About Moisture & Mould Risk Calculator
The moisture and mould risk calculator assesses the risk of surface condensation and mould growth on the warm side of a construction using the surface temperature factor (fRsi) method per BS 5250, ISO 13788, and ASHRAE 160. It is used by designers and assessors to confirm that internal surfaces stay warm enough to avoid mould.
Build the wall assembly, set monthly indoor and outdoor temperature and relative humidity, and the tool computes the fRsi of the construction, the resulting surface relative humidity each month, and a pass/fail verdict against the 80% mould-growth threshold.
How It Works
- Compute the total assembly resistance and the surface temperature factor fRsi = 1 - Rsi / R_total.
- For each month, derive the internal surface temperature Tsi from fRsi and the indoor/outdoor temperatures.
- Calculate the relative humidity at that surface from the indoor vapour pressure and the surface saturation pressure.
- Flag any month where the surface RH exceeds 80% and compare the actual fRsi against the standard requirement.
Worked Example
A construction with Rsi = 0.13 and R_total = 2.50 has fRsi = 1 - 0.13/2.50 = 0.948; with indoor 20 C and outdoor 0 C the surface sits at about 18.9 C, keeping surface RH below 80% so the month passes.
Formulas
- Surface temperature factor
fRsi = 1 - (Rsi / R_total)- Internal surface temperature
Tsi = fRsi * (Ti - Te) + Te- Surface relative humidity
RH_surface = RHi * Ps(Ti) / Ps(Tsi)
Standards & References
- BS 5250
- ISO 13788
- ASHRAE 160
Frequently Asked Questions
What is the temperature factor fRsi?
fRsi = 1 - Rsi / R_total is a property of the construction that describes how close the internal surface stays to the indoor temperature. A higher fRsi means a warmer surface and lower mould risk.
Why is 80% surface relative humidity the mould threshold?
Mould can germinate and grow once the relative humidity at a surface stays above about 80% for a sustained period, even without visible liquid condensation, which is why standards use 80% as the limiting criterion.
What fRsi do I need to pass?
It depends on the standard and climate. The tool uses the standard minimum (commonly 0.75) and the value required by the actual monthly climate data, then checks that no month exceeds 80% surface RH.
Is the moisture and mould risk calculator free to use?
Yes, it runs entirely in your browser at no cost and stores none of your input data.