Insulation Calculator
Work out how much insulation your attic, walls, or floor needs — R-value to add, inches of depth, and bags or bundles to buy — with DOE climate-zone targets, U.S. pricing, a material comparison, and a downloadable PDF.
Targets follow DOE climate zone recommendations. Bag and bundle coverage varies by brand — always check the label. Air seal before you insulate; insulation over unsealed gaps wastes most of its value. Estimates use 2024-2025 U.S. prices — for planning purposes only.
| Material | R / inch | Depth Needed | Quantity | Material Cost |
|---|
To add R-30 over 1,116 sq ft. Prices are typical U.S. retail — spray foam is normally quoted installed, not DIY.
How to use the insulation calculator
Four quick steps turn your area and target R-value into a materials order — thickness, bags or rolls, and cost — with your climate zone, material, and waste dialed in, a material comparison, and a downloadable PDF.
Enter the area
The square footage of the attic, wall, or floor you’re insulating.
Set the target R-value
Pick your climate zone and surface, or type the R-value your code requires.
Choose the material
Fiberglass, cellulose, foam, or rigid board — the thickness updates to match.
Read results & save a PDF
Get the thickness, bags or rolls, a cost breakdown, and a downloadable material list.
How the insulation estimate is calculated
It all rests on R-value, the measure of thermal resistance. The calculator turns your target R-value into a thickness for your chosen material, then uses coverage to get the bags or rolls. Here is the exact math.
Worked example — a 1,000 sq ft attic to R-38 with fiberglass
Thickness: 38 ÷ 3.2 ≈ 11.9 in of fiberglass
Coverage: ~40 sq ft per batt bag at R-38
Bags: 1,000 ÷ 40 × 1.10 ≈ 28 bags
≈ 12 inches of fiberglass and about 28 bags of batts for a 1,000 sq ft attic at R-38.
Insulation R-value charts
Handy lookups for the questions people ask most — R-value per inch by material, recommended R by surface, thickness for a target R, and R-value by climate zone. Confirm with your local energy code and product charts.
| Material | R per inch | Notes |
|---|---|---|
| Blown fiberglass | ~R-2.5 | Common attic blow-in |
| Fiberglass batt | ~R-3.2 | Cheap, DIY-friendly |
| Rock / mineral wool | ~R-3.3 | Fire & sound resistant |
| Blown cellulose | ~R-3.5 | Recycled, fills cavities |
| Open-cell spray foam | ~R-3.6 | Air seals, softer |
| XPS rigid board | ~R-5.0 | Basements, sheathing |
| Polyiso board | ~R-6.0 | High R, foil-faced |
| Closed-cell spray foam | ~R-6.5 | Highest R, air & vapor seal |
Higher R per inch means less thickness for the same R-value. Foams also air-seal, which batts and blow-in don’t do on their own.
| Surface | Typical R-value | Notes |
|---|---|---|
| Attic | R-38 – R-60 | Highest R; biggest heat loss |
| 2×6 wall | R-19 – R-21 | Deeper cavity |
| 2×4 wall | R-13 – R-15 | Standard stud wall |
| Floor | R-25 – R-30 | Over unconditioned space |
| Crawlspace | R-19 – R-30 | Walls or floor above |
| Basement wall | R-11 – R-15 | Rigid board common |
General ranges — your climate zone and local energy code set the actual target. Colder zones sit at the higher end.
| Target | Fiberglass | Cellulose | Closed-cell foam |
|---|---|---|---|
| R-13 | ~4.1 in | ~3.7 in | ~2.0 in |
| R-19 | ~5.9 in | ~5.4 in | ~2.9 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. Make sure the cavity or attic has room for the depth — foam hits the target in the least space.
| Climate zone | Attic R-value | Example areas |
|---|---|---|
| Zones 1–2 | R-30 – R-49 | Hot south, Gulf, south TX/FL |
| Zone 3 | R-30 – R-60 | Mild south, southern CA |
| Zone 4 | R-38 – R-60 | Mid-Atlantic, mid states |
| Zone 5 | R-49 – R-60 | Cold, northern states |
| Zones 6–7 | R-49 – R-60 | Very cold, far north |
Based on U.S. Department of Energy guidance. Always confirm against your local energy code, which may set a specific minimum.
thermal resistance
R ÷ R-per-inch
bags per area
Sizing insulation is a chain: pick the R-value, turn it into a thickness for your material, then into bags for your area.
- R-value — the target resistance for your surface and climate zone.
- Thickness — the target R divided by the material’s R per inch.
- Coverage — area divided by the bag or roll coverage, plus waste.
Everything the calculator works out
One area and target R-value gives you a complete insulation order — the thickness, the bags, the cost, and a material comparison.
Key figures behind insulation
Built for any insulation project
From topping up an attic to insulating a whole build, the same calculator scales to the surface, the R-value, and the material.
Adding insulation to a chilly attic and wants the right depth and number of bags to hit code R-value.
- Check your zone’s target R
- Add over dry existing insulation
- Keep clear of vents & hot lights
Insulating open walls or floors during a remodel and comparing materials on depth and cost.
- Match R to the cavity depth
- Compare batt vs foam thickness
- Air-seal before insulating
Speccing insulation across a new build or a client’s home and needs materials and cost per surface.
- Set R by code and zone
- Total bags across all surfaces
- Save the PDF for the quote
7 tips for insulating right
Small habits keep your R-value real and your insulation working, whether it’s batts, blow-in, or foam.
Insulation calculator FAQ
The R-value, thickness, material, and coverage questions people ask most before buying insulation.
Start with the target R-value for your surface and climate zone, then work out the material. Divide the target R-value by the material’s R-value per inch to get the thickness, and use the coverage per bag or roll to get how many you need.
For example, a 1,000 sq ft attic insulated to R-38 needs about 12 inches of fiberglass, which is roughly 28 bags of batts once you add a little for waste.
It depends on the surface and your climate zone. The U.S. Department of Energy recommends roughly R-30 to R-60 for attics, R-13 to R-21 for walls, and R-25 to R-30 for floors, with colder zones at the higher end.
Attics in cold northern zones often target R-49 to R-60, while mild southern zones may only need R-30 to R-38. Check your zone and surface, since local energy codes may set a minimum.
Multiply the thickness in inches by the material’s R-value per inch. Fiberglass is about R-3.2 per inch, cellulose about R-3.5, and closed-cell spray foam about R-6.5, so the same R-value takes very different thicknesses.
For instance, 10 inches of fiberglass is about R-32, while 10 inches of closed-cell foam is about R-65. To hit a target R-value, divide the target by the R per inch to get the thickness you need.
There’s no single best type; each suits different jobs. Fiberglass batts are cheap and DIY-friendly for open framing; blown cellulose fills attics and irregular cavities well; and spray foam gives the highest R-value per inch and air-seals, but costs more and is usually professionally installed.
Rigid foam boards suit basement walls and exterior sheathing. Match the material to the surface, your budget, and whether you also need air sealing.
For a common R-38 attic, plan on about 12 inches of fiberglass or blown cellulose. For R-49, which is recommended in colder zones, you’re looking at roughly 15 to 16 inches of fiberglass.
Blown insulation is installed to a marked depth to hit the R-value, while batts come in set thicknesses. Higher R-value means more depth, so make sure your attic has the room and that soffit vents aren’t blocked.
Divide the area you’re insulating by the coverage per bag at your target R-value, then add about 10% for waste. Each bag of blown insulation lists a coverage chart showing square feet per bag at each R-value and depth.
Higher R-values cover fewer square feet per bag because the layer is thicker. A 1,000 sq ft attic at R-38 might take around 28 to 35 bags depending on the product, so always check the bag’s chart.
Up to a point. Adding insulation gives diminishing returns, since going from no insulation to R-13 saves far more than going from R-38 to R-49. Beyond your zone’s recommended level, the extra savings get small.
Air sealing often matters as much as R-value, because gaps let heated or cooled air escape regardless of insulation. Aim for the recommended R-value for your zone and seal air leaks rather than piling on insulation past the point of value.
Usually yes. You can add unfaced batts or blown insulation on top of existing attic insulation to raise the R-value, as long as the old insulation is dry and not damaged or moldy. New layers should be unfaced, since a second vapor barrier can trap moisture.
Don’t compress the existing insulation, and keep insulation clear of recessed lights not rated for contact and of soffit vents. If the old insulation is wet or contaminated, remove it first.
General Estimating Notice: This calculator estimates insulation thickness, quantity, and cost from your area, target R-value, material, waste, and the prices you enter, using typical R-per-inch values and standard guidance. Actual R-values, coverage, and depth vary by product — always follow the manufacturer’s coverage chart — and recommended R-values depend on your climate zone and local energy code. Keep insulation clear of soffit vents and non-rated recessed lights, and have foam and any complex installs done by a qualified professional. For planning purposes only.