The calculator derives indoor water-vapor pressure and dew point from air temperature and relative humidity. It then estimates relative humidity directly at the colder surface. Liquid condensation becomes possible at or below the dew point; sustained surface humidity around 80% or higher can support mold even before visible condensation appears.
The optional temperature factor fRsi compares the indoor surface temperature with indoor and outdoor air temperatures. Interpretation depends on climate, construction and the applicable building standard, so use it as supporting information rather than a universal pass/fail value.