Insulation Calculator: How Much Insulation Do I Need?
Work out how much insulation an attic, wall or floor needs from its area and target R-value, in batts, rolls or bags, with cost. Pick the climate zone and the material, and see the R-value to add, the depth, and how many bags, bundles or rigid foam sheets to buy.
Your numbers
The IECC zone for your county; it sets the R-value the page starts with.
For a wall, its height.
Optional. Overrides length × width; add up several rooms here.
Empty: the ENERGY STAR level for the zone (walls: R-13, a full 2x4 cavity).
Typical allowance for cutting and uneven areas; edit it.
Depth of the old insulation; 0 if there is none.
Typical for older loose fill; 2.2 to 3.7 depending on the material.
From the bag's chart at your R-value. Empty: the maker's chart named below.
Empty: a typical value for the material. The product label wins.
Bags to buy
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- R-value to add
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- Total R-value after
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- Area with the extra
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- ENERGY STAR, Zone 4A or 4B
- R60 (add R49 over 3–4 in)
- Material cost
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Depth and bags come from Greenfiber's SANCTUARY cellulose chart (25 lb bag). Your bag's chart wins; the maker says coverage can vary by as much as 10%.
An estimate. Seal air leaks before you insulate; the local code and the maker's instructions have the final word.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How much insulation do you need?
Start with the area: length × width for an attic floor or a floor, length × height for a wall. Then pick the R-value you want and subtract what is already there: measure the depth of the old insulation and multiply by its R per inch. What is left is the R-value to add, and the material decides how deep that is and how many bags, bundles or sheets it takes.
Worked example: an attic floor of 40 × 25 ft is 1,000 sq ft. In climate zone 4A the ENERGY STAR level is R60. Three inches of old loose fill at about R-2.5 per inch is R-7.5, so you add R-52.5. On Greenfiber's SANCTUARY cellulose chart that sits between R-49 (13.3 in, 70.8 bags per 1,000 sq ft) and R-60 (16.1 in, 86.0 bags): about 14.2 in and 75.6 bags per 1,000 sq ft. With 5% extra that is 1,050 sq ft and 80 bags.
Recommended R-values by climate zone
ENERGY STAR's table for adding insulation to an existing wood-framed home, based on the 2021 International Energy Conservation Code. Find your zone on the IECC climate zone map for your county.
| Zone | Uninsulated attic | Attic with 3–4 in already (add) | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A and 4B | R60 | R49 | R19 |
| 4C, 5 and 6 | R60 | R49 | R30 |
| 7 and 8 | R60 | R49 | R38 |
Walls: when the siding comes off, blow insulation into empty cavities, and in zones 3 to 8 add insulated sheathing (R5 in zone 3, R5 to R10 in zones 4 to 8). The calculator starts walls at R-13, a full 2x4 cavity.
R-value per inch by material
Montana State University Extension gives glass-fiber batts about R-3.2 per inch, mineral wool R-3.1, and loose fill from R-2.2 per inch for glass fiber to R-3.7 for cellulose. Rigid boards go further: expanded polystyrene (EPS) R-4.0 to 4.5, extruded polystyrene (XPS) R-5.0 to 5.5, and polyisocyanurate R-6 to 7.5. The calculator uses the low end for foam; type the R printed on your board if it is higher.
That is why the same R-value comes out at different depths. R-38 is about 10.4 in of settled cellulose on Greenfiber's chart but 13.5 in of blown fiberglass on Owens Corning's AttiCat chart, and under 7 in of polyiso.
Read the bag's coverage chart
Every bag of blown-in insulation has a chart: for each R-value it lists the minimum thickness and the minimum number of bags per 1,000 sq ft. Both count: the makers say the stated R-value needs the minimum bag count as well as the depth, and fewer bags give a lower R-value.
The calculator reads Greenfiber's SANCTUARY cellulose chart (25 lb bags) or Owens Corning's AttiCat fiberglass chart (27.5 lb bags) and draws a straight line between the rows. If your product is different, type the coverage per bag from its chart for your R-value; for batts, type the coverage on the bundle.
On the site visit
Measure the attic floor between the outside walls, not the roof. Check the depth of the old insulation in a few places and note recessed lights, the attic hatch and any wet or damaged spots. Air-seal gaps around pipes, wires and the chimney before the new insulation goes in.
Then write down what the numbers do not show: which rooms are cold or hot, the age of the furnace, what the homeowner wants to spend and whether they asked about rebates. Those answers decide the quote as much as the bag count.
Sources, checked October 2026. All results are estimates; the local code and the maker's instructions decide:
- ENERGY STAR: Recommended Home Insulation R-Values (based on the 2021 IECC)
- Montana State University Extension: Home Insulation Choices (R-value per inch)
- Greenfiber SANCTUARY cellulose attic card, coverage chart (PM-6.3330-SDBZ Rev A 04/26)
- Owens Corning AttiCat blown-in fiberglass application instructions, coverage chart (Pub. 10009393-E)
More for HVAC, solar and building-materials sales: HVAC sales training, Solar sales, Building materials sales.
The R-value is on the bag. The reasons are in the conversation.
On an energy audit or a sales visit, the homeowner tells you which rooms are cold, what the bills look like, what they want to spend and when. Record the visit in ParrotNotes and get the summary and next steps, so the quote matches what they asked for.
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