In simple glazing systems, why is the maximum U-value allowed by DesignBuilder 5.8 W/m2K when ASHRAE 90.1 has a minimum standard of 7 W/m2K?

This is a known issue with energy modelling programs where the ASHRAE 90.1 standard sets U-values that are based on metal frames with "finning" which mean that the effective heat exchange area for the window assembly is actually higher than the area of the opening in the wall. Complex 2D shapes like window frames can have U-values as high as 15 if they are thermally unbroken metal and have a high ratio of wetted (exposed) area to projected area. So for the purposes of the energy model with its 2-D surfaces, the U-values are not physically realistic. For this reason, physics-based energy modelling programs like EnergyPlus which base their surface heat exchange on 2-D surfaces are not easily able to model such values envisaged by the standard.

To maintain agreement with the ASHRAE 90.1 standard DesignBuider still implements the code minimum values in the glazing data in the baseline model. However, this raises the maximum glazing U-value exceeded error due to the limits imposed by the EnergyPlus calculation engine. Although EnergyPlus's maximum allowable U-value input is 7.0 W/m2·K (1.23 Btu/h-ft2-F), the effective upper limit of the simple glazing generated by the model internally within EnergyPlus is around 5.8 W/m2·K (1.029 Btu/h-ft2-F).

Note that although EnergyPlus accepts U-value inputs up to 7.0 W/m2·K (1.23 Btu/h-ft2-F), the DesignBuilder glazing dialog has its own limit set to 5.8 W/m2·K (1.029 Btu/h-ft2-F) in recognition of the limitation within the EnergyPlus engine.

DesignBuilder is in discussion with EnergyPlus developers and researchers on this issue. Dr Peter Lyons has published a paper on the subject proposing a way to work around the issue. We hope that the EnergyPlus development team will implement his proposed method in a future version of EnergyPlus. 

LEED auditors are generally aware of this issue.