Windows: How to model an arched window?

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Assessor Question:

I would like some guidance on how to model an arched window. The lower part of the window is openable, and the top part is fixed glass and in an arched shape.

TechLink Response:

• To be scrupulously accurate, the window could be broken into two portions: fixed, and casement sash. 
• That may be considered more detail than necessary to arrive at an acceptable understanding of the total energy rating result. 
• Accordingly, a sash window with average height of approximately 1750mm, and openability of around 75 per cent, would appear to be reasonable.