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Insulation calculator for attics and walls

The insulation R-value recommended for your climate zone, and how much to buy for an attic, wall or floor. Nothing you type leaves your browser.

Measure in

1. Recommended R-value

Not sure? Find your county in the DOE climate zone guide.
Where

Choose a zone to see the recommended R-values.

2. How much to buy

Area
ft
ft
R
Choose a zone above to fill this in, or type your own.
R
From the old label, or about 3.2 per inch of fiberglass batt (DOE).
Insulation
Coverage depends on the product. Copy it from the label or bag chart: the label decides.
The row for the R-value you are adding.
$
Your entries are not saved or sent
Bags to buy
0bags
Enter the area and target R-value to start.
Area
0
Target
0
There now
R0
R-value to add
R0
Total after
R0
Coverage used
0

R-values of layers add up, so the R-value to add is the target minus what is there now.

An estimate. The product label decides coverage and installed thickness, and local building codes can require more, so check both before you buy.

How to use the insulation calculator

  1. Choose your climate zone and where you are insulating: attic, walls or floor. The recommended R-values for that zone appear, and the target R-value is filled in. Type over it if you want a different target.
  2. Enter the area as length x width, or as a total if you have already measured it. The square footage calculator helps with odd shapes.
  3. Enter the R-value there now, if any. The calculator takes it off the target.
  4. Choose blown-in or batts and copy the coverage from the product label: bags per 1,000 sq ft or coverage per bag from the bag's chart, or the R-value and square feet of one package of batts.

The result is the number of bags or packages to buy, rounded up. The calculator does not guess coverage, because it is different for every product: the label decides.

What R-value insulation do I need?

It depends on your climate zone. ENERGY STAR publishes this table of "Recommended insulation levels for retrofitting existing wood-framed buildings", and says the guidance "is based on the 2021 International Energy Conservation Code (IECC)". The zone labels are as printed on its page:

ZoneAttic with no insulationAttic with 3 to 4 in alreadyFloor
1R30R25R13
2R49R38R13
3R49R38R19
4A and 4BR60R49R19
6, 5, and 4CR60R49R30
7 and 8R60R49R38

The attic columns sit under the heading "Add Insulation to Attic", so they are amounts to add: R60 on a bare attic floor, or R49 on top of 3 to 4 inches already there. Zones cover counties, not whole states, so look yours up in the DOE-funded Guide to Determining Climate Zone by County, which helps you identify the appropriate climate zone designation for each county in the United States. Your local code can ask for more than this table.

Insulation calculator for attic

Most attics already have some insulation, so you add the difference. DOE's Insulation Fact Sheet says: In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added. The same fact sheet says to multiply the thickness in inches by 3.2 to estimate the R-value of a fiberglass batt you find, or use the label if you can see it.

Step, zone 5 attic, 30 x 40 ftWorkingResult
Area30 x 401,200 sq ft
TargetENERGY STAR, zone 5, no insulationR60
There nowabout 3.5 in of fiberglass batt, 3.5 x 3.2R11
To add60 - 11R49
Blown-in bags1,200 x 20 / 1,000, at 20 bags per 1,000 sq ft24 bags

The R49 to add matches ENERGY STAR's own figure for an attic with 3 to 4 inches already. The 20 bags per 1,000 sq ft is only an example figure: read the row on your own bag. The same fact sheet warns that insulation which is compressed will not give you its full rated R-value, which can happen if you add denser insulation on top of lighter insulation.

Blown in insulation calculator

Blown-in (loose-fill) coverage is set by the product, not by a rule. The FTC's R-Value Rule requires the label on loose-fill insulation to show "the minimum settled thickness, initial installed thickness, maximum net coverage area, number of bags per 1,000 square feet, and minimum weight per square foot at R-values of 13, 19, 22, 30, 38, and 49." Find the row for the R-value you are adding and copy one of two figures:

  • Bags per 1,000 sq ft: bags = area x that number / 1,000.
  • Coverage per bag: bags = area / that coverage. At 45 sq ft per bag, 1,200 sq ft needs 1,200 / 45 = 26.7, so 27 bags.

If the R-value you need falls between two rows, use the higher row. Blow to at least the initial installed thickness on the bag, so the insulation still reaches the settled thickness later.

Insulation calculator for walls

ENERGY STAR does not give a single wall R-value. Its advice is for when siding comes off: for an uninsulated wood-frame wall, Drill holes in the sheathing and blow insulation into the empty wall cavity before installing the new siding, then add R5 insulative wall sheathing in zone 3 or R5 to R10 in zones 4 to 8. For an insulated 2x4 wall in zones 4 to 8 it says to add R10 sheathing. For the cavity itself, the calculator uses DOE's Consumer Guide to Home Insulation, which lists R13 for an uninsulated wood-frame wall in zones 1 and 2, and R20 in zones 3 to 8.

A wall cavity holds one layer of batts, so pick the batt made for your stud depth. DOE's fact sheet gives the example of R-19 batts rated for 6-1/4 in squeezed into a 5-1/2 in wall cavity: they lose R-value. For basement or crawlspace walls, ENERGY STAR lists R5 sheathing or an R13 batt in zone 3, R10 or R13 in zones 4A and 4B, and R15 or R19 in zones 4C and 5 to 8.

How much insulation do I need?

For batts and rolls, the FTC rule says the package must show "the R-value, length, width, thickness, and square feet of insulation in the package". The calculator works:

  1. Layers = R-value to add / R-value of one batt, rounded up.
  2. Packages = area x layers / square feet per package, rounded up.

A zone 4A floor of 24 x 30 ft is 720 sq ft. ENERGY STAR's floor figure there is R19, so one layer of R19 batts. If the package label says 40 sq ft, that is 720 / 40 = 18 packages. Insulating a room well also changes how much cooling it needs; the BTU calculator sizes an air conditioner from the room.

Insulation calculator for garage

Neither table has a garage line. DOE's Insulation Fact Sheet names an uninsulated floor over a garage or crawlspace as a place worth insulating, so a room over a garage can use the Floor figure for your zone. For walls between a garage and the house, or garage walls themselves, the official pages we used give no separate figure. The calculator still works out how much to buy for any area and target you type, and your local building department can tell you what your code requires.

Loft insulation in the UK

UK products are sold by thickness and area rather than US R-values. The Energy Saving Trust says current building regulations recommend a minimum depth of 270mm for mineral wool insulation. For a loft with easy access and regular joists it describes laying a first layer of mineral wool rolls between the joists, then another at right angles over them to reach the depth. Use the calculator in meters with the coverage per roll in m² from the label, and set the target and current figures to match the R-values printed on the product, or skip them and work from the area alone.

In Excel or Google Sheets

With the area in square feet in A2 and the bag chart's bags per 1,000 sq ft in B2, the bags to buy are:

=ROUNDUP(A2*B2/1000, 0)

Microsoft's help page says ROUNDUP "Rounds a number up, away from 0 (zero)", and Google's says it rounds "always rounding up to the next valid increment". For batts, use =ROUNDUP(A2*C2/D2, 0) with the layers in C2 and the square feet per package in D2.

Questions and answers

What R-value do I need for my attic?

ENERGY STAR recommends R30 in zone 1, R49 in zones 2 and 3, and R60 in zones 4 to 8 for an attic with no insulation. With 3 to 4 inches already there, it recommends adding R25, R38 or R49.

Can I add new insulation on top of old insulation?

Yes. R-values of layers add up, so you add the difference between the target and what is there. Avoid laying dense insulation on top of light insulation that squashes easily, because compressed insulation loses R-value.

How many bags of blown-in insulation do I need?

Area x the bags per 1,000 sq ft on the bag chart, divided by 1,000. A 1,200 sq ft attic at 20 bags per 1,000 sq ft needs 24 bags. Every product is different, so use the figure on your own bag.

How do I find my climate zone?

Look up your county in the Guide to Determining Climate Zone by County from DOE's Building America program. Zones run from 1 (hottest) to 8 (coldest), and zone 4 is split into 4A, 4B and 4C.

What R-value do walls need?

DOE's Consumer Guide lists R13 for an empty wood-frame wall cavity in zones 1 and 2 and R20 in zones 3 to 8. When siding comes off, ENERGY STAR also suggests R5 to R10 insulating sheathing in colder zones.

How deep should loft insulation be in the UK?

The Energy Saving Trust says current building regulations recommend a minimum depth of 270mm for mineral wool insulation.

Sources

Checked on 13 October 2026. If a rule has changed, please tell us.

Square footage calculatorArea in feet and inches for rooms, floors and odd shapes.BTU calculatorThe air conditioner size a room needs in BTU per hour, from its area, sun, people and use, using the ENERGY STAR chart.Electricity cost calculatorWhat an appliance costs to run per hour, day, month and year.

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