Insulation Calculator
Find the ENERGY STAR R-value for your attic, floor or walls by IECC climate zone, the R to add over existing insulation, and the depth, batts or bags to buy.
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About this toolWhat is the Insulation Calculator?
How much insulation you need depends on where you live and what is already there. ENERGY STAR publishes recommended R-values for each IECC climate zone, from R30 in an uninsulated attic in Zone 1 to R60 across Zones 4 to 8. This insulation calculator looks up that target for an attic, a floor over unheated space, a basement or crawlspace wall or an exterior wall, subtracts the R-value of the insulation you have, and tells you how much to add.
Then it turns the shortfall into material. For blown cellulose or fiberglass it gives the settled depth and the number of bags, from either a density estimate or the coverage chart printed on the bag. For batts it gives the layers of the product you choose and the packs, and for spray foam the depth, board feet and kits.
R-values are shown with their metric RSI equivalents, and areas can be entered in square feet or square metres. Nothing leaves your browser.
- ENERGY STAR targets for nine climate zone groups and five locations
- Existing insulation credited from depth and type
- Blown cellulose and fiberglass by density or bag chart, batts by layers, spray foam by board feet
- R-value and RSI side by side; imperial or metric areas
- Material cost and a recommendation table for your zone
How to use the Insulation Calculator
- Pick your IECC climate zone (example cities are listed) and where the insulation goes.
- Enter the area. For an attic this is the attic floor, usually the house footprint.
- Enter the type and depth of any existing insulation. The target R-value fills in from ENERGY STAR; type your own to override it.
- Choose the material to add: blown cellulose or fiberglass, batts, or open- or closed-cell spray foam, and fill in its details.
- Read the R to add, the depth and the bags, packs or kits, then add a price to see the material cost.
Worked examples
Zone 5 attic with 3 inches of old batts
Input
1,000 ft², existing 3 in fiberglass batts, blown cellulose at R-3.5 per inch, 1.6 lb/ft³, 25 lb bags
Output
Target R-60, existing R-9.3, add R-50.7: 14.5 in settled, about 78 bags
The existing layer is 3 x 3.1 = R-9.3. Adding R-50.7 of cellulose at 3.5 per inch needs 14.5 inches. Check the bag chart for your product before buying.
Zone 3 attic in batts
Input
800 ft², uninsulated, R-30 batts, 50 ft² per pack
Output
Target R-49: two layers of R-30 (R-60), 32 packs
One layer of R-30 falls short of R-49, so two layers are needed. Lay the second layer at right angles to the joists to cover them.
Rim joists with closed-cell foam
Input
200 ft², target R-13, closed-cell foam at R-6.5 per inch, 200 board-foot kits
Output
2 in of foam, 400 board feet, 2 kits
Board feet are square feet times inches, which is how DIY spray foam kits are sized.
Background and insight
The recommendations come from the ENERGY STAR "Recommended Home Insulation R-Values" table, which is based on the 2021 International Energy Conservation Code, Table R402.1.3. For an uninsulated attic it gives R30 in Zone 1, R49 in Zones 2 and 3, and R60 in Zones 4 to 8; where 3 to 4 inches already exist it suggests adding R25, R38 or R49. Floors over unconditioned space range from R13 in the warmest zones to R38 in Zones 7 and 8. The page also lists basement and crawlspace wall options of R5 to R15 continuous sheathing or an R13 to R19 batt from Zone 3 north.
Loose-fill R-value depends on the material and on settling. Typical settled values are about R-3.2 to 3.8 per inch for cellulose and R-2.2 to 2.7 per inch for loose fiberglass, so reaching R-60 takes roughly 16 to 19 inches of cellulose or 22 inches or more of fiberglass. The US Federal Trade Commission R-Value Rule requires loose-fill bags to carry a coverage chart that lists, for each R-value, the minimum settled thickness, the minimum weight per square foot and the coverage per bag. That chart is the authority for your product, and the calculator lets you use it directly.
Existing insulation is estimated from its depth and type: old fiberglass batts about R-3.1 per inch, loose fiberglass about 2.5 and cellulose about 3.5. Compressed, wet or patchy insulation performs worse, so measure in several places. Air sealing the attic floor before adding insulation often saves as much energy as the insulation itself.
Common use cases
Attic top-ups
See how far an older attic is from today's recommendation and how many bags close the gap.
Floors over crawlspaces and garages
Find the floor R-value for your zone and the batts that fit the joist depth.
Basement walls
Compare continuous rigid sheathing with a cavity batt on a framed basement wall.
Rebate and contractor quotes
Check that a quote reaches the R-value that utility programmes and tax credits usually ask for.
Tips and limitations
- Air-seal top plates, plumbing and wiring holes and attic hatches before adding insulation.
- Keep loose fill away from recessed lights that are not IC-rated and from chimneys, and keep soffit vents open with baffles.
- Use unfaced batts when laying over existing insulation, so you do not add a second vapour retarder.
- Mark the target depth on rulers stapled to the joists before blowing insulation.
- Spray foam R-values vary by product; use the data sheet figure, and hire a pro for large areas.
Frequently asked questions
What R-value should my attic insulation be?
ENERGY STAR recommends R30 in Zone 1, R49 in Zones 2 and 3, and R60 in Zones 4 to 8 for an uninsulated attic, based on the 2021 IECC. If you already have 3 to 4 inches, it suggests adding R25, R38 or R49 respectively.
How many inches of blown-in insulation make R-60?
About 16 to 19 inches of settled cellulose at R-3.2 to 3.8 per inch, or about 22 to 27 inches of loose fiberglass at R-2.2 to 2.7 per inch. The coverage chart on your bag gives the exact minimum thickness for your product.
How many bags of blown insulation do I need for 1,000 square feet?
Read the coverage chart on the bag: it lists the area one bag covers at each R-value. As a rough estimate, cellulose at about 1.6 lb per cubic foot needs about 78 bags of 25 lb to add R-50 over 1,000 square feet.
How do I work out the R-value of my existing insulation?
Measure its depth in several places and multiply by R per inch: about 3.1 for fiberglass batts, 2.5 for loose fiberglass and 3.5 for cellulose. Compressed or damp insulation is worth less.
Can I put new insulation on top of old insulation?
Usually yes, if the old insulation is dry and not mouldy. Use unfaced batts or loose fill so you do not trap moisture with a second vapour retarder, and fix roof leaks first.
What is the difference between R-value and RSI?
RSI is the metric thermal resistance in m²·K/W. Multiply an R-value by 0.1761 to get RSI, so R-60 is about RSI 10.6.
How do I find my IECC climate zone?
Climate zones follow county lines and are mapped in the IECC and by the US Department of Energy. Roughly, the Gulf Coast is Zone 2, the southern band from Atlanta to Los Angeles Zone 3, the mid-Atlantic and Seattle Zone 4, the northern states Zones 5 and 6, and northern Minnesota and Alaska Zones 7 and 8.
Is there an in-depth guide to the Insulation Calculator?
Yes. “Attic Insulation R-Values by Climate Zone, and How to Get There” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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