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BEopt uses a combination of series and parallel heat flows to calculate the construction's total assembly R-value. Parallel path heat flows are calculated for layer comprised of framing and cavity (insulation), and then this layer is in series with other layers (e.g., sheathing, roofing material, etc.).

You can find details on these approaches in the ASHRAE Handbook of Fundamentals or an example overview at https://www.greenbuildingadvisor.com/article/the-fundamentals-of-series-and-parallel-heat-flow (for wood framed walls, though wood framed roofs are similar).

BEopt uses a combination of series and parallel heat flows to calculate the construction's total assembly R-value. Parallel path heat flows are calculated for the layer comprised of framing and cavity (insulation), and then this layer is in series with other layers (e.g., sheathing, roofing material, etc.).

You can find details on these approaches in the ASHRAE Handbook of Fundamentals or an example overview at https://www.greenbuildingadvisor.com/article/the-fundamentals-of-series-and-parallel-heat-flow (for wood framed walls, though wood framed roofs are similar).