One wall can condense moisture while another stays dry because its surface temperature falls below the dew point of adjacent indoor air. Thermal bridges, insulation gaps, exposure and restricted airflow may cool one surface more than another. Humidity supplies the water vapor, but a damp patch can also come from a leak or penetrating moisture: appearance alone is not proof of condensation.
Cross-check the dew point rather than relying on the thermal image
Measure room temperature and relative humidity at the same time as the wall surface. Estimate dew point from those air readings, then compare it with an emissivity-corrected surface measurement. A cold image pattern supports a surface-temperature hypothesis; it does not prove moisture is condensing or distinguish condensation from a hidden plumbing leak. For practical setup details, see our thermal-camera emissivity guidance.
What measurements establish condensation risk?
Measure air temperature, relative humidity and each wall’s surface temperature under the same conditions. Use the air readings to determine dew point, then compare that value with the observed surface temperature. The closest surface to or below the dew point is more susceptible to surface condensation. Localized air conditions may differ beside an exterior wall or behind furniture.
Example with two walls
Hypothetical: 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 18 °C; Wall B measures 12 °C. Wall B is about 1.9 °C below the dew point and is therefore at condensation risk, while Wall A is above it. This is an illustration, not a reading taken in a building. Surface-temperature uncertainty and localized air conditions also matter.
Field comparison procedure
- Record the moisture pattern, time, indoor activities and exterior weather.
- Measure room temperature and RH close to the affected zone without contacting the cold wall.
- Calculate or display dew point using the instrument’s documented method.
- Compare several surface temperatures on affected and unaffected walls, with consistent emissivity and distance.
- Repeat under comparable conditions, including a humid period and a drier period.
- If dampness persists independently of humidity, investigate leaks, rain penetration, plumbing or other moisture sources rather than assigning a condensation diagnosis.
Testo instruments for the diagnosis
The testo 605i measures room temperature and RH and calculates dew point through the Smart App. The testo 805i measures apparent surface temperature by infrared. For broader pattern mapping, the testo 868s thermal imager visualizes apparent surface-temperature differences. None of these instruments directly measures hidden wall moisture or identifies a leak source from the image alone.
Common sources of misleading results
| Observation | Why interpretation needs care |
|---|---|
| Cold thermal-camera stripe | May be a thermal bridge, reflection or differing surface finish |
| Wetness only near floor | Consider plumbing, penetrating or foundation-related moisture |
| Condensation behind a cabinet | Restricted airflow and locally cooler surface may matter |
| Normal room RH at midday | Earlier overnight conditions may have been different |
What to correct—and when to seek help
Improve appropriate ventilation and moisture-source control, then reassess temperatures and humidity. Do not cover persistent water damage with a coating before finding its cause. Mold, staining, musty odors, peeling paint or recurring dampness warrant professional assessment. The U.S. EPA emphasizes controlling indoor moisture as a primary way to control mold growth. For a separate assessment of persistent dampness, see our building-moisture inspection guide.