Free Insulation R-Value calculator
Size your insulation three ways in one tool — the R-value your climate zone needs, how many bags or batts to buy to hit a target R-value, and the total R-value of a layered wall — all from DOE and ENERGY STAR data and updated live, as you type.
On this page16 sections
| Zone | Attic | Wall | Floor |
|---|---|---|---|
| 1–2 | R30–49 | R13 | R13 |
| 3 | R49 | R20 | R19 |
| 4–6 | R60 | R20 | R19–30 |
| 7–8 | R60 | R21 | R38 |
Per ENERGY STAR / 2021 IECC for an uninsulated home. Top up to the recommended total if some insulation already exists.
R-value per inch and bag coverage vary by product. How accurate is this?
Results are estimates. Consult a professional.
How the insulation R-value calculator works
Insulation is rated by R-value, its resistance to heat flow, and the calculator answers three questions at once. It tells you the R-value your climate zone needs for an attic, wall or floor. It works out how much material to buy to reach a target R-value, in bags of blown-in or in batts. And it adds up the R-value of a layered assembly, because the R-values of stacked layers sum together.
What the result actually means
Three numbers do the work. The recommended R-value is your target — the depth your local code and climate ask for. The bag or batt count is what you carry out of the store to hit that target over a measured area. The assembly R-value tells you whether the layers you already have, or plan to stack, actually reach the number. The calculator shows all three so you can size the job before you buy.
What R-value means in insulation
R-value is resistance to conductive heat flow. The DOE puts it plainly: "the higher the R-value, the greater the insulating effectiveness." A material's R-value depends on its type, its thickness and its density, so the same product gives more R-value the thicker you install it.
Two rules make R-value easy to work with. First, R-value scales with thickness: doubling the depth roughly doubles the R-value for the same material. Second, R-values add. A wall built from drywall, a batt and exterior sheathing has a total R-value equal to the sum of each layer's R-value. That additive rule is what lets the calculator total a layered assembly.
How much insulation do I need by climate zone?
The R-value you need depends on where you live and which part of the home you are insulating. The DOE divides the United States into eight climate zones, from Zone 1 on the Gulf Coast and South Florida to Zone 8 in northern Alaska. Colder zones need higher R-values, and attics always need more than walls.
| Climate zone (example region) | Attic | Wall | Floor |
|---|---|---|---|
| Zone 1–2 (S. Florida, Gulf Coast) | R30–R49 | R13 | R13 |
| Zone 3 (Lower South) | R49 | R20 | R19 |
| Zone 4 (mid-Atlantic, Pacific NW) | R60 | R20 | R19 |
| Zone 5 (much of the Midwest) | R60 | R20 | R30 |
| Zone 6 (northern Midwest, New England) | R60 | R20 | R30 |
| Zone 7–8 (northern Minnesota, Alaska) | R60 | R21 | R38 |
Recommended R-values for an uninsulated home, per ENERGY STAR / 2021 IECC. Attic Zone 1 is R30; Zones 2–3 are R49; Zones 4–8 are R60. Wall figures are wood-frame cavity plus sheathing combined.
Pick your zone, read across to the area you are insulating, and that R-value becomes your target. If your attic already has a few inches of old insulation, you need less new material than an empty attic — top up to the recommended total rather than adding the full amount on top.
A worked example: insulating a 1,000 sq ft attic
Maria has a 1,000 sq ft attic in a Zone 5 (Midwest) home. ENERGY STAR recommends about R-49 for an attic that already has a few inches of old insulation, so she plans to blow in loose-fill cellulose to hit that target and wants the bag count plus a 10% margin.
Step 1 — Set the target R-value
For a Zone 5 attic with some existing insulation, the target is R-49. An empty attic in the same zone would aim for the full R-60.
Step 2 — Read coverage per bag at that R-value
From the cellulose coverage label, one 30 lb bag covers 14.9 sq ft at R-49. Coverage shrinks as the target rises: the same bag covers 26.5 sq ft at R-30.
Step 3 — Divide area by coverage
1,000 ÷ 14.9 = 67.1, rounded up to 68 bags for the bare attic.
Step 4 — Add the waste margin
With 10% added, 67.1 × 1.10 = 73.8, rounded up to 74 bags. Maria buys 74 bags to blow her 1,000 sq ft attic to R-49.
How much insulation to buy: bags and batts
Insulation is sold two ways, and the math differs for each. Blown-in loose fill comes in bags, and one bag covers a published number of square feet at a given R-value. Batts and rolls come in packages that cover a fixed area regardless of R-value, because the R-value is set by which batt you pick off the shelf.
Blown-in bags
Each bag has an FTC coverage chart printed on it. Find your target R-value, read the net square feet per bag, then divide your area by that number and round up. The table below is the published cellulose chart the calculator uses. Coverage falls as the target R-value climbs, so a deep attic eats bags fast.
| Target R-value | Net coverage per bag | Bags per 1,000 sq ft | Settled depth |
|---|---|---|---|
| R-13 | 66.7 sq ft | 15.0 | 3.6 in |
| R-19 | 44.2 sq ft | 22.6 | 5.3 in |
| R-30 | 26.5 sq ft | 37.8 | 8.2 in |
| R-38 | 20.1 sq ft | 49.7 | 10.3 in |
| R-49 | 14.9 sq ft | 67.3 | 13.2 in |
| R-60 | 11.6 sq ft | 86.4 | 16.1 in |
Greenfiber INS510LD loose-fill cellulose, net coverage at settled thickness, 30 lb bag (FTC coverage label). Other products and bag weights differ — confirm the chart on your bag.
Batts and rolls
A batt package lists its coverage in square feet — for example, an R-13 bundle covering about 88 square feet. Divide your wall or floor area by that coverage, round up, and add waste for trimming around studs, wiring and outlets. You choose the R-value by buying the right batt, so the only variable in the count is area.
Total R-value of a layered wall assembly
A real wall is several layers, and its R-value is the sum of them. Because R-values add, you find an assembly's total by multiplying each layer's thickness by its R-value per inch, then adding the results. This is how the calculator tells you whether a planned wall actually reaches its target.
Take a 3.5-inch fiberglass batt at R-3.2 per inch and add 1 inch of XPS rigid foam at R-5.0 per inch. The batt contributes 3.5 × 3.2 = R-11.2; the foam contributes 1 × 5.0 = R-5.0. The assembly totals R-16.2.
Adding exterior rigid foam is a common way to push a 2x4 wall past the cavity's limit and cut thermal bridging through the studs. The calculator's assembly mode lets you stack layers and watch the total climb until it meets the recommended R-value for your zone.
R-value per inch by insulation type
Every material delivers a different R-value per inch, and that figure decides how thick a layer must be to hit a target. Foam boards pack the most R-value into the least depth; loose fiberglass the least. These are mid-range published values; the exact rating varies by product and density.
| Insulation type | R-value per inch | Typical use |
|---|---|---|
| Blown fiberglass (loose fill) | 2.5 | Attics, open cavities |
| Fiberglass batt | 3.2 | Walls, floors between joists |
| Blown cellulose | 3.5 | Attics, dense-pack walls |
| Open-cell spray foam | 3.6 | Walls, sound control |
| Mineral (rock) wool | 3.7 | Walls, fire and sound |
| EPS rigid foam | 3.8 | Sheathing, below grade |
| XPS rigid foam | 5.0 | Sheathing, foundations |
| Polyiso rigid foam | 6.0 | Roof, exterior sheathing |
| Closed-cell spray foam | 6.5 | Rim joists, tight spaces |
Representative published R-value per inch (DOE / Home Depot / manufacturer charts). Polyiso loses some R-value in cold weather; confirm the rating on your product.
To find the depth a target needs, divide the target R-value by the material's R per inch. Reaching R-49 in cellulose at R-3.5 per inch takes about 14 inches; in closed-cell foam at R-6.5 per inch it takes under 8 inches.
Waste factor: how much extra insulation to buy
Buy the exact calculated amount and you will come up short. Batts get trimmed around studs, wiring and outlets, and the offcuts rarely fit elsewhere. Blown-in settles and some blows past the target depth in spots. A modest overage covers it.
The 10% rule
For most jobs, add 10%. On a clean open attic with blown-in, you can trim that to 5%. On cut-up walls with lots of framing, wiring and fixtures, 10% is the floor and some installers go to 15%. The calculator's waste field defaults to 10% and adjusts from 0 to 20%.
Why running short costs more
Coming up a few bags short means a second trip, and a missed patch in an attic shows up as a warm stripe on the ceiling in winter. The overage is cheap insurance against an under-insulated section you would have to reopen to fix. When in doubt, round up.
Insulation definitions
Sources and references
Every figure on this page is drawn from a published authority. The recommended R-values come from ENERGY STAR and the 2021 IECC, the additive-layer rule from the U.S. Department of Energy, and the bag coverage chart from the Greenfiber FTC product label.
ENERGY STAR — Recommended Home Insulation R-Values by climate zone (2021 IECC).U.S. Department of Energy, Energy Saver — Insulation: R-value definition and the additive-layer rule.Greenfiber INS510LD loose-fill cellulose — FTC coverage label (net coverage and settled thickness per R-value).How accurate is this insulation calculator?
The R-value math is exact. Layer thickness times R-value per inch, summed across layers, is the precise calculated R-value of an assembly, and the recommended R-values come straight from the ENERGY STAR / 2021 IECC tables. If your measurements and material ratings are right, the totals are right.
The quantities are sound estimates. Bag coverage follows the published cellulose FTC label, but other brands and bag weights cover slightly different areas, and settling, equipment and technique shift real coverage by up to 10%. R-value per inch varies by product and density, and rigid foams like polyiso lose some R-value in cold weather. Treat the bag and batt counts as planning figures, confirm the coverage chart printed on your product, and check your zone's requirement against local code. For related material take-offs, see our drywall calculator and concrete calculator. When in doubt, order to the high side — running short of an insulation job is the costlier mistake.
Frequently asked questions about the free Insulation R-Value calculator
About this insulation R-value calculator
This insulation R-value calculator runs entirely in your browser. Nothing you enter is sent anywhere — every R-value, bag count and assembly total is computed live on your device as you change the inputs.
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