Measure the room conditions that accompany the complaint
Indoor humidity measurements should pair relative humidity with air temperature, a recorded location and a time. Use an appropriate instrument, allow it to stabilize and repeat the readings under relevant operating conditions. A single percentage from a sensor beside a window is too little evidence to diagnose a stuffy bedroom or choose a dehumidifier.
The Wet Bulb Calculator can estimate wet-bulb temperature from paired dry-bulb temperature and relative humidity. Use readings taken together. The calculation does not test the sensor, locate a moisture source or establish the cause of a comfort complaint.
Define the complaint and choose the sampling locations
Start by recording what occupants notice and when it happens. A room that feels uncomfortable after a shower, a bedroom that feels stuffy overnight and condensation beside a window need different sampling plans. Record the activity without assuming that humidity explains every symptom.
Choose a repeatable position that represents the occupied room, then label additional investigation points separately. Avoid confusing a targeted reading beside a cold surface with the average conditions where people sit or sleep. A sketch or photograph of the instrument position makes a return visit easier to compare.
Vaisala’s placement guidance describes errors from heat-conducting walls, poor locations and body heat during handheld measurements. Keep the probe away from your breath and body. Also record nearby windows, heating outlets and other conditions that could make its local environment different from the room.
Treat a suspect corner as a separate measurement
A cold corner may be the correct place to investigate condensation risk, while being the wrong place to represent occupied-room air. Label its result as a local observation. If surface conditions need assessment, use an appropriate surface-measurement method and a building investigation rather than treating an air hygrometer as a surface-moisture test.
This distinction is useful for landlords comparing rooms and for contractors checking a reported damp patch. It prevents an apparently contradictory pair of measurements from being discarded when the instruments are actually observing different locations.
Check the instrument and let it stabilize
Check the sensor’s stated measurement range, accuracy, resolution and any calibration information. A display showing tenths of a percent does not establish accuracy to a tenth. Record the instrument model and identifier so an unexplained change can be traced to a device change rather than attributed immediately to the building.
Allow the probe and surrounding air to reach a stable condition after moving the instrument from a car, outdoors or another room. Vaisala’s HM40 instructions advise watching the measurement graph until it levels out. Follow the actual instrument instructions instead of imposing one settling time on every sensor and airflow condition.
A wet or contaminated probe needs attention under its manufacturer’s procedure. Do not record a number simply because the screen is active. Note a rejected sample and its reason, then repeat under acceptable conditions. Keeping the rejected observation visible is more useful than quietly replacing it with a convenient result.
Temperature mismatch can overwhelm a small room difference
Vaisala’s HM70 documentation gives a specific example: at 20°C and 100% RH, a 1°C probe-to-environment temperature difference produces about 6 percentage points of RH error. That example describes stated conditions near saturation. It is not a universal correction to subtract from every room reading.
The practical response is to correct the measurement setup. A small apparent change after carrying the instrument between rooms should not be used to prove that a ventilation adjustment worked until stabilization and placement have been checked.
Record conditions and repeat the measurements
At each reading, record temperature, relative humidity, time, location and instrument. Add whether windows and doors were open, whether heating or cooling was operating and whether bathing, cooking or drying clothes had recently occurred. Keep the notes short enough that the person collecting readings will complete them consistently.
For a hypothetical initial survey, choose three indoor locations and one sheltered outdoor comparison point, with three observation periods on each of two days. That produces 4 × 3 × 2 = 24 paired temperature-and-humidity observations. It is a sampling example, not a standard duration or proof that two days cover every problem.
If the complaint happens overnight or intermittently, a suitable logger can capture the relevant periods more effectively than daytime spot readings. Record its interval and clock setting. Retain peaks, the time spent elevated and the associated activity; an average can conceal the event that prompted the investigation.
| Record item | What to write | Why it affects interpretation |
| Instrument and accuracy | Model, identifier and stated uncertainty | Small differences may not be distinguishable |
| Location and temperature | Repeatable position and paired air temperature | Relative humidity depends on local conditions |
| Time and operation | Occupancy, windows, extraction and HVAC state | A reading belongs to a particular event |
| Stabilization | Stable, rejected or still changing | Moving sensors can produce misleading comparisons |
Use the Wet Bulb Calculator only after accepting the paired readings. Keep the original values beside any derived result so another investigator can reproduce the comparison.
Compare uncertainty before choosing an intervention
Suppose two hypothetical instruments read 52% RH and 54% RH, and each has a stated accuracy of plus or minus 3 percentage points under the relevant conditions. Their simple accuracy bands are 49% to 55% and 51% to 57%. The bands overlap. A two-point display difference alone does not establish a real difference between rooms.
These bands are an illustration of the instrument specification, not statistical confidence intervals or a complete uncertainty budget. Calibration, response time and placement can add complications. A side-by-side comparison can help identify inconsistent behavior, but it does not turn an unverified instrument into a calibrated reference.
EPA’s indoor-air guidance recommends keeping indoor RH below 60%, ideally 30% to 50%. Treat that as moisture-control guidance, not a diagnosis or a guarantee that every surface is free of condensation. The agency also notes that warm, humid outdoor air can add moisture even when its relative humidity is lower than indoors.
A monitored Portland retrofit shows why location matters
A DOE and PNNL Portland case study, tested in 2011 and published in 2012, investigated attic condensation in a 1912 house. The report recorded attic RH around 60% and identified incomplete air-barrier arrangements and movement of moist indoor air into colder attic areas.
The response combined enclosure work and ventilation changes. That documented project supports investigating the building and its separate spaces. It does not establish that every humid bedroom needs spray foam, an HRV or the same retrofit. A room-only number would have missed part of the problem described in that house.
Hand over a record that can support the next decision
For an occupant, the immediate benefit is a clearer account of when the room becomes uncomfortable. For an HVAC contractor, paired readings and operating notes help separate an instrument problem from a repeatable building condition. A moisture investigator can then decide what additional surface, ventilation or enclosure checks are needed.
Before changing equipment:
- Repeat the suspect reading from a representative, stable position.
- Check whether the difference exceeds the instrument limitations.
- Review moisture-generating activities and equipment operation at that time.
- Preserve unusual readings and the reasons for accepting or rejecting them.
Use the Wet Bulb Calculator to compare accepted paired inputs within its stated range. Keep equipment sizing and diagnosis tied to the wider investigation rather than a single derived temperature.
