Indoor Humidity & Mold Risk Calculator
Assess your indoor relative humidity, estimate dew point, and evaluate mold growth risk based on temperature and humidity conditions.
Formulas Used
Dew Point (Magnus Formula – Alduchov & Eskridge 1996):
γ(T, RH) = ln(RH/100) + (a·T)/(b+T)
Td = b·γ / (a − γ)
where a = 17.625, b = 243.04 °C, T in °C
Absolute Humidity:
AH = [6.112 · exp(17.67·T/(T+243.5)) · RH · 2.1674] / (273.15 + T) [g/m³]
Saturation Vapor Pressure:
es = 6.1078 · exp(a·T/(b+T)) [hPa]
Mold Risk Threshold:
Critical RH ≥ 80% at T ≥ 20°C (rises ~0.5%/°C below 20°C, per Viitanen model). Surface condensation occurs when Tsurface ≤ Tdew point.
Assumptions & References
- Magnus formula coefficients from Alduchov & Eskridge (1996), valid for −40°C to +60°C with <0.1% error.
- Mold growth thresholds based on ASHRAE Standard 160-2016 (Criteria for Moisture-Control Design Analysis in Buildings).
- Viitanen mold index model: sustained RH ≥ 80% at ≥ 20°C initiates mold growth within 1–2 weeks on susceptible materials.
- ASHRAE recommended indoor RH range: 30–60% for occupant comfort and IAQ (ASHRAE 55-2020).
- Absolute humidity formula from WMO Technical Document No. 8 (CIMO Guide).
- Surface condensation check assumes uniform surface temperature; thermal bridges may cause localized condensation at lower overall RH.
- Mold risk also depends on material type, nutrient availability, and exposure duration — this tool provides a general risk estimate only.