The U-value for the rooflights (which are on a horizontal plane) are adjusted in accordance with BR 443 for the BRUKL U-Value check (which considers all elements to be on the vertical plane).
In more detail - Apache assumes that the U-value input in the model is for the horizontal plane and this U-value is used for the simulation. However, in BRUKL Criterion 2 U-value check, the limiting U-values are supplied and assessed in the vertical plane. This is why the adjustment needs to be made. The conversion of the U-value is done in accordance with BR 443 (see chapter 11 of the document – U-values for windows, roof windows and rooflights) and it converts from the horizontal plane to the vertical plane by applying a correction of 0.5 W/m2K. There is no difference for sloped roofs (unlike in the standard where the incremental adjustments are made for each 10° slope between 0° and 90°) and the correction is always the same. The converted U-value is for the BRUKL display purposes only.