BEopt - Roof and Ceiling Assembly R-value calculation method

asked 2021-04-23 10:30:15 -0500

updated 2021-04-26 12:19:31 -0500

I wonder how BEopt calculated the assembly R-value for roof and ceiling separately. Like for the roof and ceiling that shares same nominal R-value, the assembly R-values are different. And the assembly R-value of ceiling almost equals to its nominal R-value while for roof, there's difference between these two. Thanks!

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Maybe related to assumptions in roof (finished, unvented) vs ceiling (unfinished attic, vented) structures (despite both being spaced 24-in on-center). For ceilings, seems fair to assume truss designs, where insulation is likely interrupted in only 3, 5 or 7 spots (vis-à-vis eaves & gusset plates) - ceiling insulation would remain ~uninterrupted (entirely covering 2x4 ceiling joists). Much more compact roof rafters on the other hand would need to support the ceiling below, and would thus interrupt insulation quite regularly (e.g. every 12 inches?) - hence lower assembly R-values.

Denis Bourgeois's avatar Denis Bourgeois  ( 2021-04-27 08:02:09 -0500 )edit