How to calculate insulation
Insulation packages list the square feet they cover, so the job is to find the area you are insulating and divide by that number. Batts and rolls fit between studs or joists, so the framing itself takes a small share of the area. Blown-in insulation is sold by the bag, and how far a bag goes depends on how deep you blow it, which is set by your target R-value.
Insulation area = area × (1 − framing %)
Packages = insulation area × (1 + extra %) ÷ sq ft per package
Blown-in: bags = attic floor area × (1 + extra %) ÷ sq ft per bag at your R-value
Worked example: batts for 40 ft of 8 ft wall
- Wall area: 40 × 8 = 320 sq ft.
- Subtract about 10% for studs and plates at 16 in. on center: 320 × 0.90 = 288 sq ft.
- Add 5% extra: 288 × 1.05 = 302.4 sq ft.
- Packages that cover 40 sq ft each: 302.4 ÷ 40 = 7.56, so buy 8 packages.
Worked example: blowing a 1,000 sq ft attic to R-49
- Suppose your bag chart says one bag covers 30 sq ft at R-49.
- Add 5% extra: 1,000 × 1.05 = 1,050 sq ft.
- Bags: 1,050 ÷ 30 = 35 bags.
- Depth: at about R-2.5 per inch for loose-fill fiberglass, R-49 is roughly 49 ÷ 2.5 = 19.6 in. deep.
How to read a blown-in insulation bag chart
Every bag of loose-fill insulation carries a coverage chart, and it is the only reliable source for how many bags you need. Coverage per bag varies a lot between products, between fiberglass and cellulose, and with bag weight. Each row of the chart is an R-value, and the columns typically give:
- Bags per 1,000 sq ft: divide your area by 1,000 and multiply. Or divide 1,000 by this figure to get sq ft per bag for the calculator.
- Maximum coverage per bag: the square feet one bag covers at that R-value.
- Minimum thickness: how deep the insulation must be to reach the R-value. Cellulose charts usually list an initial installed thickness and a lower settled thickness.
- Minimum weight per sq ft: what installers use to check that the insulation was not blown too fluffy.
If you are adding to existing insulation, R-values add together. Subtract what you have from your target and read the row for the difference. If there is no exact row, use the next higher one.
Recommended attic R-values by climate zone
ENERGY STAR publishes recommended attic insulation levels by climate zone, and the U.S. Department of Energy points homeowners to the same guidance. The first column of levels is for an uninsulated attic; the second is what to add when about 3–4 in. of insulation is already there.
| Climate zone | Example areas | Uninsulated attic | Existing 3–4 in.: add |
|---|---|---|---|
| 1 | South Florida, Hawaii | R-30 | R-25 |
| 2 | Gulf Coast, central Florida, Houston, Phoenix | R-49 | R-38 |
| 3 | Atlanta, Dallas, Las Vegas, much of the South | R-49 | R-38 |
| 4 | Mid-Atlantic, Kansas City, Seattle and the Pacific Northwest coast | R-60 | R-49 |
| 5–8 | Chicago, Denver, the upper Midwest, northern New England, Alaska | R-60 | R-49 |
Recommendations are updated from time to time, and older charts list somewhat lower minimums, so check the current ENERGY STAR chart and your local energy code. Your county's climate zone is on the code maps; it does not always follow state lines.
Common batt sizes and R-values
| Where | Cavity depth | Common batts (about) |
|---|---|---|
| 2×4 walls | 3-1/2 in. | R-13, or R-15 high-density |
| 2×6 walls | 5-1/2 in. | R-19 (compresses to about R-18), R-20 or R-21 high-density |
| Floors over crawlspaces or garages | 2×8 to 2×12 joists | R-19 to R-30, depending on joist depth and code |
| Attic floors between joists | Varies | R-30 or R-38, often topped with a second layer laid across the joists |
Batts come 15 in. wide for framing 16 in. on center and 23 in. wide for 24 in. on center. Precut batts are commonly 93 in. long for 8 ft walls and 105 in. for 9 ft walls; rolls are longer and you cut them to fit.
Why subtract framing?
At 16 in. on center, a 15 in. batt covers about 15/16 of the wall width, and plates at the top and bottom take a few more inches of height. Together that works out to roughly 10% of the wall area. At 24 in. on center it is closer to 8%. Extra studs at corners, windows and doors push it higher, so these figures lean toward buying slightly more, not less. The calculator lets you switch the deduction off or enter your own.
Tips before you insulate
- Don't compress batts. Squeezing a thick batt into a shallow cavity lowers its R-value. Cut around wires and pipes instead of stuffing behind them.
- Air-seal the attic first. Seal gaps around plumbing, wiring and top plates before blowing insulation. Leaks carry heat and moisture right through loose fill.
- Keep soffit vents open. Install baffles at the eaves so insulation doesn't block ventilation.
- Mind clearances. Keep insulation away from chimneys, flues and recessed lights that are not rated for insulation contact (IC-rated).
- Mark the depth. Staple depth rulers to a few joists so you can blow to an even level.
- Finishing a wall? After insulating, the drywall calculator estimates sheets, and the paint calculator covers primer and paint.
Frequently asked questions
How many bags of blown-in insulation do I need?
Divide your attic floor area by the square feet per bag listed on the bag chart for your target R-value. The number varies a lot by product, so always use the chart on the bag you are buying. Add about 5% for uneven joists and corners.
What R-value should my attic have?
For an uninsulated attic, ENERGY STAR recommends R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8, which covers most of the country. Check your climate zone and local code.
How deep should blown-in insulation be for R-49?
Roughly 13 to 15 in. for cellulose (about R-3.2 to R-3.8 per inch) and roughly 18 to 22 in. for loose-fill fiberglass (about R-2.2 to R-2.7 per inch). The bag chart lists the exact minimum thickness for each product.
How many square feet does a bag of insulation batts cover?
It depends on the R-value and batt size, so check the package. Thicker batts have fewer pieces per bag and cover fewer square feet. Remember that batts sized for 16 in. framing are 15 in. wide, so the framing takes up part of the wall area.
Can I add new insulation on top of old insulation?
Usually, yes, as long as the old insulation is dry and there are no signs of moisture or pests. R-values add together, so subtract the existing R-value from your target and add the difference. Don't put faced batts on top of existing insulation; use unfaced batts or loose fill.
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