Why does increasing air gap in double glazing increase cooling energy in my DesignBuilder model?

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Not a definitive answer ...

West-facing spaces are typically more problematic (than other orientations) when it comes to solar/overheating. It is not unusual to see west-facing fenestration SHGC specs below 30% - not up to 76%. Of course context matters (e.g. fenestration area, internal loads, regional practices), but cutting down solar loads (e.g. revised SHGC, exterior shading) may have an order of magnitude more impact on cooling loads than improvements in fenestration U-factors. I understand this is not what you're after, but I think it's worth underlining.

I suggest you go over comments/answers to related UMH posts (here, here & here). What you may be coming up against is hindered radiative cooling from building envelope U-factor improvements. One way of looking at this: improvements in fenestration U-factor == improved greenhouse, yet with a thwarted capacity to free-cool itself (when conditions are suitable). This can be observed even in cold climates, with overheating solutions ranging from natural or mechanical free-cooling (e.g. economizers) to mechanical heat transfer elsewhere in the building.

I suggest you dive deeper to track when increased mechanical cooling is occurring over the year. For instance, is the increase occurring at night and/or during shoulder seasons? This should provide some insight.

Hope this helps.