Florida Mold & Mildew Risk Calculator

Estimates mold and mildew growth risk based on indoor relative humidity, air temperature, estimated surface temperature, and ventilation. Uses dew point proximity and the ASHRAE 160 moisture design criterion framework.

Typical cold surfaces: AC vents ~10–15°F, exterior walls ~3–8°F, windows ~5–20°F below air temp.

Formulas Used

1. Dew Point (Magnus Formula – Lawrence 2005):
α = ln(RH/100) + (17.625 × T) / (243.04 + T)
Td = (243.04 × α) / (17.625 − α)
where T is air temperature in °C, RH is relative humidity in %.

2. Estimated Surface Relative Humidity:
RHsurface = 100 × exp(a × Td / (b + Td)) / exp(a × Tsurface / (b + Tsurface))
where a = 17.625, b = 243.04. This estimates the effective RH at a cooler surface given the ambient dew point.

3. Risk Score (0–100):
Score = Surface RH Score (0–50) + Dew Point Margin Score (0–20) + Ventilation Penalty (0–20) + Duration Score (0–10)
• Surface RH Score = ((RHsurface − 60) / 40) × 50, clamped to [0, 50]
• Dew Point Margin Score = ((10 − margin°C) / 10) × 20, for margin ≤ 10°C, clamped to [0, 20]
• Ventilation: Poor = 20, Moderate = 10, Good = 0
• Duration Score = (hours / 24) × 10

4. Risk Thresholds: Low < 20 | Moderate 20–39 | High 40–64 | Very High ≥ 65

Assumptions & References

  • Based on ASHRAE Standard 160-2016: Design Criteria for Moisture Control in Buildings – mold risk threshold is surface RH ≥ 80% sustained over time.
  • Dew point calculated using the Magnus formula (Lawrence, 2005, American Meteorological Society), accurate to ±0.35°C for typical indoor conditions.
  • Surface temperature is estimated as a user-defined offset below air temperature. Actual surface temperatures vary by material, insulation, and HVAC proximity.
  • Florida climate context: ASHRAE Climate Zone 1A (hot-humid). Indoor RH above 60% is considered elevated risk per EPA Mold Guidelines and Florida Building Code moisture management provisions.
  • Mold species common in Florida (e.g., Cladosporium, Aspergillus, Stachybotrys) can begin colonizing within 24–48 hours at surface RH ≥ 80% and temperatures 40–100°F (AIHA, 2008).
  • Ventilation scoring reflects the role of air movement in reducing boundary-layer humidity at surfaces (ASHRAE Fundamentals Handbook, Chapter 25).
  • This calculator provides a risk estimate only and does not replace professional mold inspection or industrial hygienist assessment.

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