How can I divide a fenestration surface into smaller parts?

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If you happen to be using OpenStudio to make your model you can use this measure that splits existing rectangular windows into horizontal bands. It could be expanded to break into grid of surfaces vs. stacked bands.

In EnergyPlus you would create two or more objects (such as Fenestration:Detailed) that each portion has vertex coordinates that correspond to the smaller parts that you are using. For example, if the original window goes from 1 m to 2 m in height you might need to define a smaller fenestration object using vertices that go from 1 m to 1.5 m and the second to go from 1.5 m to 2 m.

If you are using also using a WindowProperty:FrameAndDivider object it is trickier since those are attached to each fenestration object defined and you might need to estimate the impact if you are using multiple fenestration objects.

Depending on the version of EnergyPlus that you are using, you may be able to visualize it using Euclid.