Real-time Mold Risk Calculator

Combine indoor humidity, outdoor temperature, and assembly performance to see how close your interior surfaces are to dew point. The tool highlights surface RH—the most reliable predictor of mold growth.

✓ Surface relative humidity is the trigger for mold growth—not just room humidity.
✓ Cold surfaces from thermal bridges or poor insulation spike surface RH above 80%.
✓ Drywall, wood, and concrete all respond differently to prolonged moisture exposure.

Why surface RH matters

Mold colonization accelerates once surface RH stays above 80% for more than a few days. At 95%+, condensation begins and spores germinate almost immediately. By warming the surface—or dropping indoor RH—you can keep the envelope out of the danger zone.

We assume interior and exterior surface films of Rsi 0.13 and Rse 0.04 m²·K/W per ISO 13788 for quick comparisons. For detailed hygrothermal modeling, pair these results with data loggers or ASHRAE 160 moisture criteria.

Model your surface conditions

Choose an envelope assembly or enter a custom U/R-value to estimate the interior surface temperature. The calculator blends your indoor humidity and dew point to reveal when surfaces cross the mold-growth threshold.

Indoor conditions

Enter indoor air temperature, humidity, and outdoor conditions to estimate the risk of condensation on interior surfaces.

Indoor air temperature Typical comfort range: 18–24°. Indoor relative humidity (%)

Outdoor air temperature Used to estimate interior surface temperature. Surface type (optional)

Assembly heat flow

Assembly preset

Choose a preset to auto-fill the surface temperature estimate.

Surface film resistances assumed: Rsi = 0.13 m²·K/W, Rse = 0.04 m²·K/W.

Turning insights into action

Staying below 80% surface RH drastically slows mold growth. Use the recommendations to fine-tune ventilation, insulation, and moisture control.

Moisture control: Balance ventilation, sealed combustion, and dehumidifiers to keep indoor RH between 40–55%.
Thermal upgrades: Add continuous exterior insulation, insulated rim joists, or radiant barriers to warm cold surfaces.
Monitoring: Pair this tool with data loggers to track time of wetness and validate ASHRAE 160 criteria over seasons.

Need help interpreting the results?

Our certified remediators can review data logs, identify thermal bridges, and implement targeted repairs. From rim joist insulation to whole-home dehumidification, we keep your surfaces dry.

Call 877-320-4994 or request an assessment to build a mitigation plan.