Insulation estimating starts with a requirement you don't get to choose freely: the R-value required for a given assembly (wall, ceiling, floor) is typically set by local energy code and climate zone, not by preference — so the first step in any insulation takeoff is confirming the required R-value for each assembly type before pricing material.
Coverage area
Coverage area follows the same wall/ceiling square footage you'd use for drywall, minus framing member area (studs and plates don't get insulated) — batts and rolls are typically sold by coverage area per package, so the takeoff converts square footage directly into a package count once the R-value and product line are selected.
Material selection
Material selection affects both cost and technique: fiberglass batts are the most common and least expensive option for standard stud-bay applications; blown-in cellulose or fiberglass suits irregular cavities and attic spaces where batts don't fit well; spray foam costs significantly more per square foot but achieves higher R-value per inch of thickness, which matters in space-constrained assemblies.
Easy-to-miss line items
All of these show up in a code-compliant insulation package but not in a simple square-footage-only takeoff.