When the thermal boundary is being changed, you have to model the roof as already conditioned in order for TREAT to calculate the savings properly. You can do this by either changing the attic to conditioned, or moving the slopes from the attic into the first or second floor of the building model. The insulation on the ceilings will need to be transferred to the corresponding slope and then you can eliminate those ceiling surfaces altogether. The existing roof R-value should be an average of the ceiling and the roof to represent the overall R-value of the whole attic.
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