How insulation is calculated
Enter each attic, wall or floor as its own area with “+ Add another area”. The areas are added together and the batts or bags are rounded once for the whole job. Every area uses the same insulation and R-value, so run a different product, such as batts in the walls and blown-in in the attic, as a separate calculation.
Batts are counted by framing bay. One batt fills one cavity, so each batt covers the stud spacing times the batt length, including the framing it sits between.
wall covered per batt (ft²) = (spacing ÷ 12) × (batt length ÷ 12)
batts = area ÷ wall covered per batt, rounded up
bags = batts ÷ 10, rounded up
nominal thickness (in) = R-value ÷ 3.2
Blown-in insulation is sized by depth. Divide the target R-value by the R per inch of the material to get the settled depth, then use the coverage per bag for that R-value to count bags.
depth (in) = R-value ÷ R per inch (2.5 blown fiberglass, 3.6 cellulose)
volume (ft³) = area × depth ÷ 12
coverage per bag (ft²) = bag chart value, or typical coverage at R-38 × 38 ÷ R-value (51 ft² fiberglass, 24 ft² cellulose)
bags = area ÷ coverage per bag, rounded up
The typical coverage assumes a fiberglass bag that covers 51 ft² at R-38 (a cellulose bag about 24 ft²) and spreads thinner or thicker in proportion. Real bag charts fall off a little faster than that at high R-values (a few percent at R-60 for fiberglass, about 10% for cellulose), and brands differ, so enter the figure from your bag's chart under advanced options for an exact count.
Worked example: 1,000 ft² attic to R-38
The calculator opens on a 1,000 ft² attic floor with blown fiberglass to R-38.
- Depth: 38 ÷ 2.5 = 15.2 in settled.
- Volume: 1,000 × 15.20 ÷ 12 = 1,266.7 ft³.
- Coverage per bag: 51 × 38 ÷ 38 = 51.0 ft².
- Bags: 1,000 ÷ 51.0 = 20 bags.
The same 1,000 ft² of wall in R-13 batts at 16 in on center with 93 in batts: each batt covers 1.333 × 7.75 = 10.33 ft², so you need 97 batts, or 10 bags of 10. The nominal thickness works out to 4.1 in, but a real R-13 batt is a denser 3-1/2 in product made to fill a 2×4 wall.
Minimum R-values by climate zone
These are the IECC 2021 prescriptive minimums for new homes, with the settled depth each ceiling value takes in blown fiberglass and cellulose. Find your zone by county in IECC Table R301.1. In zone 5, for example, ceilings call for R-60 and floors R-30.
| Zone | Ceiling | Blown fiberglass, in | Cellulose, in | Wood-frame wall | Floor |
|---|---|---|---|---|---|
| 0 and 1 | R-30 | 12.0 | 8.3 | 13 or 0+10ci | R-13 |
| 2 | R-49 | 19.6 | 13.6 | 13 or 0+10ci | R-13 |
| 3 | R-49 | 19.6 | 13.6 | 20 or 13+5ci or 0+15ci | R-19 |
| 4 except Marine | R-60 | 24.0 | 16.7 | 30 or 20+5ci or 13+10ci or 0+20ci | R-19 |
| 5 and Marine 4 | R-60 | 24.0 | 16.7 | 30 or 20+5ci or 13+10ci or 0+20ci | R-30 |
| 6 | R-60 | 24.0 | 16.7 | 30 or 20+5ci or 13+10ci or 0+20ci | R-30 |
| 7 and 8 | R-60 | 24.0 | 16.7 | 30 or 20+5ci or 13+10ci or 0+20ci | R-38 |
“13+5ci” means R-13 in the cavity plus R-5 continuous insulation over the framing. Section R402.2.1 lets R-38 satisfy an R-49 ceiling, and R-49 satisfy R-60, where full-height insulation covers the whole ceiling and runs over the top plates at the eaves. States and cities amend the code; confirm the edition in force with your building department.
Batt thickness by R-value
| R-value | Thickness | Type | Typical cavity |
|---|---|---|---|
| R-13 | 3-1/2 in | Standard | 2×4 wall |
| R-15 | 3-1/2 in | High-density | 2×4 wall |
| R-19 | 6-1/4 in | Standard | 2×6 wall (compressed to 5.5 in it performs at about R-18) |
| R-21 | 5-1/2 in | High-density | 2×6 wall |
| R-30 | 9-1/2 in to 10 in | Standard | 2×10 floor or attic |
| R-38 | 12 in to 12-1/2 in | Standard | 2×12 or attic |
| R-49 | 13-1/2 in to 14 in | Standard | Attic (single R-49 batt, or two layers) |
| R-60 | 19 in | Standard | Attic: two layers (for example R-30 + R-30); no common single R-60 batt |
Measuring and buying tips
- For an attic, measure the ceiling below, length times width, and skip the eaves past the walls. For walls, multiply length by height and subtract big openings like garage doors and patio doors; leave ordinary windows in, since the offcuts fill around them.
- Match the batt to the framing: 15 in wide batts for 16 in on center, 23 in for 24 in. Use 93 in precut batts for 8 ft walls and 48 or 96 in for floors and attics.
- Blown-in settles. The bag chart lists both the installed and the settled depth; size to the settled depth and set depth rulers on the joists before you start.
- Air-seal top plates, plumbing and wiring holes, and the attic hatch before insulating, and keep loose fill back from recessed lights that are not IC-rated and from flues.
- Keep soffit vents open with baffles so the attic still breathes once the fill is deep.
When to hire a pro
Get an insulation contractor for knob-and-tube wiring, vermiculite (it can contain asbestos), signs of moisture or mold, or an attic too low to work in safely. A blower rental is fine for an open attic, but a pro will dense-pack closed walls and can run a blower-door test to find the air leaks.
Sources
- International Code Council, 2021 International Energy Conservation Code, Table R402.1.3 (insulation minimum R-values) and Section R402.2.1 (ceilings with attics); 2021 values as tabulated by PNNL Building America Solution Center
- R-value per inch: fiberglass batt 3.2 (manufacturer data); loose fill within the DOE/PNNL ranges of 2.2–2.7 (fiberglass, 2.5 used) and 3.2–3.8 (cellulose, 3.6 used)
- Blown fiberglass bag chart: Owens Corning AttiCat, about 51 ft² per bag at R-38 (other brands 52–61)
- Blown cellulose bag chart: GreenFiber INS765LD, about 24 ft² per bag at R-38
- Batt packaging: typical 10 batts per bag (Owens Corning and Knauf packages run 6 to 13 depending on R-value)