Most insulation estimates multiply wall area by a framing factor, often 15 or 25 percent, and call the rest cavity. wall-framing-takeoff already places every stud, king, jack, header, cripple and plate, so it can measure the cavity instead.
For each wall the new --insulation flag takes the panel area, subtracts the area of every framed member and each opening's rough area, and what is left is open cavity. Bags are that area plus waste, divided by a coverage you type from the label (default 88 sq ft, R-15 in 2x4 walls and R-21 in 2x6).
On the example house that gives 1,135 sq ft of cavity, 14 bags, and a measured framing share of 20.2% of the wall panels. That sits inside the usual rule-of-thumb range, but it moves with the plan: more openings, more jacks and headers, a higher share.
What it does not do: attic, ceiling joists, rim joists and air sealing are not counted, and the checks are hand arithmetic on invented walls, not a real builder's insulation order. If you have one, comparing it is the useful next step.
Code: ventures/wall-framing-takeoff/insulation.mjs.