How does OpenStudio assign roof constructions?

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Look at this recent question about interior doors. Roofs work in a similar fashion. If part of your building has an attic zone (without insulation at the exterior surface) and part of your model has a insulated built up roof, you would set the most common as the building default. Then for specific spaces or surfaces you would assign the other type of roof construction. This is similar approach to what you would take if different parts of your building have different exterior wall constructions.

There is one extra step for attic spaces. You need to create a a new construction set for attic spaces with an interior floor construction that has the desired insulation. You don't have to do anything to the space below, if it doesn't have a space or surface level ceiling assignment then the attic floor will be mirrored and applied to adjacent space.