Insulation Calculator
This insulation calculator estimates how many packages of batts or rolls, or how many bags of blown-in insulation, you need for an attic floor, walls or a floor over an unheated space. Add the areas, pick your climate zone for a suggested target R-value, enter the R-value you already have, and type the package coverage or the bag chart row from your product label. It rounds up to whole packages or bags.
Quick answer: R-value to add = target R − existing R. Batts: layers = R to add ÷ R per layer, rounded up, then packages = area × layers ÷ coverage per package, rounded up. A 40 × 30 ft attic is 1,200 sq ft. For R-60 with R-30 batts that is 2 layers, and 1,200 × 2 ÷ 53.33 = 45.003, so 46 packages. Blown-in: 1,200 ÷ 1,000 × 31.5 bags = 37.8, so 38 bags.
How much insulation to buy starts with the R-value you need to add: the target R-value minus the R-value already there, because the R-values of layers add up. For batts or rolls, divide that by the R-value of one layer and round up to whole layers, then divide the area times the layers by the square feet in one package and round up. For blown-in insulation, multiply the area in thousands of square feet by the bags per 1,000 sq ft for the R you add, and round up. Package coverage and the bag chart come from your product label.
Example: a 40 × 30 ft attic is 1,200 sq ft. To reach R-60 from nothing with R-30 batts that is 2 layers, and 1,200 × 2 ÷ 53.33 = 45.003, so buy 46 packages. With blown-in at 31.5 bags per 1,000 sq ft, 1.2 × 31.5 = 37.8, so buy 38 bags.
Method
How this calculator works
The area and R-value math is exact. The suggested targets are government guidance for adding insulation to an existing house, and the package and bag figures are product-specific, so they come from your label.
Area and R-value to add
The net area is the sum of length × width for every area, minus any area you leave out, such as an attic hatch, a chimney chase or the windows and doors in a wall. R-values of layers add up, so what you need to add is the target minus what is already there, and never below zero.
Net area = Σ (Length × Width) − Area to leave outR to add = Target R − Existing RSuggested target R-value
For an attic floor or a floor over an unheated space the calculator suggests the ENERGY STAR table for retrofitting existing wood-framed buildings, which says it is based on the 2021 IECC Table R402.1.3. For walls it uses the DOE Consumer Guide to Home Insulation (DOE/EE-2476), and it shows the DOE range for the attic next to the suggestion. This is guidance for adding insulation, not code. New construction has no sourced default, so type the target and check the energy code adopted where you build.
Target = suggested value for your zone, or the value you typeBatts and rolls
Layers are the R-value to add divided by the R-value of one layer, rounded up, and each layer is laid across the one below. Packages are the area times the layers, divided by the coverage of one package, rounded up. The 30 and the 53.33 sq ft in the fields are an example (one R-30 pack from an Owens Corning fact sheet), so use the figures on your own label. The extra allowance starts at 0% because no source gives one.
Layers = round up (R to add ÷ R per layer)Packages = round up (Net area × Layers × (1 + Extra ÷ 100) ÷ Coverage per package)Blown-in bags
Bag charts list bags per 1,000 sq ft for each R-value. Enter the row for the R-value you are adding, not the total. The calculator uses net coverage, the more conservative choice, and rounds up. The chart also gives a minimum installed thickness. Owens Corning says to provide both the number of bags and at least the minimum thickness, or the R-value comes out lower. The 31.5 bags and 20.5 in are an example (one maker's R-60 row).
Bags = round up (Net area ÷ 1,000 × Bags per 1,000 sq ft × (1 + Extra ÷ 100))Suggested targets by climate zone
| Climate zone | Attic, none yet | Attic, 3 to 4 in already there | Floor over unheated space | Wall, uninsulated wood frame |
|---|---|---|---|---|
| Zone 1 | R-30 (DOE range R30 to R49) | R-25 (DOE range R19 to R38) | R-13 | R-13 |
| Zone 2 | R-49 (DOE range R38 to R60) | R-38 (DOE range R30 to R49) | R-13 | R-13 |
| Zone 3 | R-49 (DOE range R38 to R60) | R-38 (DOE range R30 to R49) | R-19 | R-20 |
| Zone 4 (not marine) | R-60 (DOE range R49 to R60) | R-49 (DOE range R30 to R49) | R-19 | R-20 |
| Zone 4 marine or 5 | R-60 (DOE range R49 to R60) | R-49 (DOE range R38 to R49) | R-30 | R-20 |
| Zone 6 | R-60 (DOE range R49 to R60) | R-49 (DOE range R38 to R49) | R-30 | R-20 |
| Zone 7 or 8 | R-60 (DOE range R49 to R60) | R-49 (DOE range R38 to R49) | R-38 | R-20 |
What each field does
| Field | What to enter |
|---|---|
| How will you insulate? | Batts or rolls (fiberglass or mineral wool) or Blown-in (loose fill, by the bag). |
| Where | Attic floor, no insulation yet; Attic floor, 3 to 4 in already there; Walls; or Floor over an unheated space. |
| Climate zone | Used only for the suggested target. Find yours on the DOE climate zone map. |
| Target R-value | Leave empty to use the suggested value for your zone, or type your own. Local code may require more. |
| Existing R-value | 0 or empty if there is none. R-values of layers add up. |
| R-value of one layer | For batts. The 30 in the field is an example, so use the R on your batt package. |
| Coverage per package | For batts. The 53.33 sq ft (4.95 m² in the metric view) is an example, so use the square feet on your label. |
| Bags per 1,000 ft² for the R you add | For blown-in. The 31.5 is an example (an R-60 chart row), so use your bag chart row for the R-value you add. |
| Minimum installed thickness | Optional, from the same chart row. You must reach both the bag count and this depth. |
| Areas: Length, and Width or wall height | One row per area, up to 20 (use Add area). For a wall, its length and its height. |
| Area to leave out | Optional. Windows and doors in a wall, an attic hatch or a chimney chase. |
| Extra allowance | Starts at 0%, because no source gives one. Packages and bags always round up. |
Worked example: a 40 × 30 ft attic
Arithmetic checkedThese are the calculator's own example values: an attic floor with no insulation, Zone 4 (not marine), a target of R-60, R-30 batts at 53.33 sq ft per package, and a blown-in chart row of 31.5 bags per 1,000 sq ft.
| Step | Math | Result |
|---|---|---|
| Area to insulate | 40 × 30 | 1,200 sq ft |
| Suggested target | ENERGY STAR, Zone 4 (not marine); DOE range R49 to R60 | R-60 |
| R-value to add | 60 − 0 | R-60 |
| Layers of batts | 60 ÷ 30 | 2 layers |
| Packages, before rounding | 1,200 × 2 ÷ 53.33 | 45.003 |
| Packages to buy | rounded up | 46 packages |
| Blown-in instead: bags, before rounding | 1,200 ÷ 1,000 × 31.5 | 37.8 |
| Blown-in instead: bags to buy | rounded up, with a 20.5 in minimum thickness | 38 bags |
| You would see | 46 packages to buy | |
Disclaimer
Results are planning estimates based on the dimensions and assumptions you enter. They are not engineering, structural or building-code advice. Product yields, densities and coverage vary by manufacturer, batch and site conditions. Check product labels, your supplier and your local building department before you order materials or start work. Read the full calculation disclaimer.
Accuracy
What affects your estimate
The formula is simple. What changes the answer is what is already there, the package or bag chart you buy and how whole layers land against your target.
Existing insulation lowers what you add
Take a 1,200 sq ft attic in Zone 3 with R-19 already there (Greenfiber's guide calls about 6 in of fiberglass batt R-19). The suggested target is R-38, so you add R-19: one layer of R-19 batts at 48.96 sq ft per package, and 1,200 ÷ 48.96 = 24.51, so 25 packages.
Whole layers can overshoot the target
In Zone 2 the suggested target for an uninsulated attic is R-49. R-30 batts give 49 ÷ 30 = 1.63, which rounds up to 2 layers and R-60, so you buy 46 packages and the calculator warns that the layers reach R-60. A different R per layer may land closer.
Blown-in charts differ by product
For R-60 over 1,200 sq ft, the Owens Corning AttiCat chart row of 31.5 bags per 1,000 sq ft gives 38 bags, and the Greenfiber Sanctuary row of 92.5 gives 111. Different products, bag weights and blowing machines have different charts, so use your own bag and the row for the R you add.
Walls: the package already counts the framing
A 20 × 8 ft wall is 160 sq ft, minus 44 sq ft of windows and doors, 116 sq ft. R-13 batts at 117.34 sq ft per package give 116 ÷ 117.34 = 0.99, so 1 package. Batt sizes are based on the 16 or 24 in on-center spacing, so no framing factor is applied. For walls the calculator counts one layer of batts, because a stud cavity takes one; if that layer is below the target, it says so.
The extra allowance
No source gives a waste figure for batts or bags, so it starts at 0%. Entering 5% on the 1,200 sq ft attic gives 1,200 × 2 × 1.05 ÷ 53.33 = 47.25, so 48 packages instead of 46. The result always rounds up to whole packages.
Metric view
A 12 × 9 m attic is 108 m². With R-30 batts at 4.95 m² per package and a target of R-60, that is 2 layers, and 108 × 2 ÷ 4.95 = 43.6, so 44 packages. Package coverage is entered in m², and the suggested targets stay in R-values.
Suggested R-values are ENERGY STAR and DOE guidance for adding insulation to an existing wood-framed house, not code, and local code may require more. Air-seal first. Keep attic vents clear (use baffles), keep insulation 3 in from recessed lights unless they are IC-rated, and away from flues and chimneys. Do not insulate around knob-and-tube wiring. Follow local code for vapor retarders, and do not put a vapor barrier between layers. Spray foam, rigid board and sound batts are not calculated, and this is not an energy audit. See the calculation disclaimer.
Common mistakes
Leaving the example label values in the fields
The R-30 layer, the 53.33 sq ft package and the 31.5 bags per 1,000 sq ft are examples from one maker. The calculator warns while they are unchanged, so enter the figures from your own package or bag chart.
Reading the suggested R-value as code
The suggestion is ENERGY STAR and DOE guidance for retrofits, and your local code may require more. For new construction, type the target and check the energy code adopted where you build.
Using the chart row for the total R
Bag charts are read at the R-value you are adding. If you have R-19 and want R-38, use the row for R-19, not R-38.
Hitting the bag count but not the depth
Blown-in insulation has to reach the minimum thickness as well as the bag count. Fewer bags spread too thin, or the right bags at too little depth, give a lower R-value.
Squeezing batts into the wrong cavity
The DOE warns that compressed insulation does not give its full rated R-value, for example R-19 batts rated for 6 1/4 in pushed into a 5 1/2 in cavity. Match the batt thickness to the cavity.
Covering vents, lights and flues
Blocking attic vents, burying recessed lights that are not IC-rated or packing insulation against flues causes problems the quantity cannot fix. Use baffles and keep the clearances on the label.
Questions people ask
How do I calculate how much insulation I need?
Find the area, subtract the R-value you already have from your target, and divide by the coverage of one package for batts or use the bag chart row for blown-in. Round up to whole packages or bags. The guide on how much insulation you need works through 1,200 sq ft attic examples.
What R-value do I need in my attic?
ENERGY STAR suggests R-30 to R-60 for an uninsulated attic depending on the climate zone, and less if 3 to 4 in is already there. The guide on R-value for attic insulation compares the ENERGY STAR and DOE tables by zone.
How many bags of blown-in insulation per 1,000 square feet?
It depends on the product and the R-value. One Owens Corning chart gives 14.6 bags per 1,000 sq ft for R-30 and 31.5 for R-60. Another maker's chart is different, so use the row on your bag.
Does it work for cellulose and blown-in fiberglass?
Yes for any loose fill with a bag chart. Pick Blown-in and enter the bags per 1,000 sq ft and the minimum thickness from your bag, for the R-value you are adding.
Can I use it for new construction?
You can, but the suggested target is for adding insulation to an existing house. No new-construction minimum is built in, so type your target and check the energy code adopted where you build.
Does it calculate spray foam or rigid board?
No. It covers batts, rolls and blown-in loose fill. Spray foam, rigid board and sound-attenuation products are not calculated, and neither is the R-value of a product.
Should there be a vapor barrier between layers?
No. The DOE says not to put a vapor barrier between a new layer and existing insulation. Whether a wall needs a vapor retarder at all is a question for your local code, so the calculator does not decide it.
Related calculators and guides
Sources
Where the numbers come from
- ENERGY STAR, recommended home insulation R-values attic and floor targets by zone, based on the 2021 IECC Table R402.1.3, accessed October 17, 2026.
- DOE, Consumer Guide to Home Insulation (DOE/EE-2476) R-value ranges and wall targets by zone, accessed October 17, 2026.
- DOE/CE-0180, Insulation Fact Sheet R-values of layers add, compression and vapor barrier warnings, accessed October 17, 2026.
- Owens Corning PINK Next Gen Fiberglas Insulation fact sheet example batt package coverage, such as 53.33 sq ft for R-30, accessed October 17, 2026.
- Owens Corning AttiCat blown-in insulation data sheet example bag chart rows and the bags-plus-thickness rule, accessed October 17, 2026.
- Greenfiber, do-it-yourself attic installation instructions example coverage chart and the R-19 of about 6 in of batts, accessed October 17, 2026.
- NIST Handbook 44 (2026), Appendix C exact area conversions for the metric view, accessed October 17, 2026.