Quick answer: Use the testo 625 to measure room-air temperature and relative humidity and read the calculated dew point. Then measure the suspect wall’s surface temperature with a separate suitable instrument. If the surface is at or below the local dew point, surface condensation is possible. The testo 625 does not directly measure wall moisture or prove that mold is present.
What the testo 625 measures—and what it does not
The current testo 625 (0563 1625) is a portable thermohygrometer with a humidity/temperature probe. Testo specifies air temperature and relative humidity measurement, calculated dew point and wet-bulb temperature, and support for the testo Smart App for saving and documenting readings. The current published humidity accuracy is 2.5 percentage points of RH in a specified central measuring range; that is not the same as an absolute humidity measurement. For a comparable instrument covering the same assessment, see the testo 605i dew-point guide.
The instrument does not measure a wall’s actual surface temperature, liquid water inside plaster or mold spores. A cold wall needs a separate contact or infrared surface thermometer, used with the appropriate surface and emissivity precautions. A moisture meter and building inspection may be needed if the area remains damp. For related guidance, see 868 Thermal Camera Guide: Setup, Emissivity and Safe Checks.
Repeatable room and wall check
- Record conditions. Note outdoor weather, heating operation, recent showers/cooking and where dampness appears.
- Measure the room air. Hold the 625 away from body heat, vents, sunlight and contact with the damp wall. Let its temperature and RH display stabilize.
- Read the dew point. Save air temperature, RH, dew point, location and time in the Smart App when supported, or record them manually.
- Measure the actual surface. Use a suitable independent thermometer at the affected patch and on a comparable dry wall. For shiny materials, address reflections and emissivity.
- Compare and repeat. A surface temperature at or below the dew point supports a condensation mechanism, but a single observation cannot distinguish a hidden leak from all other moisture sources.
- Escalate warning signs. Persistent staining, visible mold, deteriorating plaster or recurring dampness near the floor calls for building assessment rather than ventilation advice alone.
Worked example: dew point versus wall temperature
Hypothetical readings: indoor air is 22 °C (71.6 °F) at 60% RH. Its approximate dew point is 13.9 °C (57.0 °F). Wall A measures 17 °C, while wall B measures 12 °C (53.6 °F). Wall B is about 1.9 °C below the air dew point, so condensation is plausible there; wall A is above it. The figures are illustrative and assume the sampled room air represents the air next to each wall. For a fuller explanation of the wall-to-wall difference, see why condensation forms on one wall.
Interpret trends rather than guessing a universal RH limit
| Finding | Meaning | Next step |
|---|---|---|
| High RH, cold wall, visible droplets | Surface condensation plausible | Review moisture generation, ventilation and insulation |
| Normal room RH, very cold corner | Local thermal bridge still possible | Measure surface temperatures at more locations |
| Wall stays damp independently of RH | Leak or penetrating moisture possible | Investigate building fabric and services |
| RH differs between two meters | Placement, stabilization or accuracy may differ | Co-locate and verify instruments |
What the Smart App adds
For compatible current units, the testo Smart App can display a second screen, help document sites and save measured values. Confirm pairing and export format in the current manual. A screenshot or app graph is useful documentation, not a substitute for recording surface temperature and a clear inspection method. For pairing and compatible-probe details, see the Testo Smart Probes compatibility guide.
Official reference
Testo 625 official product specifications and instruction manual. If the wall is persistently wet or mold is present, arrange a qualified building/moisture investigation rather than inferring a defect from the dew-point calculation alone.