Pick your state, then how much insulation is up there now. The target R-values come straight from the U.S. Department of Energy's recommendations for existing wood-framed homes (DOE/CE-0180, Oak Ridge National Laboratory) — they are not our numbers.
A state can span two or three climate zones. Pick whichever line names a town nearest you. For a definitive answer, look your county up in the 2021 IECC county map.
Measure against a joist. A standard 2×6 joist is 5½ inches deep — if the insulation sits well below the top of the joist, you are in the second case at best.
Roughly your home's footprint. A 1,200 sq ft single-storey house has about 1,200 sq ft of attic floor.
How to read the number you just got
R-value measures resistance to heat flow, and it stacks: R30 of existing insulation plus R19 of new insulation gives you R49. That is why the "existing 3–4 inches" answer changes the target — you are not starting from zero, so you need less new material to land in the same place.
The thing most people get wrong is treating R-value as the whole story. It is not. A perfectly specified R60 attic with air leaking through recessed light housings, the attic hatch and every plumbing penetration will underperform a sealed R38 attic. Air sealing first, insulation second — in that order, every time. We wrote up what that actually looks like in the one number that actually matters.
Depth is not the same as performance
Blown products are rated at a settled density. Blow cellulose in too light and it will drift down over the first winter, taking your R-value with it — the bag tells you how many square feet it covers at a given R-value, and that number assumes you use the whole bag. Count bags, not inches.
Batts fail differently. They are rated for a clean, correctly sized cavity, and any gap, compression or bunching around wiring drops the effective R-value of that bay hard. If your joist bays are irregular, blown material will outperform batts no matter what the label claims.
Where the money actually goes
The table gives materials only. Installed pricing depends far more on access than on square footage: a walk-up attic with a floor already down is a different job from a 3-foot crawl over loose joists, and quotes reflect that. If you are getting it done, get the air sealing quoted as a separate line item so you can see whether it is actually included.
Related reading from our own attempts at this: attic insulation costs in detail, what blowing cellulose is really like, fiberglass batts and the itch, why DIY spray foam kits go wrong, and what four inches of settled insulation was costing us every month.
Source and method
Target R-values are the U.S. Department of Energy's published recommendations for existing wood-framed houses (DOE/CE-0180, Insulation Fact Sheet, Oak Ridge National Laboratory), which give a cost-effective range per climate zone rather than a single figure. Climate zones follow the 2021 IECC map. State-to-zone matching here is approximate because most states span more than one zone — for a definitive answer, look up your county in the 2021 IECC county map. R-per-inch figures are standard published values for each material; cost ranges are materials-only US averages and are deliberately wide.