Attic Insulation Calculator: How Much Insulation Do I Need?

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In climate Zones 4 to 8, ENERGY STAR recommends adding R-49 to an attic that already has 3 to 4 inches of insulation, and R-60 to an attic that has none. With Owens Corning AttiCat blown fiberglass, R-49 means at least 17 in deep and at least 25 bags per 1,000 sq ft, so a 40 × 30 ft (1,200 sq ft) attic takes about 31 bags. Zones 1 to 3 call for less, from R-25 to R-49.

Our attic insulation calculator also works as a blown-in insulation calculator: enter your attic size, climate zone, and current insulation to get the R-value to add, the minimum depth, and the bag count.

How much attic insulation do you need?

Enter the attic floor size, your climate zone, and the insulation there now. Results in R-value to add, depth, and bags of blown fiberglass.

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Recommended levels from ENERGY STAR (2021 IECC). Find your zone on the ENERGY STAR climate map. Keep insulation 3 in from recessed lights unless they are IC-rated, and out of the air space around flues and chimneys.

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 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).

The attic insulation formula

  • Attic area = length (ft) × width (ft)
  • R-value to add = ENERGY STAR's figure for your zone and current insulation
  • Chart row = the first AttiCat chart row at or above that R-value
  • Bags = area ÷ maximum net coverage per bag for that row, rounded up

Why a bag chart instead of an inches-per-R shortcut? DOE explains that R-value depends on the type of insulation, its thickness, and its density, and that deeper loose-fill settles denser under its own weight, so its R-value "does not change proportionately with thickness."

Worked example (the calculator's default values)

Say your attic floor is 40 × 30 ft, you live in Zone 5, and about 3 to 4 in of old insulation is already there.

  1. Area: 40 × 30 = 1,200 sq ft
  2. R-value to add: Zones 4 to 8 with 3 to 4 in now = R-49
  3. Chart row: R-49 = at least 17 in, up to 39.9 sq ft per bag
  4. Bags: 1,200 ÷ 39.9 = 30.08, rounded up to 31 bags

The calculator shows the same result: "Add about R-49: at least 17 in of blown fiberglass, about 31 bags." The detail line adds: "1,200 sq ft of attic floor. ENERGY STAR recommends adding R-49 in Zones 4 to 8 when there are about 3 to 4 in of insulation now. Depth and bags use the R-49 line of one blown fiberglass bag chart (AttiCat, 39.9 sq ft per 27.5 lb bag); cellulose and other products differ, so check your bag."

The chart's shortcut of 25 bags per 1,000 sq ft suggests 30 bags. The calculator lands one higher because 1,000 ÷ 39.9 is really 25.06, and it always rounds up to a whole bag.

Other combinations for the same 1,200 sq ft attic

Climate zoneInsulation nowR-value to addChart row usedMinimum depthBags
Zone 1NoneR-30R-3010.75 in18
Zone 13 to 4 inR-25R-269.5 in16
Zones 2 and 3NoneR-49R-4917 in31
Zones 2 and 33 to 4 inR-38R-3813.5 in23
Zones 4 to 8NoneR-60R-6020.5 in38
Zones 4 to 83 to 4 inR-49R-4917 in31

An uninsulated attic in Zones 4 to 8 is 1,200 ÷ 31.8 = 37.7, so 38 bags. Zones 2 and 3 with 3 to 4 in is 1,200 ÷ 52.6 = 22.8, so 23 bags. Zone 1 with existing insulation needs R-25, which isn't on the chart, so the calculator uses the next row up, R-26: 1,200 ÷ 79.3 = 15.1, or 16 bags. It still reads "Add about R-25: at least 9.5 in of blown fiberglass, about 16 bags."

How much insulation to add to a 40 by 30 foot atticCross-section of an attic floor: ceiling drywall, ceiling joists, and about 3 to 4 inches of old insulation between the joists. A new layer of blown fiberglass at least 17 inches deep covers the old layer and the joists, measured with a ruler from the ceiling. For a 40 by 30 foot attic, 1,200 square feet, in climate Zones 4 to 8 with 3 to 4 inches now, ENERGY STAR recommends adding R-49. On the AttiCat bag chart R-49 means at least 17 inches and up to 39.9 square feet per bag, so 1,200 divided by 39.9 is 30.08, rounded up to 31 bags.R-49 ON A 1,200 SQ FT ATTIC481216200inAdd at least 17 inR-49 of blown fiberglass3 to 4 in there nowceiling joist1,200 sq ft40 ft × 30 ft attic floorAdd R-49Zones 4 to 8, 3 to 4 in now31 bags1,200 ÷ 39.9 sq ft per bagZones 4 to 8 with 3 to 4 in now: add R-49 (ENERGY STAR)AttiCat chart, R-49 row: at least 17 in, up to 39.9 sq ft per bag1,200 sq ft ÷ 39.9 = 30.08 → 31 bags of blown fiberglass
In Zones 4 to 8 with 3 to 4 in already there, ENERGY STAR recommends adding R-49. On the AttiCat chart that is at least 17 in deep, and 1,200 sq ft ÷ 39.9 sq ft per bag = 30.08, so 31 bags.

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.

Climate zoneUninsulated attic: add3 to 4 in already there: add
1R-30R-25
2R-49R-38
3R-49R-38
4A and 4BR-60R-49
4C, 5, and 6R-60R-49
7 and 8R-60R-49

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.

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).

R-valueMinimum thicknessMaximum net coverage per bagMinimum bags per 1,000 sq ft
R-3010.75 in68.5 sq ft14.6
R-3813.5 in52.6 sq ft19
R-4917 in39.9 sq ft25
R-6020.5 in31.8 sq ft31.5

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.

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.
  • 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.

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
  • 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.

Frequently asked questions

How much insulation do I need in my attic?

ENERGY STAR recommends adding R-49 to R-60 in Zones 4 to 8, R-38 to R-49 in Zones 2 and 3, and R-25 to R-30 in Zone 1; the lower number applies when you already have 3 to 4 in. For a 1,200 sq ft attic needing R-49, that is at least 17 in of AttiCat, or about 31 bags.

How many bags of blown-in insulation do I need per 1,000 sq ft?

For AttiCat fiberglass, the chart minimum is 14.6 bags for R-30, 19 for R-38, 25 for R-49, and 31.5 for R-60. Other products have their own charts.

How deep should blown-in insulation be for R-49?

For AttiCat, at least 17 in, with at least 25 bags per 1,000 sq ft. R-60 takes 20.5 in and R-38 takes 13.5 in. Cellulose and other brands list different depths.

Is the ENERGY STAR R-value the total or the amount to add?

The amount to add. The table is headed "Add Insulation to Attic," with one column for an uninsulated attic and one for an attic with 3 to 4 in already in place.

Can I blow new insulation over old insulation?

Usually. ENERGY STAR's do-it-yourself guide says you can add loose fill on top of fiberglass batts or blankets, and DOE says the R-values of layers add up. Call a pro first if the old insulation is wet, and leave vermiculite alone until it has been tested.

Does this insulation calculator work for cellulose?

The R-value target works for any product, but the depth and bag count come from one blown fiberglass chart. For cellulose, use your bag's chart for that R-value.

What climate zone am I in for insulation?

Use the U.S. map on the ENERGY STAR insulation R-values page or on DOE's Energy Saver insulation page. 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, and a letter after the number doesn't change the attic figures.

Should I air seal before adding attic insulation?

Yes. ENERGY STAR says, 'EPA recommends air sealing the attic before adding attic insulation,' and publishes a separate attic air sealing guide. Keep soffit vents open with rafter vents (baffles) so outside air can still flow in.

How close can insulation be to recessed lights and flues?

The AttiCat data sheet says not to insulate on top of or within 3 in of recessed light fixtures unless they carry an IC rating. ENERGY STAR notes that codes usually require 1 in from metal flues and 2 in from masonry chimneys to combustibles, and recommends an insulation dam around the flue.