How to Measure Humidity and Dew Point at Home (Hygrometer Guide)

EPA notes relative humidity can be measured with a small, inexpensive moisture or humidity meter (hygrometer) available at hardware stores^[1].

What accuracy is good enough?

For professional condensation checks, ISO 8502-4 names hygrometers accurate to ±3% RH and air thermometers accurate to ±0.5 °C^[2]. Look for those figures on a spec sheet; many cheap meters don't state accuracy at all.

How to measure properly

1. Let the meter acclimatise in the room for a while before reading — moving a sensor from a warm pocket to a cold room gives false values.
2. Measure at the height and place you care about (bedroom wall, basement corner), away from heaters and direct sun.
3. Log over several days; humidity swings with cooking, showers, weather and occupancy.
4. For condensation, also measure the surface temperature — ISO 8502-4 specifies a contact thermometer for the surface^[2].
5. Enter air °C, RH and surface °C in the condensation risk calculator.

What the numbers mean

• Above 60% RH: above EPA's recommended limit^[1].
• Surface below the dew point: condensation is forming.
• Surface RH ≥ 80%: mould risk zone used in building physics (see thermal bridges).

Sources

1. US EPA – A Brief Guide to Mold, Moisture and Your Home
2. ISO 8502-4:2017 public preview (iTeh)