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?
Posted by Andy Tindale on 06-Jul-20 05:23 PM
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.
LEED auditors are generally aware of this issue.
Last Updated 06 July 2020