Condensation and dew-point checker

By the Breezewold team

Sources awaiting check

Work it out for your home

Worked example

For a room at 20°C and 60% relative humidity, with window glass measuring 11°C:

  • Dew point: 12°C.
  • The glass is 1°C below the dew point, so condensation is likely on it.
  • First steps: open the trickle vents, wipe the glass each morning, and dry washing outdoors or in a ventilated room.

Source: Alduchov and Eskridge (1996), Journal of Applied Meteorology, awaiting check; NHS, awaiting check

Enter the room temperature and relative humidity from a hygrometer. The checker works out the dew point: the temperature at which that air starts to drop water. Any surface at or below it gets wet.

How it works

The dew point comes from the Magnus formula, a standard approximation used in meteorology, with the coefficients shown in our sources. Add a surface temperature, for example from an infrared thermometer pointed at the window glass, to see whether that surface is at risk.

We call a surface "borderline" when it's within 1°C of the dew point. That's our rule of thumb, because hand-held meters aren't exact.

FAQs

What humidity should my home be?

We'll add sourced guidance on indoor humidity here once it's checked.

Can I trust a cheap hygrometer?

Readings vary between devices. Use the checker as a guide, and look at what actually happens on your windows and walls.

Read the guide: Why are my windows wet in the morning?

Sources