does selecting the truss,cantilever option result in uniform insulation across the attic? for example, if 15 inches of cellulose is selected, with BEopt consider a raised heel truss so there is no compression of insulation?

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BEopt v3 does not by default model the reduced level of insulation at the edge of the attic perimeter. The provided insulation R-value is used without any adjustment and is assumed to be uniform across the entire attic. If you did want to calculate an adjustment, equations can be found in ANSI 301 Section 4.2.2.2.

(Note that BEopt v2.8 did estimate the adjustment based on roof type/slope, eave depth, and insulation thickness. I wouldn't recommend using BEopt v2.8 because there have been numerous and substantial modeling improvements across the board in BEopt v3. We hope to bring the roof perimeter insulation adjustment calculation back to BEopt v3 at some point.)