Ground Floor U-values in Criterion 2 of Part-L BRUKL report

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Why does the building fail on Criterion 2, Walls or Floors, when all the external constructions are correctly insulated?

Ground floor U-values used in constructions adjacent to ground can either be entered as being corrected for 3-D conduction effects or not:

In medium & large buildings the ground acts as a significant heat store, reducing heat losses and the effective U value.                                                                                                                                           

For uncorrected ground floor U-values (for example those calculated by DesignBuilder using the Layers Definition method and based on BS EN ISO 6946) SBEM automatically corrects the U-value based on the floor area of the building and the external wall perimeter length using the equations in BS EN ISO 13370. This 'corrected' U-Value is used in the calculation and reported on the BRUKL. 

If you have been provided with U-Values from a professional body, check if these have been corrected and if so make sure the box is ticked.

Reporting differences in SBEM and DSM

SBEM calculates the adjusted U-values for the individual zones

For a building 9m x 6m (all exposed perimeter) with unadjusted U-Value 0.25.

The adjusted U-Values for corner zones (Perimeter/Area = 6/9) are 0.19. For zones with one edge exposed (Perimeter/Area = 3/9) the U-Values are calculated as 0.16. The area weighted average U-Value for the whole block is 0.18:

DSM calculates the adjusted U-Value for the whole block

Thus it uses a Perimeter/Area for the whole block of 30/54 which generates a U-Value of 0.18. This is also the reported maximum U-Value as only one calculation is performed: