How the attic R-value calculator works
Depth × R-per-inch = what you have
Loose-fill insulation settles and slumps, so its R-value lives in the depth: cellulose delivers R-3.2 per inch, blown fiberglass R-2.7, batts near their nominal rating only when uncompressed and split-faced around wiring. Six inches of 1990s blown fiberglass is roughly R-16 — a number that explains a lot of upstairs bedrooms.
The gap to target
Your climate zone sets the ceiling: R49 for most of the mixed-climate band, R60 up north. The calculator computes the missing R and converts it to inches of your chosen add-on — typically 8–12 inches of cellulose turns a R-16 attic into a code-current one.
Before you insulate: air sealing
Insulation slows conduction; it does nothing about air leaks — the recessed lights, bath-fan housings, and plumbing chases that leak heated air straight into the attic. Seal first (caulk, foam, flashings), then insulate: the combination outperforms either alone, and blown insulation over unsealed leaks buries the problem instead of fixing it.
Multiply the R-values, not the insulation layers
R-values add linearly across layers — R-16 plus R-19 of new cellulose is R-35. What does NOT add: compressing old batts under new ones (crushed fiberglass loses R per inch fast) or the framing itself (see the whole-wall discussion in our guides).