Formula
R-value is resistance to heat flow. Layers add. Batts: net area is the gross area minus openings, floored at zero. Packages are that net times one plus waste, divided by the coverage on the package, rounded up. The package already states coverage for a framed wall at the spacing it names, so the studs are not subtracted a second time.
packages = ceil(net × (1 + waste ÷ 100) ÷ coverage)
R added = target − existing
depth = R added ÷ R per inch
bags on the chart = ceil(area × bags per 1,000 ÷ 1,000)
Blown-in has two paths. The bag chart is the one to order from: bags per 1,000 square feet for the R you are adding, required on loose-fill labels by the FTC R-value Rule. The other path is an estimate. Settled depth is the added R divided by R per inch. Volume is area times that depth. Weight is volume times settled density. Bags are the weight divided by the bag weight, when you enter one. RSI is R times 0.1761. Spray foam is a different product and is not calculated here.
Worked examples
Walls, 120 ft × 8 ft. 960 sq ft. Two doors at 20 and eight windows at 15 are 160 sq ft of openings. Net 800. At 5 percent and 40 sq ft per package, 21 packages.
Cellulose, 1,000 sq ft, existing R-11, target R-49. Add R-38. At 3.5 per inch the settled depth is 10.86 in. Volume is 904.8 ft³. At 1.5 lb/ft³ that is 1,357 lb, 55 bags of 25 lb. A chart row of 55.6 bags per 1,000 sq ft for that same add is 56 bags. The 55.6 figure is only an example.
Fiberglass loose fill, add R-38 at 2.5 per inch. 15.2 in settled. R-49 is RSI 8.63. R-13 is RSI 2.29.
Reference
ENERGY STAR retrofit targets for an uninsulated attic, and the 2021 IECC Table R402.1.3 code minimums for new construction. Zone Zone 5 is the default on the form. Change the zone and the target R follows the attic column. You can type over it.
| Zone | Attic, empty | Attic, 3–4 in | Floor | 2021 IECC ceiling | 2021 IECC wood wall | 2021 IECC floor |
|---|---|---|---|---|---|---|
| Zone 1 | R-30 | R-25 | R-13 | R-30 | R-13 or R-0 + R-10 ci | R-13 |
| Zone 2 | R-49 | R-38 | R-13 | R-49 | R-13 or R-0 + R-10 ci | R-13 |
| Zone 3 | R-49 | R-38 | R-19 | R-49 | R-20 or R-13 + R-5 ci or R-0 + R-15 ci | R-19 |
| Zone 4A or 4B | R-60 | R-49 | R-19 | R-60 | R-30 or R-20 + R-5 ci or R-13 + R-10 ci or R-0 + R-20 ci | R-19 |
| Zone 4C (marine) | R-60 | R-49 | R-30 | R-60 | R-30 or R-20 + R-5 ci or R-13 + R-10 ci or R-0 + R-20 ci | R-30 |
| Zone 5 | R-60 | R-49 | R-30 | R-60 | R-30 or R-20 + R-5 ci or R-13 + R-10 ci or R-0 + R-20 ci | R-30 |
| Zone 6 | R-60 | R-49 | R-30 | R-60 | R-30 or R-20 + R-5 ci or R-13 + R-10 ci or R-0 + R-20 ci | R-30 |
| Zone 7 | R-60 | R-49 | R-38 | R-60 | R-30 or R-20 + R-5 ci or R-13 + R-10 ci or R-0 + R-20 ci | R-38 |
| Zone 8 | R-60 | R-49 | R-38 | R-60 | R-30 or R-20 + R-5 ci or R-13 + R-10 ci or R-0 + R-20 ci | R-38 |
The current ENERGY STAR retrofit page does not publish a 2×4 or 2×6 cavity range. It tells you to fill an empty cavity and, in zones 3 through 8, to add insulating sheathing when the siding is off. R-13 and R-15 are common 2×4 batt labels. R-19 and R-21 are common 2×6 batt labels. Wall note for the default zone: fill the cavity, and in zones 4–8 the page also calls for about R-5 to R-10 of sheathing on an uninsulated wall when the siding comes off.
Zone 5 attic RSI at the empty-attic target is 10.57. Cellulose starts at 3.5 per inch, 1.5 lb/ft³, 25 lb a bag. Fiberglass loose fill starts at 2.5 per inch, 0.5 lb/ft³, 27.5 lb a bag.
Questions
How much blown-in insulation do I need for 1,000 sq ft?
Read the row on the bag for the R-value you are adding. This page’s estimate, which is not that chart, adds R-38 of cellulose at 3.5 per inch: 10.86 inches settled, about 1,357 lb, 55 bags at 25 lb. A label row of 55.6 bags per 1,000 sq ft rounds up to 56. Use the row for your product.
How many bags of insulation do I need?
For batts, net area times one plus waste, divided by the square feet on the package, rounded up. The 120 × 8 ft wall with 160 sq ft of openings is 800 sq ft net, 840 with 5 percent waste, 21 packages at 40 sq ft each. For loose fill, the bag’s bags-per-1,000 figure is the count to order.
What R-value do I need in my attic?
ENERGY STAR’s retrofit table, based on the 2021 IECC, calls an uninsulated attic R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. An attic that already has 3–4 inches is a lower add-on. Counties inside a state can sit in different zones. The map on the ENERGY STAR page is the lookup.
How deep should blown-in insulation be for R-49?
Divide the R you are adding by the R per inch. Cellulose at 3.5 per inch is about 14 inches for a full R-49 from an empty attic, and 10.86 inches when you are only adding R-38 over an existing R-11. The settled thickness on the bag is the number that counts. Fiberglass loose fill at 2.5 per inch is 15.2 inches for R-38.
Can I put new insulation over old?
Yes, when the old layer is dry and the target is higher than the R already there. Subtract the existing R. If the existing R already meets the target, this page orders nothing. Air-seal before you cover the attic floor, and keep the soffit vents open with baffles.
R-13 vs R-15 vs R-19 walls?
A 2×4 cavity is commonly filled with an R-13 or R-15 batt. A 2×6 cavity is commonly R-19 or R-21. Those are product labels, not the ENERGY STAR retrofit table. The 2021 IECC Table R402.1.3 is the code minimum for new construction, and it often adds continuous insulation outside the cavity. Local code wins.
Does insulation R-value go down if compressed?
Yes. A batt stuffed into a cavity thinner than its label loses R-value. DOE’s insulation guidance says the labeled R assumes the full thickness. Use the batt made for the cavity depth.
Sources
- ENERGY STAR, Recommended Home Insulation R-Values, the table “Recommended insulation levels for retrofitting existing wood-framed buildings,” based on the 2021 IECC Residential Provisions Chapter 4 Table R402.1.3. Attic and floor numbers on this page are that table. energystar.gov. The climate-zone map is on the same site. Counties inside one state can differ.
- 2021 International Energy Conservation Code, Table R402.1.3, Insulation Minimum R-Values and Fenestration Requirements by Component. The ceiling, wood-frame wall, and floor columns here are that table, labelled as the code minimum for new construction. Local amendments win.
- DOE, via the Building America loose-fill insulation summary of the Department of Energy fact sheet “Loose-Fill Insulations.” Settled cellulose is 3.2–3.8 R per inch and 1.5–2.0 lb/ft³. Settled loose-fill fiberglass is 2.2–2.7 R per inch and 0.5–1.0 lb/ft³. This page starts cellulose at 3.5 and 1.5, and fiberglass at 2.5 and 0.50, inside those ranges. Compressed batts lose R-value because the labeled R assumes full thickness. DOE Energy Saver, “Types of Insulation” and “Where to Insulate,” is the plain-language companion, including air sealing and soffit baffles.
- Greenfiber Sanctuary cellulose: published settled density 1.5 lb/ft³, nominal bag weight 25 lb. Owens Corning AttiCat Expanding Blown-In PINK Fiberglas: nominal bag weight 27.5 lb. The AttiCat coverage card’s minimum weight and thickness imply about 0.45–0.51 lb/ft³, which is why the fiberglass default is 0.50 rather than 0.6. R-38 on that card is 13.5 in, about 2.8 per inch, so 2.5 per inch is the DOE-range estimate and the card is the order quantity.
- FTC R-value Rule, 16 CFR Part 460. Loose-fill labels have to show the coverage chart: bags per 1,000 square feet and minimum thickness for each R-value. That chart is the preferred count on this page.