What is the temperature factor fRSI?

With the introduction of the Energy Saving Regulations (EnEV) in 2002, the temperature factor fRSI has become increasingly more important in the planning and installation or replacement of constructions components in the building envelope. The fRSI temperature factor is use to determine the transition area between the interior of window frames and the plaster reveal. Therefore, it is the temperature in the corner. This is also known as the geometric and constructive heat bridge.

In constructive terms, the lowest temperatures are recorded in this area. Low temperatures on wall surfaces cause mould.

The fRSI temperature factor is a figure indicating whether the temperatures at this spot are critical or not. The fRSI temperature factor is expressed as a coefficient/ratio. If this ratio is greater than 0.7 then the temperature at this point complies with the regulations. If the factor is smaller than 0.7 then the temperature at this point is too low.

A low surface temperature indicates a critical climate for mould. This is critical because the temperature is allowing condensation to build in this area and if the relative humidity is around 80%, then this is enough for mould to bloom. Mould is classed as a visible defect and leads to legitimate complaints.

DIN 4108-2 specifies the calculation for the fRSI temperature factor. In order to make the calculation, the relationship between the room temperature, the external temperature and surface temperature at the transition point between the frame and the wall is determined.

An optimum temperature ratio is when the external temperature is -5°C, the room temperature is 20°C and the surface temperature at the transition point between the window frame and the wall is at least 12.6°C. The fRSI temperature factor is then exactly 0.7.

In isothermal calculations, the temperatures are visible over the temperature lines. These temperature lines for structural attachments are planned in compliance with the regulations.

If you cannot calculate the temperature factor or you don’t want to, then you can use a technical standard solution from the thermal bridges catalogue: DIN 4108 supplementary sheet 2.

The ift installation tool has the option of calculating the fRSI temperature factor for many different individual, self-designed connections. You can access the ift installation tool under Tools.