Thermal transmittance “U” and R-value


The thermal transmittance “U” (in W/m2K) is the reciprocal value of R under consideration of the internal and external surface resistance, that depend on the final application of the element. The table with the surface resistance factor can be found in EN ISO 6946. The following formula needs to be used when calculating the U-value of a sandwich element.


Example U-Value Calculation 

Cross section of a 4-layer sandwich panel 

Figure 05: cross section of a 4-layer sandwich panel


Product name Thickness d (mm) λ (W/mK) R (m2K/W)
LAYER 1 Aluminium 1 230 0,000
LAYER 2 OSB 2 0,13 0,015
LAYER 3 STYROFOAM™ RTM-X 51 0,029* 1,759
LAYER 4 Aluminium 1 230 0,000
TOTAL   55   1,774
* λD (lambda declared)

Table 01: R-value calculation based on figure 05