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Space cooling penalties with lower U-value can make total sense for fairly warm climates. For (very) warm climates, you might actually end up having more of a cooling penalty than heating savings.

This is actually counter intuitive to most people until you start thinking about it, but that's really where energy modeling has a value!

The short story is that by having a lower U-value, you're "trapping heat" in the summer. Your building becomes a lot better at not transmitting this heat that would have been dissipated to the outdoors especially at night when the temperatures outside are lower.

I've actually had a reviewer asking for the same thing on a model I submitted recently, and I gave him relatively the same answer. To illustrate my point, I used the DOE Commercial Prototype Building Model of the same type (multifamily in my case), and simulated it in the same location as my project and varied the window U-value. It exhibited exactly the same behavior.

Now a couple more things:

• You should still make sure that the increase in cooling is still reasonable and not way too big to be believable
• Another characteristics of windows is the Solar Heat Gain Coefficient. This will have quite a big impact for summer cooling (the lower the better for cooling, the opposite for heating...).