The thermal performance of an insulating part in a node is characterised by the linear thermal transmittance Ψ (in W/mK). This thermal transmittance indicates which penalty (difference between the numerical two-dimensional reference based on the overall dimensions) must be imposed on the thermal transmission calculated on the basis of U-values. Ψ-values can be negative when the reference calculation overestimates the actual heat flow.
The temperature factor f is an indicator of the lowest internal surface temperature Θsi at a feature, where Θi is the internal temperature and Θe the external temperature. The temperature factor f has a value between 0 and 1.
If the internal surface temperature is too low, surface condensation and mould growth may occur. Depending on the building type, minimal values are presupposed for f ≥ 0.80.
By way of example, the calculation of both the Ψ-value and the temperature factor f for a typical application such as the foundation is presented below, with use of the Marmox THERMOBLOCK® R2 nano and Styrodur® C in cavity (80 mm) and floor (40 mm). A graphical depiction of the feature with isotherms and heat streamlines can be obtained using Physibel’s KOBRA software, which is free to download from the WTCB.