Condensation risk
Internal surface temperature and fRsi (mould risk, BS 5250 / ISO 13788)
- where
- internal surface temperature (°C)
- internal temperature (°C)
- internal surface resistance for mould checks (m²K/W)
- U-value of the element or point (W/m²K)
- external temperature (°C)
- temperature factor
- internal vapour pressure (Pa)
- internal relative humidity (%)
- lowest saturation pressure that keeps the surface at or below 80% RH (Pa)
- minimum surface temperature for 80% RH (°C); below 0 °C the constants 21.875 and 265.5 are used
- fRsi needed for these conditions
Explain this equation
Works out how warm the inside surface of a wall or roof stays, and its temperature factor fRsi, to judge the risk of mould and surface condensation.
Heat flowing out has to cross the internal surface resistance first, so the surface is cooler than the room by R_si × U × ΔT. fRsi expresses the surface temperature as a fraction of the way from outside to inside, so it does not depend on the weather on the day; 0.75 is the usual minimum for dwellings. Mould grows when the surface relative humidity stays at 80% or more, so the room's vapour pressure divided by 0.8 gives the saturation pressure the surface must stay above; reversing the Magnus formula gives that minimum temperature and the matching f_m.
Surface temperature follows from the resistance split; the mould check uses the saturation pressure curve.
Sense check: an uninsulated wall at U = 1.0 with R_si 0.25 has fRsi 0.75, about 15 °C on the surface at 20 °C inside and 0 °C outside. At 60% RH the surface needs about 15.4 °C, so mould is likely.
Watch out: corners, reveals and junctions are colder than the plain wall. Check thermal bridges separately; this is for the plain element or a known point U.
Works out how warm the inside surface of a wall or roof stays and its temperature factor fRsi, then compares it with the 0.75 dwelling criterion and with the value needed to keep the surface below 80% relative humidity.
Enter your values to see the result.
Good to know
fRsi is the surface-to-outside temperature difference divided by the inside-to-outside difference, so it does not depend on the weather on the day. For a plain element it equals 1 − Rsi × U. BS EN ISO 13788 and BS 5250 use Rsi = 0.25 m²K/W for mould assessment on opaque surfaces (allowing for corners and furniture) and 0.13 for glazing. BRE IP 1/06, referenced by Approved Documents C and L, sets fRsi ≥ 0.75 for dwellings, so any U-value worse than about 1.0 W/m²K is borderline. Mould needs about 80% RH at the surface for several days, hence the second check. Use the fRsi from a thermal-bridge model (BS EN ISO 10211) for junctions and reveals.
Common mistake
Insulating the main wall but leaving reveals, window heads and floor/ceiling junctions untreated. These become the coldest surfaces and grow mould first, even though the wall U-value is now good.
Limits
One-dimensional; junctions need a 2D/3D model to get their fRsi. Steady state at the temperatures and RH entered. The 80% surface RH limit is the BS EN ISO 13788 mould criterion; it says nothing about interstitial condensation.
Sources
- BS EN ISO 13788:2012 Hygrothermal performance of building components – Critical surface temperature to avoid mould growth and interstitial condensation (Glaser method; Annex E saturation vapour pressure)
- BS 5250:2021 Management of moisture in buildings – Code of practice (uses BS EN ISO 13788 for surface and interstitial condensation risk)
- Schöck: The surface temperature factor fRsi (BRE IP 1/06 criterion fRsi ≥ 0.75 for dwellings; 0.5 offices and retail; 0.9 swimming pools)
- BRE Information Paper IP 1/06 Assessing the effects of thermal bridging at junctions and around openings (critical temperature factor 0.75 for dwellings), referenced by Approved Documents C and L
- University of Trieste lecture notes, Psychrometry (saturation pressure as used in the UNI EN ISO 13788 calculations: 610.5 exp(17.269 t/(237.3 + t)) for t ≥ 0 °C, 610.5 exp(21.875 t/(265.5 + t)) below 0 °C)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.