Insulation Calculator

Inches of insulation by type to hit your target R-value, with cost. Updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Depth needed = target R-value ÷ R-per-inch for your material. Cost estimates use 2026 installed averages: fiberglass batts ~$0.90/sq ft, blown cellulose ~$1.45/sq ft (attic at R-49), open-cell spray foam ~$0.55 and closed-cell ~$1.25 per board foot (1 sq ft at 1 inch). DIY blown-in with a rented machine typically halves the cellulose figure.

The waste allowance

Insulation is not ordered with a percentage waste factor the way concrete or tile is, and adding one is usually the wrong instinct. Batts are cut to fit and the offcuts get used in short bays and around obstructions, so the practical allowance is small. Blown-in material is the opposite problem: coverage is printed on the bag for a target depth, and the real risk is buying too few bags because the chart was read as though the attic were bare. What you should build in instead is depth. Settling, joists that are not level with one another and a hatch or two that never gets covered all pull the finished average below the number you aimed for, so mark the target depth on the rafters and blow to the mark rather than to the bag count.

R-value per inch is the whole calculation

Every depth figure this calculator produces comes from one division: the target R-value divided by the R-value per inch of the material. The Department of Energy publishes those per-inch figures as ranges, because product density and formulation vary within each category. Standard fiberglass batt runs R-2.9 to R-3.8 per inch, and high-performance batt R-3.7 to R-4.3. Blown fiberglass sits lower at R-2.2 to R-2.7, and blown cellulose higher at R-3.2 to R-3.8.

Spray foam splits sharply by cell structure. Closed-cell is about R-6.2 per inch and open-cell about R-3.7, which is close to a good batt. Rigid foam board is published by DOE as a single category at R-4 to R-6.5 per inch. DOE does not break that out into XPS, EPS and polyiso individually, so if you are designing to a specific rigid foam assembly you need the manufacturer's data sheet for the actual product rather than the category range.

The other thing that governs real R-value is loft. Fiberglass insulates because it holds still air in a designed thickness. Squeeze an R-19 batt into a cavity shallower than it was made for and you do not get R-19 in less space, you get less than R-19, because the material has lost the loft it depends on. Match the batt to the cavity depth rather than forcing it.

What ENERGY STAR recommends by climate zone

Target R-values are set by climate, and ENERGY STAR publishes them by zone with two attic numbers rather than one. The higher figure applies to an uninsulated attic, and the lower to an attic that already has some insulation and is being topped up. That distinction matters more than it looks: it is the difference between a full depth of new material and a top-up layer over what is already there.

For attics, Zone 1 is R30 uninsulated or R25 topping up. Zones 2 and 3 are R49 or R38. Zones 4A and 4B, Zone 4C together with 5 and 6, and Zones 7 and 8 all carry the same attic recommendation of R60 uninsulated or R49 topping up. In other words, once you are north of the mild southern zones, the attic target does not keep climbing with the climate.

Floors follow a different curve. Zones 1 and 2 call for R13, Zone 3 through 4B for R19, Zones 4C, 5 and 6 for R30, and Zones 7 and 8 for R38. These are recommendations for energy performance, not code minimums, and your local energy code may require something different. Check both before you buy.

Blown-in coverage comes off the bag

There is no universal figure for how many square feet a bag of blown insulation covers, and anyone publishing one is guessing. Coverage depends on the product, its settled density and the depth you are blowing to, and it differs between manufacturers and even between products from the same manufacturer.

You do not have to guess, because the number is printed on every bag. The FTC's R-value Rule requires the coverage chart on the packaging, giving bags per 1,000 square feet, minimum thickness and the resulting R-value. That chart is the authoritative figure for the product in your hands, and it is the one to plan against.

The trap in those charts is that they assume you are covering bare joists to a target depth. If the attic already has six inches of old fiberglass and you are adding to it, the bag count from the chart is for the full depth, not for the depth you still need. Work out the additional inches required to reach your target, then read the chart at that depth. Ignoring the existing layer is the most common way people end up with a pallet of bags they did not need, or several too few.

Cost per R, and where spray foam earns its keep

Spray foam gets treated as an automatic upgrade, and the comparison that actually matters is rarely made. Closed-cell foam has the highest R per inch of anything in common residential use, but it also costs far more per unit of R than open-cell foam or batts. Where depth is genuinely limited, paying that premium buys something real. In an open attic floor with unlimited depth available, it buys very little that cheaper material could not.

The two foams are also not interchangeable in function. Closed-cell foam acts as a vapour retarder at typical installed thicknesses; open-cell does not, and assemblies designed around closed-cell behaviour can misbehave if open-cell is substituted for cost reasons. That is an assembly decision, not a price decision.

The practical way to choose is to work out cost per R for each option over the actual area, then let the constraints decide the rest. Cathedral ceilings, rim joists and shallow cavities reward high R per inch. Accessible attic floors reward cost per R. Most houses want both approaches in different places.

Recommended R-values by climate zone

ENERGY STAR recommendations. The two attic figures are for an uninsulated attic and for topping up an attic that already has some insulation.

Attic and floor recommendations from ENERGY STAR's insulation guidance by climate zone. Per-inch R-values used elsewhere on this page are from the US Department of Energy. These are performance recommendations, not code minimums.

What goes wrong insulating

Compressing a batt into a shallow cavity. Fiberglass works by holding still air at its designed loft. An R-19 batt squeezed into a shallower bay delivers less than R-19, not R-19 in less space.

Reading the bag chart as though the attic were bare. Blown-in coverage charts assume you are covering bare joists to a target depth. With existing insulation in place, work out the additional inches you need and read the chart at that depth.

Assuming spray foam is simply better. Closed-cell has the best R per inch and the worst cost per R. Where depth is not constrained, batts or blown cellulose reach the same target for far less.

Substituting open-cell for closed-cell to save money. Closed-cell acts as a vapour retarder at typical thicknesses and open-cell does not. Swapping one for the other changes how the assembly handles moisture, not just its price.

Using a category R-value for a specific board. DOE publishes rigid foam as a single R-4 to R-6.5 per inch range without breaking out XPS, EPS and polyiso. For a real assembly, use the manufacturer's data sheet for the product you are buying.

Frequently asked questions

Why your state changes the price (but not the quantity)

How much insulation you need is physics — it doesn't care where you live. What it costs does. Labor rates, delivery distance, fuel, and local demand move installed prices by roughly 45 points from end to end: Arkansas, Mississippi, North Carolina sit near the bottom of the range, while Alaska, Hawaii, New York sit at the top. The Price this for selector at the top of the calculator applies a published construction cost index for your state to the 2026 national averages, so the dollar figures move and the material list stays put.

Treat the result as a budgeting range, not a quote. Two metros inside the same state can differ by more than the statewide index suggests, and a single busy contractor can outweigh both. See the full cost index for all 50 states and where the price data comes from.