Most US attics should be insulated to R-49 or R-60 — R-30 only in zone 1 — per ENERGY STAR's recommended levels, which follow the 2021 IECC. This insulation calculator finds the recommended R-value for your climate zone and the part of the house you're insulating, then counts the batts, layers and bags you need and prices the job.
It's for homeowners topping up an attic, remodelers insulating new walls, and anyone comparing batts against blown-in insulation. You need your climate zone, the area in square feet, the framing spacing and the batt product you plan to use.
How to use the insulation calculator
- Find your IECC climate zone in PNNL's county guide or the DOE climate zone lookup, and pick it.
- Choose where you're insulating: an empty attic, an attic with 3–4 in already, a floor over a crawlspace or garage, a wall, or a basement wall.
- Enter the area. For walls, subtract windows and doors.
- Pick the framing spacing (16 or 24 in on center), the batt R-value and the batt length. For rolls, enter the roll length in inches.
- Read the recommended R-value, the layers needed to reach it, the batt count and depth. Add batts per bag and the price from the label to get bags and cost.
Recommended R-values by climate zone
Attic, floor and basement levels below are ENERGY STAR's recommendations for existing wood-framed homes, which it bases on Table R402.1.3 of the 2021 IECC. Wall values are the 2021 IECC prescriptive options as summarized by the U.S. Department of Energy for new walls. Your local code may be older or amended, so check with the building department before a permitted job.
| Zone | Attic (empty) | Attic (has 3–4 in) | Floor | Wood-frame wall (2021 IECC) | Basement / crawl wall |
|---|---|---|---|---|---|
| Zone 1 | R-30 | R-25 | R-13 | R-13, or R-10 continuous | Not listed |
| Zone 2 | R-49 | R-38 | R-13 | R-13, or R-10 continuous | Not listed |
| Zone 3 | R-49 | R-38 | R-19 | R-20, or R-13 + R-5 ci, or R-15 ci | R-5 ci or R-13 cavity |
| Zone 4A / 4B (4 except Marine) | R-60 | R-49 | R-19 | R-20 + R-5 ci, or R-13 + R-10 ci, or R-20 ci | R-10 ci or R-13 cavity |
| Zone 4C (Marine) | R-60 | R-49 | R-30 | R-20 + R-5 ci, or R-13 + R-10 ci, or R-20 ci | R-15 ci or R-19 cavity |
| Zone 5 | R-60 | R-49 | R-30 | R-20 + R-5 ci, or R-13 + R-10 ci, or R-20 ci | R-15 ci or R-19 cavity |
| Zone 6 | R-60 | R-49 | R-30 | R-20 + R-5 ci, or R-13 + R-10 ci, or R-20 ci | R-15 ci or R-19 cavity |
| Zone 7 | R-60 | R-49 | R-38 | R-20 + R-5 ci, or R-13 + R-10 ci, or R-20 ci | R-15 ci or R-19 cavity |
| Zone 8 | R-60 | R-49 | R-38 | R-20 + R-5 ci, or R-13 + R-10 ci, or R-20 ci | R-15 ci or R-19 cavity |
"ci" means continuous insulation, typically foam or mineral-wool board over the sheathing, which stops heat flowing through the studs. A Delaware state energy office summary of the 2021 IECC also lists an R-30 cavity-only wall option and lets R-60 attic insulation drop to R-49 where full depth extends over the wall top plates at the eaves; those are code details to confirm locally.
Batt R-values, thickness and sizes
The thickness of a fiberglass batt sets which framing it fits. These are Owens Corning EcoTouch values; Johns Manville's data sheet lists some ratings at different thicknesses (for example R-30 at 10-1/4 in), so read your bag.
| Batt | Thickness (in) | Typical use |
|---|---|---|
| R-11 | 3.5 | 2×4 walls (3-1/2 in cavity) |
| R-13 | 3.5 | 2×4 walls (3-1/2 in cavity) |
| R-15 | 3.5 | 2×4 walls (3-1/2 in cavity) |
| R-19 | 6.25 | 2×6 walls and floors |
| R-21 | 5.5 | 2×6 walls and floors |
| R-30 | 9.5 | Attics, floors and cathedral ceilings |
| R-38 | 12 | Attics, floors and cathedral ceilings |
| R-49 | 14 | Attics, floors and cathedral ceilings |
Owens Corning makes wall batts 15 or 23 inches wide for studs 16 or 24 inches on center, in 93-inch lengths; R-30 and thicker ceiling batts are 16 or 24 inches wide and 48 inches long. Johns Manville's standard lengths are 48, 93 and 96 inches.
How the batt count is calculated
A batt fills one framing bay, so its coverage is the on-center spacing × batt length ÷ 144 square feet: 10.33 sq ft for a 93-in batt at 16 in on center and 8 sq ft for a 48-in batt at 24 in. Batts = area × (1 + waste) ÷ coverage, rounded up. Layers = recommended R ÷ batt R, rounded up, since R-values of stacked layers add. Bags = batts ÷ pieces per bag, rounded up.
Worked examples
- Zone 5 attic, 1,000 sq ft, empty: target R-60. Two layers of R-30 (R-60) at 9.5 in each. 48-in batts on 24-in joists: 125 per layer, 250 total; with 5% waste 264.
- Zone 3 attic with 3–4 in already: ENERGY STAR's level for this case is R-38 of added insulation, so one layer of R-38 batts (12 in) meets it.
- 2×4 wall, 400 sq ft net, 16 in o.c.: R-13 batts, 93 in long: 41 batts with 5% waste.
- Floor over a zone 5 crawlspace, 600 sq ft, 16 in o.c.: R-30; 48-in R-30 batts: 113 batts.
Batts needed by area (one layer)
| Area | 93 in batts, 16 in o.c. | 93 in batts, 24 in o.c. | 48 in batts, 16 in o.c. | 48 in batts, 24 in o.c. |
|---|---|---|---|---|
| 100 sq ft | 10 | 7 | 19 | 13 |
| 250 sq ft | 25 | 17 | 47 | 32 |
| 500 sq ft | 49 | 33 | 94 | 63 |
| 750 sq ft | 73 | 49 | 141 | 94 |
| 1,000 sq ft | 97 | 65 | 188 | 125 |
| 1,500 sq ft | 146 | 97 | 282 | 188 |
| 2,000 sq ft | 194 | 130 | 375 | 250 |
Batts or blown-in?
Batts suit open wall and floor framing and attics with clear, regular joist bays. Blown-in loose fill flows around wiring, blocking and irregular spaces and reaches R-49 or R-60 in a single layer (Owens Corning's AttiCat chart lists R-60 at 20-1/2 in). To count bags of loose fill from the manufacturer's label chart, use the blown-in insulation calculator.
Measuring the area
For an attic, measure the length and width of the insulated ceiling below it, not the roof. Leave out areas over unheated porches and garages unless you're insulating them too, and add separate rectangles for an L-shaped house. For walls, multiply the perimeter of the exterior walls by the wall height, then subtract the window and door openings. For a floor over a crawlspace, measure the crawlspace footprint. If your framing isn't a standard 16 or 24 in on center, pick the closer value; a batt count per bay is then still right, and the waste allowance covers short end bays.
Odd-shaped bays, blocking and plumbing all create offcuts. A small waste allowance covers them on a clean new wall; a cluttered attic with many short bays needs more. If you're insulating between rafters in a cathedral ceiling, check the depth of the rafters first: an R-30 batt at 9.5 in won't fit a 2×8 (7-1/4 in actual) without compression.
Common mistakes
- Compressing thick batts into thin cavities. An R-19 batt squeezed into a 2×4 wall does not give R-19.
- Leaving gaps. Cut batts to fit snugly around wiring and boxes rather than stuffing them behind.
- Burying non-IC recessed lights. Both Owens Corning and Johns Manville say to keep insulation at least 3 in away from fixtures not rated for insulation contact.
- Leaving kraft or foil facing exposed. The makers' data sheets say these facings will burn and must be covered by an approved finish.
- Skipping air sealing. Insulation slows heat flow but doesn't stop air leaks; DOE's home-improvement credit guidance treats air sealing and insulation together.
Related calculators
Measure an attic with the roof pitch calculator, add up fractional wall lengths with the construction calculator, or price an exterior job with the siding calculator and renovation cost calculator.
Frequently asked questions
What R-value do I need in my attic?
ENERGY STAR recommends R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8 for an uninsulated attic. If the attic already has 3–4 inches, it suggests adding up to R-25, R-38 and R-49 respectively.
What R-value should exterior walls have?
For new 2021 IECC construction, DOE lists R-13, or R-10 continuous in zones 1–2; R-20, or R-13 + R-5 ci, or R-15 ci in zone 3; and R-20 + R-5 ci, or R-13 + R-10 ci, or R-20 ci in zones 4–8 (ci = continuous insulation outside the studs). For an existing wall, ENERGY STAR suggests blowing insulation into empty cavities and adding insulating sheathing when the siding comes off.
What R-value do floors over a crawlspace need?
ENERGY STAR lists R-13 in zones 1–2, R-19 in zones 3 and 4A/4B, R-30 in zones 4C, 5 and 6, and R-38 in zones 7–8 for floors over unheated space.
How many batts do I need for 1,000 square feet of attic?
With 48-inch batts on 24-inch joists, each batt fills 8 sq ft, so one layer takes 125 batts (132 with 5% waste). Reaching R-60 with R-30 batts takes 2 layers, so double that count.
How do I find my climate zone?
Look up your county in PNNL's Guide to Determining Climate Zone by County (IECC 2021 updates) or the DOE Building America Solution Center climate zone lookup. The Solution Center's key runs from 1A (very hot-humid) to 8 (subarctic); a C after the number marks a marine zone.
How thick is R-30, R-38 and R-49 batt insulation?
In Owens Corning's EcoTouch data sheet R-30 is 9.5 in, R-38 is 12 in and R-49 is 14 in thick. Johns Manville lists R-30 at 10-1/4 in and R-49 at 13-1/2 in, so check the bag for the exact product.
Can I put R-19 in a 2x4 wall?
No — R-19 batts are 6.25 in thick and a 2x4 stud is 3-1/2 in deep. Compressing a batt lowers its R-value; Owens Corning rates its R-19 at R-18 even in a 5-1/2 in (2x6) cavity. Use R-13 or R-15 for 2x4 walls.
Should I use 16-inch or 24-inch wide batts?
Match the framing: 15-inch batts for studs 16 in on center and 23-inch batts for 24 in on center (16 and 24 in wide for the thicker ceiling batts). Batts are sized to friction-fit between framing members.
How many bags of insulation do I need?
Divide the number of batts by the batts per bag printed on the package. Pieces per bag vary by R-value, width and brand, so the calculator asks you to read it from the label instead of guessing.
Does adding more insulation over existing insulation work?
Yes. R-values of layers add, so R-19 existing plus R-30 new gives about R-49.
Sources & method
- ENERGY STAR — Recommended Home Insulation R-Values (attic, floor and basement levels by zone, based on 2021 IECC Table R402.1.3)
- U.S. Department of Energy — Energy-Efficient Home Improvement Credit: Insulation and Air Sealing Essentials (2021 IECC ceiling and wood-frame wall R-values)
- PNNL — Guide to Determining Climate Zone by County: Building America and IECC 2021 Updates (2022)
- DOE Building America Solution Center — Climate zone lookup by county or ZIP
- Delaware DNREC — 2021 IECC changes, residential (state energy office summary)
- Owens Corning — EcoTouch PINK Fiberglas Insulation data sheet, Pub. No. 10013811-E (R-value, thickness, widths and lengths)
- Johns Manville — Formaldehyde-Free Fiberglass Insulation data sheet BID-0144 (standard sizes and lengths)
Results are estimates for general information. Found an error? It helps everyone — see our methodology.