Start from the required assembly and R-value, then use the selected manufacturer's coverage at the installed thickness and density.
Calculate by assembly
Do not combine attic, exterior wall, floor, and rim-joist areas when materials or target R-values differ.
Use installed performance
Compression, voids, wind washing, settling, moisture, and incorrect density can reduce performance. Follow tested assemblies and installation instructions.
Price the enabling work
Include air sealing, ventilation baffles, fire blocking, ignition or thermal barriers, access, protection, removal, and inspection requirements.