Insulation Calculator

R-38 is 15.2 inches of blown fiberglass and 5.8 inches of closed-cell foam

Required depth is the target R-value divided by the material's R-value per inch, which means the material you choose decides how much room the job needs before it decides anything else. At an R-38 attic target, blown fiberglass at R-2.5 per inch has to be 15.2 inches deep; closed-cell foam at R-6.5 per inch reaches the same R-38 in 5.8 inches. A higher R-per-inch product such as closed-cell foam or a rockwool batt gets to the same R-value in far less thickness than blown fiberglass, and that is the comparison people get wrong most often: three inches of one product is not three inches of another, so compare materials by R-value and never by depth.

The eight types the tool offers use representative R-value-per-inch figures: blown fiberglass ~2.5, blown cellulose ~3.5, blown mineral wool ~3.0, fiberglass batt ~3.2, mineral wool batt ~4.2, XPS rigid board ~5.0, open-cell foam ~3.7, and closed-cell foam ~6.5. Products vary within published ranges, so check your specific product's rating before you commit to a depth. The tool lays all eight out side by side in a full depth-and-quantity table, so the thickness each one would need for your target is visible next to the material you actually picked.

Rockwool, sold as mineral wool, is covered in both loose-fill (blown) and batt form. It typically reaches a higher R-value per inch than fiberglass, especially as a batt — around R-4.2 per inch against roughly R-3.2 per inch for a standard fiberglass batt — which is why the two batt options land at such different depths for the same target.

The two spray foams split the same way. Open-cell foam (R-3.5–3.7 per inch) is cheaper, flexible, vapor-permeable, and an excellent sound dampener, which makes it the usual pick for interior walls and sound control. Closed-cell foam (R-6–7 per inch) is rigid, waterproof, acts as a vapor barrier, and carries the highest R-value per inch on the list, which is what earns it rim joists, crawl spaces, and extreme climates.

Every number here is reported as R-value, because that is what insulation is actually rated and sold by in the US. R-value measures resistance to heat flow, so higher is better insulated. U-value (or U-factor) is its mathematical inverse, U = 1 ÷ R — lower is better insulated — and it is the figure used for windows, doors, and UK and European building specifications. Divide 1 by any R-value result to convert it: R-38 ≈ U-0.026.

The same R-38 attic in 32 bags, 12 packages, or 10,271 board-feet

Depth is half the answer; the other half is how much material that depth and your area add up to in the unit your product is actually sold in. Take a 1,000 sq ft attic at an R-38 target in blown fiberglass (R-2.5/in), 40 cu ft per bag, $15 a bag. Required depth: 38 ÷ 2.5 = 15.2 in. Cubic feet: 1,000 × 15.2 ÷ 12 = 1,267 cu ft. Bags at 40 cu ft each: ⌈1,267 ÷ 40⌉ = 32 bags. Estimated material cost: 32 × $15 = $480.

The same attic in fiberglass batt (R-3.2/in) at 88 sq ft per package: 38 ÷ 3.2 = 11.9 in, then ⌈1,000 ÷ 88⌉ = 12 packages. In open-cell spray foam: 38 ÷ 3.7 = 10.3 in, then 10,271 board-feet. One R-38 target, three completely different shopping lists.

Each route works differently. Loose-fill — blown fiberglass, cellulose, or mineral wool — turns depth and area into a volume, divides it by the coverage-per-bag figure you enter from the product chart, and rounds up to whole bags. Batts, rolls, and rigid board skip volume entirely, because they come in a fixed thickness per product rather than a variable-depth fill: select a batt, roll, or rigid-board option from the Insulation Type list and enter the square feet each package covers, printed on the wrapper, instead of a bag's cubic-foot coverage, and the package count is simply your area divided by that coverage area, rounded up. Spray foam is quantified in board-feet — one square foot at one inch, so area times depth in inches — because foam is sold and quoted that way rather than by the bag.

The R-value achieved is the calculated depth times the R-per-inch, which is how you confirm the depth you are about to blow lands on the target rather than near it. The optional price field then takes whatever your selected type is sold by — per bag, per package, or per board-foot, whichever applies — and multiplies it by the quantity, so the budget comes out of the same pass. Leave it at 0 to skip the estimate. It is material only, at the price you enter: no tax, no delivery, and no installation labor.

Set the target from your climate zone: R-30 in the South, R-60 in the North

The target R-value is an input here, not an output, so it is the number to settle before you pick a material. DOE recommendations by climate zone: Zone 1–2 (South) R-30 to R-49, Zone 3–4 (Mid) R-38 to R-60, and Zone 5–8 (North) R-49 to R-60. R-38 is the most common attic target for mixed-climate US homes, and EnergyStar.gov will give you the range for your specific zone. Walls generally run R-13 to R-21, floors R-25 to R-32. Measure the area to match the assembly you are targeting: conditioned floor or ceiling area for an attic, wall square footage for a wall, floor area for a floor.

Recommended and required are not the same thing. Required R-values are set by climate zone and application in the energy code, and crawl spaces and cathedral ceilings carry moisture rules of their own. Confirm the target R-value and any vapor-barrier requirement with the IECC or a local building official before you order material, and consult an insulation pro for spray foam or any moisture-prone assembly.

Measure what is already up there, or you will pay for R-38 twice

The estimate assumes a bare cavity or attic floor, and the biggest planning error on an insulation job is ignoring insulation that is already in place and ordering for the full target rather than the gap. Measure the existing depth, multiply it by that material's R-per-inch to get its current R-value, and subtract that from your target before you use the tool — you only need to add the difference. Six inches of settled blown fiberglass is already about R-15, which makes an R-38 attic a 23-point job rather than a 38-point one.

Adding on top is straightforward: blown insulation works well over existing batts, and it is the usual way to top up an attic. Two rules govern the layering. Do not add vapor barriers between insulation layers — never sandwich a second one in between — and if the new layer is batts, install them with the facing down, against the existing insulation.

The coverage chart on the bag outranks every bag count here

Coverage per bag or package depends on the specific product, and for blown insulation on the blowing machine and how much the material settles, so two products of the same nominal type can cover different areas. Always cross-check the count against the coverage chart on the product you actually buy — that chart, tuned to the real settled density, is more reliable than any generic per-bag figure, and it is the number to follow when the two disagree. Treat the quantity here as a starting point: order it, then add a spare bag or two so you are not stopped just short of the far corner of an attic.

Use the depth figure the same way. Mark your target on the rafters or on a depth gauge so you blow evenly to the right thickness everywhere, and check it with a ruler in several spots as you go, because coverage falls short in the corners and eaves if you do not watch the depth. Attics get blown short in the eaves where the roof meets the wall, which leaves a cold ring around the edge, and soffit vents get buried in the process. Install rafter baffles at the eaves so soffit ventilation keeps working after the insulation goes in, while you still hit full depth out at the wall.

This is a job most homeowners can take on themselves, whether the target is an attic, a wall, a ceiling, or a floor, and whether the material is loose-fill fiberglass, cellulose, or mineral wool, fiberglass or mineral-wool batts and rolls, rigid foam board, or spray foam. Many home improvement stores rent an insulation blower free with a minimum bag purchase, which is what makes blown fiberglass and cellulose genuinely DIY-friendly. Have a helper feed bags while you run the hose. Spray foam is the exception: it needs professional equipment and training, so it is not a rental-machine weekend.

One thing no bag count can correct is the framing. Joists and studs conduct heat around the insulation, so thermal bridging pulls the assembly's effective R-value below the insulation's rated value no matter how accurately you hit the depth.

Air-seal before you insulate: R-value cannot plug a leaky attic

Caulk and foam the gaps, top plates, and penetrations first, because insulation slows heat flow but does not stop air leaks, and a leaky attic wastes the R-value you paid for. No depth figure on this page compensates for that.

The same boundary applies to what the numbers mean. This tool sizes the material and its cost; what it does not do is estimate what you will save. Energy savings depend on your climate, your current insulation level, your heating and cooling system's efficiency, and local energy prices, all of which vary too much for a general calculator to estimate honestly. For a savings figure, use a home-energy-audit tool or a professional who can measure your specific house.

Price has an edge of its own. Spray-foam board-feet is a material quantity, not an installed price — foam is a professional install, so get a contractor quote rather than reading the board-foot number as a budget — and for every other material the optional cost covers the product alone, with installation labor separate if you are hiring the work out.

Those are the limitations worth carrying to the store: depth and achieved R-value are dependable arithmetic, coverage is a manufacturer's number to be verified, and comfort, code compliance, and the energy bill are all decided outside this page.

Sources and methodology

Recommended R-values by climate zone follow the 2021 International Energy Conservation Code (IECC) Residential Provisions, as published by ENERGY STAR (a joint EPA/DOE program).

• ENERGY STAR (EPA) — Recommended Home Insulation R-Values by Climate Zone

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