How to measure for your insulation r-value project
- Pick your target R-value. Select the location: walls in a 2×4 frame need R-13, 2×6 walls need R-21, floors over unheated spaces need R-30, and attics range from R-38 in mild climates to R-60 in the far north. When in doubt, check your local energy code — colder DOE climate zones (5–7) require higher R than zones 1–3.
- Choose your insulation material. Fiberglass batts (R-3.2/in) are the cheapest and easiest for a DIY wall cavity. Blown cellulose (R-3.5/in) is great for attics and retrofits — it fills gaps without cutting. Closed-cell spray foam (R-6.5/in) packs the most R per inch and adds an air seal, but needs professional installation. The calculator converts whichever material you pick into the exact thickness required.
- Enter your area and any existing R-value. Type in the square footage to insulate. If you already have insulation in the space, measure or estimate its current R-value and enter it in the "Existing R-value" field — the calculator subtracts it so you only buy what you actually need. The result shows thickness in inches and coverage in board feet, which matches the language on blown-in product bags.
How the insulation r-value calculator works
R-value measures resistance to heat flow — the higher the number, the slower heat moves through the material. Each insulation type has a fixed R-per-inch rating, so the thickness you need equals the R-value still to gain divided by that rating: thickness (in) = (target R − existing R) ÷ R-per-inch. Coverage in board feet is simply area × thickness, matching the unit on blown-in product bags. Worked example: a 1,000 sq ft attic has R-19 of old blown fiberglass and you want to reach R-49. You need 49 − 19 = 30 more R. With blown cellulose at R-3.5/in that is 30 ÷ 3.5 = 8.6 inches of added insulation, covering 1,000 × 8.6 = 8,600 board feet. Switching to closed-cell spray foam (R-6.5/in) instead: 30 ÷ 6.5 = 4.6 inches — less depth, but a much higher installed cost.
Which type are you estimating?
Fiberglass batts — walls and floors
The most common DIY choice. Batts fit snugly between studs or joists and are available pre-cut to 2×4 (R-13) or 2×6 (R-21) depth. Never compress a batt to fit a thinner cavity — compression reduces its R-value proportionally. To hit R-13 from bare you need about 4.1 inches; R-21 needs 6.6 inches.
Enter: R-per-inch: 3.2 | Target R: R-13 (wall 2×4) or R-21 (wall 2×6)
Blown cellulose — attic top-ups and retrofits
Cellulose (R-3.5/in) is made from recycled paper and is the go-to for blowing into existing attic floors or dense-packing closed cavities. It fills around wiring and blocking without cutting. To reach R-49 from bare you need 14.0 inches; R-60 needs 17.1 inches. Rentable blowers are available at home centers.
Enter: R-per-inch: 3.5 | Target R: R-38, R-49, or R-60 (attic by climate zone)
Closed-cell spray foam — highest R, air seal included
Closed-cell foam delivers R-6.5 per inch — more than twice the R of fiberglass per inch — and doubles as an air and vapor barrier. It is ideal for cathedral ceilings, rim joists, and crawlspaces where thickness is limited. R-49 from bare takes only 7.5 inches; R-30 only 4.6 inches. Professional installation is required.
Enter: R-per-inch: 6.5 | Target R: R-30 (floor/crawlspace) or R-49 (attic/cathedral ceiling)
DOE attic targets by climate zone
The U.S. Department of Energy recommends R-38 for attics in zones 1–3 (southern states), R-49 for zones 4–6 (most of the continental U.S.), and R-49 to R-60 for zones 6–7 (northern and mountain states). Enter your existing R-value and the calculator shows exactly what thickness to add on top — no need to pull out what is already there.
Enter: Target R: R-38 (mild), R-49 (cold), R-60 (far north) | Enter existing R to top up
Floor insulation over unheated spaces
Floors above an unheated crawlspace or garage target R-25 to R-30. Fiberglass batts are common here (R-30 from bare = 9.4 inches); friction-fit between joists and staple a vapor retarder to the warm side. If the crawlspace has moisture issues, consider rigid XPS foam (R-5.0/in) against the foundation walls instead — R-30 needs only 6.0 inches of XPS.
Enter: R-per-inch: 3.2 (fiberglass) or 5.0 (XPS) | Target R: R-30 (floor)
Tips & ways to save
- Never compress a fiberglass batt to fill a thinner cavity — a 3.5-inch batt rated R-13 crammed into a 2-inch gap loses much of its R-value because you are squeezing out the air pockets that do the insulating.
- Air sealing before you insulate pays bigger dividends than extra R-value. Caulk and foam around wiring penetrations, top plates, and recessed lights before you lay or blow in any insulation.
- Blown-in bags list coverage in square feet at a given depth — cross-check using your board-foot result. For example, 8,600 board feet ÷ your bag's sq-ft-at-depth rating tells you how many bags to buy.
- In cold climates, add the insulation on top of the existing layer rather than replacing it. The calculator's "existing R" field does this math for you — just enter what is already there.
- Wear an N95 mask and safety glasses when handling fiberglass batts or running a cellulose blower. Fine fibers and dust irritate lungs and eyes.
Insulation thickness for a target R-value (inches)
| Target R-value | Fiberglass batt | Blown cellulose | Closed-cell foam |
|---|---|---|---|
| R-13 | 4.1 in | 3.7 in | 2.0 in |
| R-21 | 6.6 in | 6.0 in | 3.2 in |
| R-30 | 9.4 in | 8.6 in | 4.6 in |
| R-38 | 11.9 in | 10.9 in | 5.8 in |
| R-49 | 15.3 in | 14.0 in | 7.5 in |
| R-60 | 18.8 in | 17.1 in | 9.2 in |
Thickness = target R ÷ R-per-inch (fiberglass batt ≈ 3.2, blown cellulose ≈ 3.5, closed-cell foam ≈ 6.5 per inch). Recommended R-values vary by climate zone — check local code.
Frequently asked questions
How much insulation do I need for an attic?
What is the R-value per inch of insulation?
How do I add insulation to an existing attic?
Does compressing insulation reduce its R-value?
What R-value do I need for my climate zone?
What is a board foot of insulation?
Sources
Related calculators
Reviewed by the BackyardCalc editorial team. Figures are computed from the formula above and checked against manufacturer yields.