Insulation is rated by R-value, its resistance to heat flow, and R-values add up: R-11 of old insulation plus R-38 of new gives R-49. How thick that new layer has to be depends on the material. This insulation calculator takes your target and existing R-values, divides the difference by the material's R-value per inch, and tells you how many inches to add, the installed volume and the number of bags based on the coverage printed on the bag. For example, topping an attic from R-11 to R-49 with blown cellulose (about R-3.5 per inch) takes about 10.9 inches.
Insulating walls during a remodel? Frame and close them up with the wall framing calculator and the drywall calculator.
How to use the insulation r-value calculator
- Enter the length and width of the attic floor, wall or crawlspace you are insulating.
- Choose a target R-value. The Department of Energy's recommendations by climate zone are in the table below; your local energy code may require a specific level for new work.
- Estimate the existing R-value. Measure the depth of what is there and multiply by roughly 2.5 to 3.5 per inch for loose fiberglass or cellulose, or read the facing of old batts. Use 0 for a bare attic.
- Choose the material you will add. R-values per inch are typical figures; the product label is the final word.
- Enter the coverage per bag at the R-value you are adding, from the bag's coverage chart, to get a bag count.
Formula
Worked example
A 40 × 30 ft attic floor has about 3.5 inches of old fiberglass (roughly R-11). The homeowner, in climate zone 5, wants R-49 and will blow in cellulose. The bag chart says one bag covers 40 ft² at R-38.
- Area: 40 × 30 = 1,200 ft².
- R to add: 49 − 11 = R-38.
- Depth: 38 ÷ 3.5 = 10.9 inches of settled cellulose on top of the old layer.
- Bags: 1,200 × 1.05 ÷ 40 = 31.5, so 32 bags.
With blown fiberglass at about R-2.5 per inch, the same R-38 would take about 15.2 inches, which is why the bag chart, not depth alone, should guide you.
Inches of insulation for each R-value
Approximate installed thickness to reach each R-value from zero, at typical R-values per inch. Product R-values vary; the label governs.
| Material | R-13 | R-19 | R-30 | R-38 | R-49 | R-60 |
|---|---|---|---|---|---|---|
| Blown fiberglass | 5.2 in | 7.6 in | 12.0 in | 15.2 in | 19.6 in | 24.0 in |
| Blown cellulose | 3.7 in | 5.4 in | 8.6 in | 10.9 in | 14.0 in | 17.1 in |
| Fiberglass batts | 4.2 in | 6.1 in | 9.7 in | 12.3 in | 15.8 in | 19.4 in |
| Mineral wool batts | 3.9 in | 5.8 in | 9.1 in | 11.5 in | 14.8 in | 18.2 in |
| EPS foam board | 3.3 in | 4.8 in | 7.5 in | 9.5 in | 12.3 in | 15.0 in |
| XPS foam board | 2.6 in | 3.8 in | 6.0 in | 7.6 in | 9.8 in | 12.0 in |
| Polyiso foam board | 2.2 in | 3.2 in | 5.0 in | 6.3 in | 8.2 in | 10.0 in |
| Open-cell spray foam | 3.6 in | 5.3 in | 8.3 in | 10.6 in | 13.6 in | 16.7 in |
| Closed-cell spray foam | 2.0 in | 2.9 in | 4.6 in | 5.8 in | 7.5 in | 9.2 in |
Recommended R-values by climate zone (DOE)
The U.S. Department of Energy's recommended total R-values for wood-framed homes:
| DOE climate zone | Uninsulated attic | Attic with 3–4 in existing | Uninsulated floor |
|---|---|---|---|
| 1 | R-30 to R-49 | R-19 to R-38 | R-13 |
| 2 | R-49 to R-60 | R-38 to R-49 | R-13 |
| 3 | R-49 to R-60 | R-38 to R-49 | R-19 |
| 4 (except Marine) | R-60 | R-49 | R-19 |
| 4 Marine and 5 | R-60 | R-49 | R-30 |
| 6 | R-60 | R-49 | R-30 |
| 7 and 8 | R-60 | R-49 | R-38 |
Walls are typically R-13 to R-20 in the cavity, with continuous exterior insulation (R-5 to R-15) recommended in colder zones. Find your zone on the ENERGY STAR or DOE map.
Labels, settling and air sealing
U.S. rules (16 CFR Part 460) require insulation packaging to state R-values and, for loose fill, a coverage chart showing the minimum settled thickness, the number of bags per 1,000 ft² and the coverage per bag for each R-value. Blown insulation settles, so installers mark depth rulers to the initial installed thickness on the bag. Before adding insulation, seal attic bypasses around light fixtures, plumbing and the attic hatch; DOE notes air sealing and insulation work together. Keep insulation clear of recessed lights that aren't IC-rated and away from chimneys, and keep soffit vents open with baffles. Upgrading the roof too? See the roofing shingles calculator; for exterior walls, the siding calculator.
Caveats and assumptions
- R-values per inch are typical figures from DOE and manufacturers; real products vary. Use the label coverage chart for final quantities.
- Compressed batts lose R-value. Don't squeeze a thick batt into a thin cavity.
- Existing insulation that is wet, moldy or damaged by pests should be removed, not covered.
- Spray foam is normally installed by professionals; its R-value per inch differs between open- and closed-cell products.
Frequently asked questions
How many inches of insulation is R-49?
About 14 inches of blown cellulose at R-3.5 per inch, or about 19.6 inches of blown fiberglass at R-2.5 per inch. Check the bag chart for the exact settled thickness.
How many inches is R-38 insulation?
About 10.9 inches of cellulose, 12.3 inches of fiberglass batts or 15.2 inches of blown fiberglass, using typical R-values per inch.
How much insulation do I need in my attic?
DOE recommends R-30 to R-49 in the warmest zone and R-49 to R-60 elsewhere for an uninsulated attic. If you already have 3 to 4 inches, it recommends adding enough to reach R-38 to R-49 depending on zone.
Can I put new insulation on top of old insulation?
Usually, yes, if the old insulation is dry and in good condition. Don't put faced (vapor retarder) batts on top of existing insulation; use unfaced batts or loose fill.
How many bags of blown-in insulation do I need?
Divide your attic area by the coverage per bag at the R-value you are adding, from the bag's chart. For 1,200 ft² at 40 ft² per bag, that is 30 bags plus a little extra.
Sources
- U.S. Department of Energy — Energy Saver Guide — recommended R-values by climate zone; insulation types
- DOE Building Science Education — Types of insulation — insulation materials and how they work
- ENERGY STAR — Recommended home insulation R-values — climate zone map and levels
- eCFR — 16 CFR Part 460, Labeling and Advertising of Home Insulation — R-value labels and loose-fill coverage charts
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.