Example Data Table
| Air Temp (C) | RH (%) | Surface Temp (C) | ACH | Material | Score | Category |
|---|---|---|---|---|---|---|
| 24 | 55 | 20 | 0.7 | Standard | 38 | Moderate |
| 26 | 70 | 18 | 0.3 | Susceptible | 78 | High |
| 22 | 45 | 21 | 1.2 | Resistant | 18 | Low |
| 20 | 80 | 16 | 0.2 | Susceptible | 92 | Severe |
Scores are illustrative and depend on exposure duration, moisture sources, and building details.
Formula Used
The calculator estimates dew point using a Magnus-type approximation, then derives surface relative humidity from the same vapor pressure at the colder surface.
- Dew point (C): computed from air temperature and indoor RH (Magnus approximation).
- Actual vapor pressure: e = RH · es(Tair)
- Surface RH (%): RHs = 100 · e / es(Tsurface) (capped at 100%).
- Likelihood score: a weighted index that increases with high Surface RH, long exposure, susceptible materials, recent wetting, and low ventilation.
This is a screening tool for construction decisions, not a lab diagnosis. For visible growth or persistent odors, follow your local remediation guidance.
How to Use This Calculator
- Measure indoor air temperature and indoor relative humidity.
- Find the coldest surface (corners, glazing, behind furniture).
- Estimate ventilation (ACH) from design data or commissioning notes.
- Select moisture level, material susceptibility, and wet-event history.
- Press Calculate Likelihood to view score and drivers.
- Download CSV or PDF to attach to inspection reports.