying weather, humidity and the water cycle.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard, in °F or °C. Up to 30 rooms. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Passion Projects space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose °F or °C and, if you can, type the temperature of the coldest window or wall.
  2. Add each room with its temperature and relative humidity.
  3. Read the highest dew point and which rooms will condense on the cold surface.
  4. Act on the rooms above 50%, then measure again later.
  5. Print or save the readings to compare across a season.

Check the rooms

How the dew point is worked out

The Magnus formula, from temperature and relative humidity.

The worked example: the bathroomBathroom72°F, 85%22.2°Cγ1.314ln(0.85) + 17.625T ÷ (243.04 + T)Dew point19.6°C= 67.2°FGlass at 50°FCondenses67°F ≥ 50°FThe worked example: the bathroomBathroom72°F, 85%22.2°Cγ1.314ln(0.85) + 17.625T ÷ (243.04 + T)Dew point19.6°C= 67.2°FGlass at 50°FCondenses67°F ≥ 50°F
Air holds more water when it is warm. The dew point is the temperature at which the water already in the air would saturate it, so any surface colder than that gets wet.

The formula

Temperatures are turned into °C. γ = ln(RH ÷ 100) + 17.625 T ÷ (243.04 + T), and the dew point is 243.04 γ ÷ (17.625 − γ). These constants, from Alduchov and Eskridge, are accurate to a few tenths of a degree across indoor temperatures. Absolute humidity, the grams of water in a cubic metre of air, comes from the saturation vapour pressure 6.112 e^(17.67T ÷ (T + 243.5)) hectopascals.

What the numbers mean

A surface colder than the room’s dew point collects water: fogged windows, sweating pipes, damp corners behind furniture. The EPA suggests keeping indoor relative humidity between 30 and 50%, and below 60% to limit mould and dust mites. Heating a room lowers its relative humidity but leaves its dew point alone; ventilation and dehumidifying lower the dew point.

Sources

  1. Alduchov, O. A., and Eskridge, R. E. (1996). Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology 35(4), 601 to 609.
  2. Bolton, D. (1980). The computation of equivalent potential temperature. Monthly Weather Review 108, 1046 to 1053: saturation vapour pressure.
  3. U.S. Environmental Protection Agency. A Brief Guide to Mold, Moisture, and Your Home (EPA 402-K-02-003): indoor humidity 30 to 50%, below 60% (epa.gov).
  4. National Weather Service. Dew point versus humidity (weather.gov).

The worked example is made up. Home hygrometers are often off by 3 to 5 percentage points; compare readings from the same device. Your readings stay in this browser. Support: team@caveatacademic.com.