How much insulation do I need?
Pick the R-value you want, divide it by the R-value per inch of the insulation, and you have the depth. For blown-in insulation, the bag's coverage chart tells you how many square feet each bag covers at that R-value.
Depth (in) = R-value to add ÷ R per inch. Bags = area × (1 + extra %) ÷ bag coverage at that R-value
Take a 40 × 25 ft attic floor, 1,000 sq ft, with no insulation, to R-49 with blown-in fiberglass. At R-2.5 per inch that's 49 ÷ 2.5 = 19.6 in deep. If a bag covers 40 sq ft at R-30, it covers 40 × 30 ÷ 49 = 24.5 sq ft at R-49. With 5% extra, 1,000 × 1.05 ÷ 24.5 = 43 bags.
Topping up instead? Enter the R-value already there. Old loose fill and batts run roughly R-2.5 to R-3 per inch, so 4 in of old fiberglass is about R-11. Bringing it to R-49 means adding R-38: 15.2 in and 34 bags.
R-value per inch by insulation type
| Insulation | Typical R per inch | Used here |
|---|---|---|
| Blown-in fiberglass | 2.2–2.9 | 2.5 |
| Blown-in cellulose | 3.2–3.8 | 3.5 |
| Fiberglass batts | 2.9–3.8 | 3.2, or the batt's nominal thickness |
| Open-cell spray foam | 3.5–3.8 | 3.7 |
| Closed-cell spray foam | 6.0–7.0 | 6.5 |
These are typical values. The product label or bag chart is what counts, so change the R per inch and coverage to match what you buy.
How much insulation do I need in my attic?
ENERGY STAR and the Department of Energy recommend attic levels from about R-30 in the warmest climate zones up to R-60 in the coldest. Colder zones, which cover much of the country, typically call for R-49 to R-60. Look up your climate zone on the ENERGY STAR or DOE insulation map before you buy.
Blown-in insulation calculator
Blown-in (or blow-in) insulation is sold in bags, and each brand prints a chart with the bags per 1,000 sq ft and the minimum settled depth for each R-value. Enter your bag's coverage at R-30 and the calculator scales it to your target. Many home centers lend a blowing machine when you buy enough bags. Ask before you rent one.
Cellulose insulation calculator
Cellulose is denser than loose fiberglass, so it reaches a given R-value at a shallower depth: R-49 is 14 in at R-3.5 per inch, against 19.6 in for fiberglass. Bags are smaller and cheaper, but you need more of them. Settled depth is what counts. Blow to the installed depth on the bag chart, which allows for settling.
Wall insulation calculator
Wall depth is set by the studs: 3.5 in in a 2×4 wall and 5.5 in in a 2×6 wall. That limits what you can reach.
| Batt | Thickness | Fits |
|---|---|---|
| R-13 or R-15 | 3.5 in | 2×4 walls |
| R-19 | 6.25 in | 2×6 walls (slightly compressed) and floors |
| R-21 | 5.5 in | 2×6 walls |
| R-30 | 9.5 in | Attics and floors |
| R-38 | 12 in | Attics |
Batt packages list their coverage in square feet. Enter it, and the calculator rounds up to whole packages. The calculator warns you when the depth won't fit the wall.
Spray foam insulation calculator
Spray foam is measured in board feet: 1 sq ft at 1 in thick. Multiply area by depth. For 1,000 sq ft of wall at R-13, closed-cell foam at R-6.5 per inch needs 2 in, or 2,200 board feet with 10% extra. Open-cell needs 3.5 in to reach the same R-value. DIY kits are labeled by board feet, and foam yield drops in cold conditions, so follow the kit's temperature range.
Pricing an insulation job
Switch to contractor mode to add labor and markup. The production rate is in square feet per hour, with an hour added for setup on blown and foam jobs. Then send it with the estimate template.
Questions
How much insulation do I need in my attic?
ENERGY STAR and the DOE recommend about R-30 in the warmest climate zones up to R-60 in the coldest. Colder zones typically call for R-49 to R-60. Check the climate zone map for your area.
How deep is R-49 blown-in insulation?
About 19.6 in for blown fiberglass at R-2.5 per inch, or 14 in for cellulose at R-3.5 per inch. The bag chart lists the exact settled depth.
How many bags of blown-in insulation do I need?
Divide the attic area by the bag's coverage at your R-value, from the chart on the bag. For example, at 24.5 sq ft per bag, 1,000 sq ft takes 41 bags, or 43 with 5% extra.
Can I add new insulation over old insulation?
Usually yes, if the old insulation is dry and not moldy. Enter its R-value as existing insulation and the calculator works out the rest.