Insulation Calculator

An insulation calculator answers three questions in a row: what R-value your climate calls for, how much of it you already have, and how many inches or bags of new material close the gap. A 1,000 sq ft Chicago attic with 3.5 inches of old fiberglass batts sits at R-11.2; reaching the recommended R-60 means adding R-48.8, which is 13.2 inches of blown cellulose, and the furnace then burns about 107 fewer therms of gas a year.

Insulation is one part of a bigger energy picture. The BTU Calculator sizes the heating and cooling equipment that the insulation protects, the Square Footage Calculator measures irregular attics and rooms, and the Drywall Calculator handles the wallboard when an open wall cavity gets insulated during a remodel.

Recommended R-values by climate zone

ENERGY STAR publishes the levels that are cost-effective when you add insulation to an existing wood-framed home, based on Table R402.1.3 of the 2021 International Energy Conservation Code. The two right-hand columns convert the uninsulated-attic level into depth with DOE's R-value per inch for blown cellulose (R-3.7) and blown fiberglass (R-2.5):

Walls follow a different rule because the cavity size is fixed. Whenever siding comes off an uninsulated wood-frame wall, ENERGY STAR recommends blowing insulation into the empty cavities, then adding insulated sheathing under the new siding: R-5 in zone 3 and R-5 to R-10 in zones 4 to 8. An already insulated 2x4 wall in zones 4 to 8 gets R-10 of sheathing. For basement and crawlspace walls the levels are R-5 sheathing or an R-13 batt in zone 3, R-10 or R-13 in zones 4A and 4B, and R-15 or R-19 in zones 4C and 5 to 8.

The formulas behind the calculator

Every result on this page comes from five short relationships. R-values of stacked layers add, which is why the insulation already in your attic counts toward the target:

HDD stands for heating degree days. EIA defines them against a 65°F base: a day with a mean temperature of 30°F contributes 35 degree days, and the year's total is a compact measure of how much heating a location needs. Multiplying by 24 turns degree days into degree hours, and dividing by R gives the Btu that leak through each square foot. The calculator adds R-2 to every case for the drywall and the still air on both faces, so an uninsulated starting point does not divide by zero.

Worked example: topping up a Chicago attic to R-60

Take a 1,000 sq ft attic in Chicago (Cook County, zone 5) holding 3.5 inches of fiberglass batts. At DOE's R-3.2 per inch that is 3.5 × 3.2 = R-11.2. ENERGY STAR's zone 5 level is R-60, so the job is to add R-48.8:

• Depth of blown cellulose: 48.8 ÷ 3.7 = 13.2 inches
• U-factor: 1 ÷ 11.2 = 0.089 before, 1 ÷ 60 = 0.017 after
• Volume: 1,000 × 13.19 ÷ 12 = 1,099 cubic feet, or 66 to 88 bags of 25 lb at 1.5 to 2.0 lb per cubic foot
• Heat loss with NOAA's 5,974 heating degree days: 1,000 × 5,974 × 24 ÷ (11.2 + 2) = 10.9 million Btu a year before, and ÷ (60 + 2) = 2.3 million Btu after

The 8.5 million Btu saved, delivered by a gas furnace at 80% AFUE, is 8,549,302 ÷ (100,000 × 0.80) = 106.9 therms. At EIA's 2025 residential average of $1.48 per therm that is $158 a year, so a $2,000 job pays for itself in 12.6 years from heating alone. These are the calculator's default inputs, so you can reproduce every figure by loading the page.

Worked example: an Atlanta attic with blown fiberglass

Now a milder climate: a 1,500 sq ft attic in Atlanta (zone 3, 2,531 heating degree days) with 6 inches of loose fiberglass, heated by a heat pump. The old insulation is 6 × 2.5 = R-15, and the zone 3 target of R-49 leaves R-34 to add. Blown fiberglass at R-2.5 per inch needs 34 ÷ 2.5 = 13.6 inches, which is 1,700 cubic feet, or 34 to 68 bags of 25 lb at 0.5 to 1.0 lb per cubic foot.

Heat loss falls from 1,500 × 2,531 × 24 ÷ 17 = 5.4 million Btu to 1.8 million Btu, saving 3.6 million Btu. A heat pump at the federal minimum of HSPF2 7.5 delivers 7,500 Btu per kWh, so the saving is 3,573,176 ÷ 7,500 = 476 kWh, or $82 a year at 17.30 cents. Against an $1,800 quote that is a 21.8 year payback. Atlanta also logs 2,051 cooling degree days a year; the summer benefit is real but not counted here.

Depth needed for common R-values, by material

The same R-value can be 2 inches thick or 24, depending on the material. This table divides each common target by the R-value per inch that DOE lists for the material, which is the same arithmetic the calculator runs:

Fiberglass batts are labeled by R-value rather than depth: DOE lists R-11 (low density), R-13 (medium) and R-15 (high density) batts for a 2x4 wall, R-21 for a 2x6 wall and R-38 batts for 12 inch spaces, so treat the batt row as a guide to stacking. Compressing a batt into a shallower space lowers its R-value, which is why an R-19 batt rated for 6¼ inches loses performance in a 5½ inch wall cavity.

How many bags of blown-in insulation you need

Loose-fill bags are sold by weight, so the estimate runs through volume and density: cubic feet = area × depth ÷ 12, pounds = cubic feet × settled density, bags = pounds ÷ bag weight. DOE's Building America Solution Center gives settled densities of 1.5 to 2.0 lb per cubic foot for loose-fill cellulose and 0.5 to 1.0 lb for loose-fill fiberglass, which is why the calculator shows a range rather than a single number.

The exact answer is on the bag. The FTC R-value Rule requires every loose-fill package to carry a chart with the minimum settled thickness, initial installed thickness, maximum net coverage, bags per 1,000 sq ft and minimum weight per square foot at R-13, R-19, R-22, R-30, R-38 and R-49. Read the row for your R-value to add, multiply by your area in thousands of square feet, and you have the count for that brand.

What climate zone are you in?

The 2021 IECC assigns a zone to every county, so many states span two or more. The table below combines DOE's county list (PNNL, 2022) with the Census Bureau's 2025 county population estimates to show the zones in each state and the zone where the largest share of residents lives. The calculator uses that zone when you pick a state.

Zone 4C is the marine climate of western Oregon, western Washington and two counties on California's north coast, Del Norte and Humboldt; ENERGY STAR groups it with zones 5 and 6. DOE's county list omits Ada County (Boise), so Idaho's share counts it in zone 5: Boise logs 5,321 heating degree days a year, fewer than zone 5 Chicago's 5,974.

How climate changes the payoff

The heat-loss formula scales with degree days, so the same upgrade is worth far more in the north. Each row uses a reference city's NOAA 1991-2020 normal for 1,000 sq ft of attic, a gas furnace at 80% AFUE and $1.48 per therm:

Going from R-11 to R-60 saves about $161 a year in Chicago but only $34 in Houston, a nearly fivefold difference from climate alone. The jump from R-30 to R-60 is smaller everywhere because each added inch removes a shrinking share of an already small loss, which is the law of diminishing returns built into 1 ÷ R.

How to use this calculator

1. Pick the area you are insulating and enter its square footage. For an attic, use the footprint of the heated rooms below; for walls, leave out windows and doors.
2. Choose your state or zone. The target R-value fills in from ENERGY STAR's table and changes when you switch zone or area; you can type your own target at any time.
3. Describe what is there now: the type of the existing insulation and its depth measured with a ruler in several spots.
4. Choose the material you plan to add. Loose fill shows bags, batts show a layer combination, rigid board shows sheets and spray foam shows board feet.
5. Set heating details: fuel, price, efficiency, degree days and your quote. The payback card updates as you type.

The calculator is built for homeowners pricing an attic top-up, DIYers deciding between cellulose and fiberglass, and contractors who want a quick, sourced number to put in front of a client.

Key terms

• R-value: resistance to heat flow, in hr·ft²·°F/Btu. Higher is better, and layers add.
• U-factor: 1 ÷ R, the rate of heat flow. Heat loss is proportional to U.
• Climate zone: the IECC's 1 to 8 rating of a county's heating and cooling needs, with A, B and C moisture regimes.
• Heating degree days (HDD): the yearly sum of how far each day's mean temperature falls below 65°F.
• Settled thickness: the depth loose fill keeps after it settles, which is the depth that delivers the labeled R-value.
• Board foot: 1 sq ft of foam 1 inch thick, the unit spray foam is priced in.
• AFUE and HSPF2: seasonal efficiency ratings for furnaces (percent) and heat pumps (Btu of heat per watt-hour).

What changes the result

• Existing depth. Every inch of old cellulose is worth R-3.7 you do not have to buy, so measure in several places; old insulation is often thinner at the eaves.
• Material choice. R-49 is 7.5 inches of closed-cell foam or 19.6 inches of blown fiberglass.
• Climate. Degree days drive the savings; a Duluth attic saves about seven times what a Houston attic saves from the same upgrade.
• Fuel price and efficiency. At the default prices, a delivered million Btu costs $18.50 with gas, $23.07 with a heat pump, $35.54 with propane, $50.70 with electric resistance and $52.54 with heating oil.

Tips before you add insulation

• Air seal first. EPA recommends sealing attic air leaks around top plates, wiring holes and chases before adding insulation, because insulation hides those leaks and does not stop air.
• Keep the soffits open. Install rafter vents (baffles) at the eaves so blown insulation cannot block attic ventilation.
• Respect recessed lights. Keep insulation at least 3 inches from can lights that are not IC-rated.
• Use unfaced batts when you lay them over existing insulation.

Limitations

The heat-loss model treats the outdoor air as the cold side. An attic or crawlspace runs warmer than outdoors in winter, so real savings are usually lower than the formula shows, and cooling savings are not counted at all. Wood framing conducts heat around the insulation and is not modeled, and the R-2 allowance for the bare assembly is an approximation that matters most when you start from zero. R-values per inch are DOE's typical figures; individual products vary, and the label decides.

Related calculators

Use the BTU Calculator to resize heating and cooling equipment after an upgrade, the Square Footage Calculator for odd-shaped attics, and the Drywall Calculator when walls are open. The Electricity Cost Calculator and the Natural Gas Cost Calculator turn the energy savings into a monthly bill, and the ROI Calculator compares the project with other uses of the money.

Sources

• ENERGY STAR (EPA), Recommended Home Insulation R-Values (2021 IECC basis) and A Do-It-Yourself Guide to Sealing and Insulating.
• U.S. Department of Energy, Energy Renovations: Insulation (PNNL-20972), Tables 2.2 and 3.2; Guide to Home Insulation (DOE/EE-0340); Insulation Fact Sheet (DOE/CE-0180).
• DOE Energy Saver, Insulation Materials (archived copy of June 2026; the page has since been retired from energy.gov).
• DOE Building America Solution Center, loose-fill insulation settled densities.
• Pacific Northwest National Laboratory for DOE, Guide to Determining Climate Zones by County: Building America and IECC 2021 Updates (PNNL-33270).
• U.S. Census Bureau, Vintage 2025 county population estimates.
• NOAA National Centers for Environmental Information, U.S. Climate Normals 1991-2020 (annual heating and cooling degree days).
• U.S. Energy Information Administration, Btu conversion factors, degree days, residential electricity price (17.30 cents per kWh, 2025), residential natural gas price ($15.34 per Mcf, 2025) and heating oil and propane prices (week of October 5, 2026).
• Electronic Code of Federal Regulations, 10 CFR 430.32 (furnace AFUE and heat pump HSPF2 minimums) and 16 CFR 460.12 (FTC R-value Rule labels).
• EPA, Asbestos-contaminated vermiculite insulation; IRS, Energy Efficient Home Improvement Credit.