Building Surveyor Salary Index

Condensation and mould risk calculator

Last reviewed September 2026

Enter the air temperature and relative humidity to get the dew point. Add a surface temperature to see whether that surface will get condensation, or is damp enough for mould to grow. Add the outside temperature too, and it will suggest whether the problem is the building or the way it’s used.

Readings

Enter an air temperature between −30 and 50 °C and a relative humidity between 1 and 100%.

Taking the readings

  • Measure air temperature and relative humidity with a thermo-hygrometer in the room, away from radiators, windows and people.
  • Measure surface temperature with a surface or infrared thermometer at the coldest points: external corners, window reveals, lintels, behind furniture and at ceiling junctions. A thermal camera helps find them.
  • A single reading is a snapshot. Humidity changes through the day with cooking, bathing and drying clothes, and the 80% mould limit in BS EN ISO 13788 is a monthly average. A reading above it points to a risk rather than proving mould will grow. Data loggers left for a week or two give a better picture.

Is it the building or the way it’s used?

The temperature factor shows how well a surface holds on to heat: the surface temperature above outside, divided by the room temperature above outside. A factor of 1 means the surface is as warm as the room, and 0 means it’s as cold as outside. It stays roughly the same whatever the weather, so it describes the building rather than the day.

BRE Information Paper IP 1/06 gives a critical factor for each type of building: surfaces below it are likely to get condensation or mould with normal use. A problem surface below the benchmark points to the building, such as a cold bridge or missing insulation. A problem surface that meets it points to too much moisture in the air.

  • Home, residential building or school: 0.75
  • Office or shop: 0.50
  • Storage building: 0.30
  • Sports hall, kitchen or canteen: 0.80
  • Swimming pool, laundry or brewery: 0.90

Take the readings in cold weather, with the heating on steadily for a few hours, and out of direct sun. The room temperature stands in for what BS EN ISO 13788 calls the operative temperature, which also allows for the warmth of the room’s surfaces, so treat the factor as an indication rather than a precise figure.

Condensation or another kind of damp?

Dew point readings help tell condensation from penetrating or rising damp, but they aren’t a diagnosis on their own. Look at the pattern of the damp, the construction and how the building is heated and ventilated, and remember that moisture meter readings are affected by salts and cold surfaces. The survey kit list covers the meters and cameras, and the books list includes guides to investigating damp.

How it’s worked out

The dew point comes from the saturation vapour pressure formulas in BS EN ISO 13788, which use different constants above and below 0 °C. The surface relative humidity is the vapour pressure of the room air divided by the saturation vapour pressure at the surface temperature. Results are shown to one decimal place, but they’re only as accurate as the instruments used: a hygrometer is typically accurate to a few per cent.

The method only covers moisture carried in the air. It doesn’t account for rain penetration, rising or ground water, leaks, or moist air leaking into the structure, so a low-risk result doesn’t rule those out.

Sources

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