The temperature factor, called fRsi factor, is a criterion for the risk of mould growth with a wall connection. Our approach for the fRsi calculation methodology is taken from BS EN ISO 13788:2002.
- Collate the outdoor and indoor temperature for the specified project to which the design belongs. This data is retrieved in the same manner as it is for the Condensation calculations.
- Adjust the internal/external air film R-values as proposed by NCC 2019 Volume One J1.2.
- If framing is present in the design, the process is altered to better represent the impact of this thermal bridge.
- Retrieve the internal surface temperature.
- Using the described equation, calculate the fRsi of the system.
If framing is present in the design, this is grounds for thermal bridging. And since thermal bridges cause additional heat losses, this also means they cause lower interior surface temperatures. It is assumed that the coldest points in the wall system occur where the framing is positioned. Since the fRsi requires this coldest point, an approach is required to find the internal surface temperature at the framing.
Since there is currently no method of isolating and retrieving the temperature at just a point on a surface, the effect is approximated replacing the entire framed insulation layer with just framing. This involves removing that current layer and replacing it with a generic homogeneous material layer with the same properties of the original framing (in terms of thermal conductivity and thickness). Since temperatures are calculated solely via R-Values, this will effectively emulate what is happening at the points on the interior surface where framing is situated.
Internal Surface Temperature
The surface temperatures throughout the whole system are calculated using the same approach as the Interstitial Condensation calculation. The temperatures are dependent on the specified layer's R-Value proportion of total system R-Value, with the interior air film internal surface being the same as the indoor air temperature.
The internal surface temperature of the system is required to calculate the fRsi, but since the outermost surface temperature is always the indoor air temperature, an alternative approach is required. Instead, the internally facing surface temperature of the layer before the air film has been selected, which is believed to be the most accurate measurement of the ‘internal surface temperature’ available in this method.
The calculation is quite simple:
θe is the external air temperature
θi is the internal air temperature
θsi is internal surface temperature.
Note that the equation is purely a ratio or percentage of the internal surface temperature of the overall temperature difference.
As the temperature is linked to the R-Value of the system and individual layers, the fRsi will remain constant throughout all temperature conditions when those values to remain constant. This is where the impact of framing is seen, as these materials typically have poor thermal performance, and so will result in a lower fRsi.
Note, fRsi results will not be displayed for warmer temperature locations; locations with annual average temperatures of over 15 degrees.