Attic Insulation Calculator: How Much Insulation Do I Need?

How to measure your attic and check existing insulation

1. Dress for it. ENERGY STAR lists safety glasses, gloves, a dust mask or particulate respirator, and a lightweight disposable coverall and hat to keep irritating insulation off your skin. Attics heat up as the day goes on, so start early and bring a drop light.
2. Step only on the framing. Walk on joists or truss chords, never on the exposed ceiling drywall or insulation between them, or you can fall through the ceiling. Use boards across the joists if needed.
3. Measure the floor area. Measure the attic floor's length and width in feet, or the ceilings of the rooms directly below. For an L-shaped house, add up the rectangles with our square footage calculator, then enter the total as the length and 1 as the width.
4. Measure the depth in several spots. Push a ruler down through the insulation until it touches the ceiling below. ENERGY STAR's do-it-yourself guide says that if your insulation is level with or below the floor joists, so you can easily see them, you should add more.
5. Look for warning signs. Note any damp insulation, mold, knob-and-tube wiring, or vermiculite (see "When to call a pro first").
6. Pick the closest option. Choose "None" for bare joists or "About 3 to 4 inches," the two cases in ENERGY STAR's table. If you have clearly more than 4 in, the "3 to 4 inches" result is more than you need.

How much insulation to add to existing insulation

R-60 is the total to reach in Zones 4 to 8, and R-49 in Zones 2 and 3: those are ENERGY STAR's figures for a bare attic, and DOE says the R-values of layers add up, so subtract what is already there. ENERGY STAR's column for 3 to 4 in follows the same math: in Zones 2 to 8 it is R-11 lower than the bare-attic figure, and Owens Corning's 3.5 in fiberglass batts are R-11 or R-13. Estimate the R-value you have from its depth with the inches-by-R-value chart further down, then add the AttiCat row at or above the difference. Depths are AttiCat's minimums, and bags are for a 1,000 sq ft attic:

The first two rows match the calculator's two choices, None and About 3 to 4 inches. Cellulose depths are settled depths from Greenfiber's chart, and all the depths come from current product charts, so treat the R-value of old insulation as an estimate; DOE notes that compressed insulation does not give its full rated R-value. Zone 1 is left out because ENERGY STAR's two figures there, R-30 and R-25, are only R-5 apart. Wet insulation or vermiculite? See "When to call a pro first" before adding anything.

How to find your climate zone

Use the U.S. map on the ENERGY STAR insulation R-values page or on the U.S. Department of Energy's (DOE) Energy Saver insulation page. For more detail, DOE's Building America Solution Center hosts the 2021 IECC climate zone map, the code edition both recommendations refer to.

Zones run from 1 to 8, some with a letter (A, B, or C) that doesn't change the attic numbers. The calculator groups zones the way ENERGY STAR's figures do: Zone 1, Zones 2 and 3 (identical values), and Zones 4 to 8 (R-60 or R-49 throughout).

Recommended R-value for attic insulation by zone

ENERGY STAR describes R-value as insulation's ability to resist heat traveling through it; higher is better. Its table is headed "Add Insulation to Attic," so these are amounts to add, not totals. The figures are based on the 2021 International Energy Conservation Code (IECC), Table R402.1.3.

DOE's Energy Saver table matches for Zones 4 to 8 but gives ranges for Zones 1 to 3 (for example, R-49 to R-60 for an uninsulated attic in Zones 2 and 3), and ENERGY STAR's numbers fall inside them. Is a room below the attic hard to cool? The AC size calculator sizes a window unit from ENERGY STAR's room air conditioner chart, and the electricity cost calculator shows how many kWh a heater or air conditioner in that room uses. Replacing a furnace? The furnace size calculator estimates the heat a home needs from its floor area and climate.

Blown-in insulation coverage chart: R-30 to R-60

These rows come from the Owens Corning AttiCat PINK blown-in fiberglass data sheet, for a 27.5 lb nominal bag. The calculator uses the full chart (R-13 to R-60); Zone 1's R-25 uses the R-26 row (9.5 in, 79.3 sq ft per bag, 12.6 bags per 1,000 sq ft).

Both numbers count. The data sheet says: "Stated R-value is achieved by installing the minimum required number of bags per 1,000 net sq. ft. at a thickness not less than the label minimum thickness." Reaching 17 in with fewer than 25 bags per 1,000 sq ft is not R-49 by this chart, and neither is enough bags with thin spots.

Cellulose and other brands have their own charts. Because R-value depends on the material, thickness, and density, keep the calculator's R-value target, then read the minimum thickness and coverage per bag from your product's chart and divide your attic area by that coverage.

Bags of blown-in insulation by attic size

Here is what the calculator gives for common attic sizes with the AttiCat chart above, each count rounded up to a whole bag. Each R-value also needs its minimum depth from the chart.

Other products differ: one blow-in cellulose chart lists 12.0 sq ft per bag at R-49, against AttiCat's 39.9, so read the coverage from your own bag.

How many inches of insulation for R-30, R-38, R-49 and R-60?

17 in of blown fiberglass, 13.68 in of settled cellulose, or 14 in of fiberglass batts gives R-49, by three makers' charts. The depth differs because R-value depends on the material, its thickness, and its density:

Cellulose takes more bags for the same R-value: Greenfiber's chart lists 46.1 bags per 1,000 sq ft for R-30, 61.1 for R-38, 83.5 for R-49, and 108.1 for R-60 (19.05 lb bags), against AttiCat's 14.6, 19, 25, and 31.5 (27.5 lb bags). Greenfiber's chart is based on settled thickness. Read the table the other way to estimate what is in your attic now: 6.5 in of batts is about R-19.

How many packages of R-30, R-38 and R-49 batts do I need?

16 packages of Owens Corning's unfaced R-38 batts, 24 × 48 in, cover a 1,000 sq ft attic floor: 1,000 ÷ 64 sq ft per package = 15.6, rounded up. Divide your floor area by the square feet printed on the package, round up, and pick the width that fits your joist spacing. Unfaced batts from the Owens Corning fact sheet:

Two layers work too, since DOE says layer R-values add up: R-30 plus R-19 is R-49 (13 + 8 = 21 packages of the 24 in batts for 1,000 sq ft), and two R-30 layers are R-60 (26 packages). ENERGY STAR says to lay the second layer over and perpendicular to the first, which helps cover the tops of the joists, and to use unfaced insulation with no vapor retarder for it.

Tips before you blow in insulation

• Air-seal first. "EPA recommends air sealing the attic before adding attic insulation," ENERGY STAR says, and it publishes a separate attic air sealing guide. Planning solar panels too? The U.S. Department of Energy also puts efficiency first, since a home that uses less electricity needs fewer panels; see the solar panel calculator.
• Keep soffit vents open. ENERGY STAR's do-it-yourself guide says never to cover soffit vents with insulation. Staple rafter vents (baffles) to the roof decking between the rafters, where the attic ceiling meets the attic floor, so outside air flows in through the soffit and out through the gable or ridge vent.
• Give recessed lights 3 inches. The AttiCat data sheet says not to insulate on top of or within 3 in of recessed light fixtures or similar electrical devices unless they carry an IC rating, meaning they are approved to be covered. ENERGY STAR gives the same 3 in rule.
• Keep insulation off flues and chimneys. The data sheet says not to place insulation in the air spaces around metal flues, chimneys, or fireplaces, and to keep the NFPA 31, 54, or 211 clearances. ENERGY STAR notes that codes usually require 1 in from metal flues (2 in from masonry chimneys) to combustibles, and recommends an insulation dam around the flue.
• Count your bags. Keep the empties and compare with the calculator; the rated R-value needs both the bags and the depth.
• Don't compress it. DOE says "insulation that is compressed will not provide its full rated R-value," so don't pack it down or store things on top of it.
• Plan for the machine. ENERGY STAR notes that loose fill needs a blowing machine; some home improvement stores rent them, but it may be wise to hire a professional.

How to air seal an attic before insulating

1/4 in is where caulk stops and foam starts: ENERGY STAR's attic air sealing guide uses silicone or acrylic latex caulk for holes of 1/4 in or less and expanding spray foam for gaps from 1/4 in to 3 in. EPA recommends sealing first and insulating after. What to use on the common attic leaks:

Pull the insulation back to reach each leak, seal it, and push the insulation back into place once the caulk or foam is dry, as the guide describes. Start with the largest openings, such as dropped soffits and open stud cavities, and keep the flue clearances and the insulation dam from the list above.

When to call a pro first

ENERGY STAR recommends consulting a professional before you proceed if you find:

• Difficult attic access and limited space to work
• Wet or damp insulation, a sign of a leaky roof; if the roof needs new shingles, the roofing calculator estimates them
• Moldy or rotted rafters or floor joists
• Kitchen, bathroom, or dryer vents that exhaust into the attic instead of outdoors
• Little or no attic ventilation
• Knob-and-tube wiring (pre-1930), which can be a fire hazard in contact with insulation

ENERGY STAR also warns that vermiculite insulation may contain asbestos. Don't disturb it unless it has been tested by an approved lab.