Insulation R-value calculator

Enter a target R-value and get the settled depth to install and the bags or bundles to buy — for fiberglass batt, blown fiberglass, or cellulose. The depth changes with the material, and that is the part most calculators leave out.

01 How the math works

R-value adds up by the inch. Every material has an R-value per inch of settled thickness, so the depth you need is the target R-value divided by that per-inch figure. Two layers stack: an R-12 layer under an R-38 layer is R-50.

The per-inch figure is where materials split. trowelmath uses representative values from the DOE ranges: fiberglass batt R-3.2/in (DOE range 2.9–3.8), blown fiberglass R-2.5/in (2.2–2.7), and blown cellulose R-3.5/in (3.2–3.8). Because the per-inch numbers differ, the same R-value adds up by the inch differently — the number on the bag is the same, the pile in your attic is not.

Blown-in bag counts work off area, not depth: bags = area × (1 + waste) ÷ the coverage per bag, rounded up. Coverage per bag at your target R is printed on the package, so read it there rather than assume a fixed number.

02 Worked example

Take a bare attic floor of 1,000 ft² (92.90 m²) with no usable insulation left, aiming for R-49 for a cold-climate attic.

Settled depth to hit R-49: 14.0 in (35.56 cm) of blown cellulose, 15.31 in (38.90 cm) of fiberglass batt, or 19.6 in (49.78 cm) of blown fiberglass. Same R, but the blown fiberglass pile is about 40% deeper — enough to bury the top of a 2×10 ceiling joist and then some.

For the bag count, say the bag's chart lists 40 ft² of coverage at R-49. With a 10% waste allowance that's ⌈1,000 × 1.10 ÷ 40⌉ = ⌈27.5⌉ = 28 bags. Read the coverage off your own product — it moves with density and target R.

03 When this calculator is wrong

This calculator uses one representative R-per-inch per material and treats every inch as equal. Real installations diverge in a few specific ways.

• Depth is not interchangeable across materials. R-49 is 14.0 in of cellulose but 19.6 in of blown fiberglass — about 40% more depth for the identical R. If your trusses or storage deck cap the depth, the low-per-inch material may not physically fit, and picking by price alone can leave you short on R.
• Compressed batts lose R-value. A nominal R-19 fiberglass batt is about 6.25 in (159 mm) thick. Squeezed into a 2×6 stud cavity (actual 5.5 in / 140 mm) it delivers roughly R-18, not R-19. Anywhere a batt is crushed — behind pipes, under wiring, over-stuffed — the installed R drops below the label. A calculator that multiplies the label rating by area overstates what you actually get.
• Loose fill has two minimums, and it settles. Blown-in settles about 15–20% after install, so coverage charts already state the settled thickness. The FTC R-Value Rule requires meeting both the minimum bags per 1,000 ft² and the minimum settled depth — hit the bag count but blow it too thin and you miss the rating. The depth here is the settled target; fluff the fresh blow higher so it lands there.
• R-per-inch is representative, not exact. Product density shifts the real figure inside the DOE range. High-density batts run higher; low-density loose fill runs lower. The number printed on the product governs the estimate, not the site default.

04 What to do with the result

Order more than the math gives you, never less. Unopened bags and unused batt bundles return easily, so the cost of one extra bag is small; the cost of stopping a blow with 80 ft² still bare, or driving back mid-install, is a lost afternoon. Round the bag count up to whole bags and add the waste factor before you buy.

Mark the settled depth on the truss faces or a couple of depth rulers stapled to the joists before you start, then blow to the line and confirm the bag count came out at or above the chart minimum. For batts, buy the width that matches the actual cavity — 15 in for 16-in-on-center framing, 23 in for 24-in — and cut around obstructions rather than crushing full batts over them.

05 Common questions

How many inches of insulation do I need for R-49?

About 14.0 in (35.56 cm) of blown cellulose, 15.31 in (38.90 cm) of fiberglass batt, or 19.6 in (49.78 cm) of blown fiberglass. The depth follows the material's R-per-inch, so a lower-per-inch material needs a deeper pile for the same R.

What R-value do I need in my attic?

ENERGY STAR and DOE recommend attic R-values by climate zone: R30–R49 in the warmest zone, R38–R60 through the middle, and R49–R60 in cold zones. The 2021 IECC ceiling minimum is R-49 for Zones 2–3 and R-60 for Zones 4–8. Use the code minimum your jurisdiction has adopted as the floor and confirm the adopted code year with your building department.

Does blown-in insulation lose R-value over time?

It settles about 15–20% in the first days, which is why manufacturer coverage charts quote the settled thickness and the bags needed to reach a rated R after settling. As long as you install the minimum bags and hit the settled depth, the rated R holds; the loss is designed into the chart, not a surprise later.

Can I add new insulation over old?

Yes. R-values add, so unfaced blown-in or unfaced batts laid over existing insulation stack to the combined R. Enter the existing depth above and the calculator returns the R already in place and the extra depth to add. Do not lay faced batts face-up over old insulation — the facing traps moisture — and do not compress the old layer.

Is cellulose or fiberglass better per inch?

Cellulose carries more R per inch — about R-3.2 to R-3.8 versus R-2.2 to R-2.7 for blown fiberglass — so it hits a target R in less depth. Fiberglass is lighter, which matters over a ceiling rated for limited load. Neither is "better" outright; the choice is depth and weight against price.