Insulation Calculator

Step 1: Choose Imperial (R-values, inches, sqft) or Metric (cm, sqm).

Step 2: Select what you are insulating. The calculator distinguishes between attic flat space, sloped/cathedral attic, 2×4 vs 2×6 framed walls, floors over crawlspaces, and basement walls. Each surface has different typical R-value targets because each one's job is different.

Step 3: Pick your US climate zone (IECC zones 1 through 8). The calculator auto-suggests the ENERGY STAR-recommended R-value for your surface in your climate. Zone 1 (Miami) needs less attic insulation than Zone 6 (Minneapolis). If you are outside the US, choose the closest match: zone 1 ≈ tropical, zone 3 ≈ Mediterranean / temperate, zone 5 ≈ northern Europe, zone 7 ≈ Scandinavia.

Step 4: Override the R-value if your local building code requires something different, or if you are choosing to exceed code for better efficiency. Leaving this blank uses the climate-zone recommendation.

Step 5: Choose your insulation material. Each has a different R-per-inch — fiberglass batts at R-3.2/in, blown cellulose at R-3.5/in, rigid foam XPS at R-5.0/in, and closed-cell spray foam at R-6.5/in. Higher R-per-inch means thinner installation for the same total R, but typically higher cost.

Step 6: Enter your area. You can enter the total area directly (sqft or sqm) if you already know it, or enter length × width and let the calculator multiply.

Step 7: Adjust the waste slider. 10 percent is the recommended default. Use 15 percent for irregular spaces with many joists, beams, and obstructions. Use 5 percent only for clean rectangular spaces.

Step 8: Optionally enter price per bag or per sheet to see total material cost.

Step 9: Click Calculate. The result shows bags or sheets needed, required thickness in inches/cm, coverage per bag, and the step-by-step math.

Insulation is sold by the bag, the roll, or the sheet — but the thing that actually matters is the R-value, which measures how much it resists heat flow. Higher R = more insulation. R-13 is barely enough for a wall, R-49 is good for an attic in most US climates, and R-60 is recommended for cold climates.

R-value scales with thickness. Every material has a specific R-value per inch of thickness. Fiberglass batts deliver about R-3.2 per inch. Closed-cell spray foam delivers R-6.5 per inch — twice as much, in half the thickness. To achieve R-30, you need 9.4 inches of fiberglass batt OR 4.6 inches of closed-cell spray foam.

Coverage per bag varies with target R-value. A bag of fiberglass batts that covers 218 sqft at R-13 only covers about 75 sqft at R-49 — the batts are thicker (and longer per bag holds fewer of them). Blown-in insulation works the same way: a bag of cellulose covers about 37 sqft at R-30 but only 22 sqft at R-49. The calculator scales the coverage automatically.

Climate zone determines the recommended R-value. ENERGY STAR's climate zones run from 1 (hot, no heating season) to 8 (subarctic). For attics, the recommendation is R-30 in zone 1, R-49 in zones 2-3, R-60 in zones 4-8. For walls and floors, the targets are lower because walls lose less heat than ceilings (heat rises) and floors are mostly insulated from below by the earth.

Different materials suit different surfaces. Fiberglass batts work in framed wall and floor cavities where the framing controls the thickness. Blown-in cellulose or fiberglass works for attics where you want to pile insulation deep on a flat floor (or above ceiling). Rigid foam works against concrete basement walls and on the outside of homes (under siding). Spray foam works in irregular spaces, around penetrations, and where air-sealing matters as much as R-value — but it costs 3 to 5 times more than batts.

Building codes set minimums. Most US jurisdictions follow the International Energy Conservation Code (IECC), which mandates minimum R-values by climate zone. The calculator's defaults match ENERGY STAR recommendations, which usually exceed code minimums by 10 to 20 percent. Always check your local building code — some jurisdictions are stricter than IECC, especially California (Title 24) and the Pacific Northwest.

Step 1: Pick target R-value. Either use the climate-zone recommendation or your local code minimum.

Step 2: Pick material. Material defines R-per-inch:
- Fiberglass batts: R-3.2/in
- Mineral wool batts: R-3.8/in
- Blown cellulose: R-3.5/in
- Blown fiberglass: R-2.6/in
- Rigid XPS foam: R-5.0/in
- Open-cell spray foam: R-3.7/in
- Closed-cell spray foam: R-6.5/in

Required thickness = Target R ÷ R-per-inch

Step 3: Compute bag/sheet count.

For fiberglass and mineral wool batts (sold by the bag of pre-cut batts):
- Bag coverage decreases as R-value increases (thicker batts = fewer per bag)
- Industry-typical bag coverage: 218 sqft at R-13, 163 at R-19, 140 at R-21, 120 at R-30, 95 at R-38, 75 at R-49

For blown-in cellulose/fiberglass (sold by the loose-fill bag):
- Coverage = baseline at R-30 × (30 ÷ target R)
- Baseline: 37 sqft/bag for cellulose, 50 sqft/bag for blown fiberglass

For rigid foam (sold in 4×8 ft / 1.2×2.4 m sheets):
- Sheet area is constant: 32 sqft / 2.88 sqm per sheet
- Thickness controls R-value, not coverage area

Step 4: Apply waste percentage.

Bags or sheets needed = ⌈(Area × (1 + waste %)) ÷ coverage per bag/sheet⌉

Example: 1,000 sqft attic, climate zone 4, fiberglass batts, 10% waste.
- Zone 4 attic recommends R-60. But fiberglass batts top out at R-49 in pre-cut bag form, so realistically use R-49 plus a second cross-batted layer to reach R-60.
- For R-49 alone: bag coverage = 75 sqft. Total area with waste = 1,100 sqft. Bags = ⌈1,100 / 75⌉ = 15 bags.
- Required thickness = 49 ÷ 3.2 = 15.3 inches.

Example 1: 1,000 sqft attic, fiberglass batts, Zone 4 (Washington DC). Zone 4 attic recommends R-60 to R-49. At R-49, bag coverage is 75 sqft. With 10% waste, 1,100 sqft ÷ 75 = ⌈14.67⌉ = 15 bags. Required thickness = 49 ÷ 3.2 = 15.3 inches.

Example 2: 1,500 sqft house — attic only. A 1,500 sqft single-story house has 1,500 sqft of attic to insulate. In Zone 4 at R-49 with fiberglass batts: with 10% waste = 1,650 sqft ÷ 75 sqft/bag = 22 bags. With blown cellulose at R-49 (coverage 22.5 sqft/bag): 1,650 ÷ 22.5 = ⌈73⌉ = 73 bags of cellulose.

Example 3: 400 sqft basement wall, rigid XPS foam, R-13. Rigid foam comes in 4×8 ft sheets (32 sqft each). With 10% waste = 440 sqft ÷ 32 = ⌈13.75⌉ = 14 sheets. Required thickness = 13 ÷ 5.0 = 2.6 inches — round up to 3-inch sheets at the store.

Match the material to the cavity. Batts go in framed cavities where the studs control depth. Blown-in goes on flat attic floors where you can pile it deep. Rigid foam goes on concrete walls or under siding. Spray foam goes in irregular spaces or where air-sealing matters as much as R-value.

Compress = lose R-value. A R-19 batt squeezed into a R-13 cavity becomes a R-13 batt, not a R-19 batt. The fluff is the insulation — crushing it eliminates the air pockets that resist heat flow.

Air-seal first, insulate second. Insulation slows conduction, not air movement. A poorly air-sealed attic with R-60 insulation can lose more heat than a well-sealed attic with R-30 because warm air leaks straight through the insulation. Caulk gaps, seal can-light holes, weather-strip the attic hatch BEFORE adding insulation.

Faced vs unfaced. Faced batts (paper kraft or foil) include a vapor retarder that should always face the warm side of the wall (interior in cold climates, exterior in hot/humid). Use unfaced batts on top of existing insulation or in walls that already have a vapor retarder.

Wear PPE. Fiberglass batts shed irritating fibers. Long sleeves, gloves, eye protection, and an N95 mask are non-negotiable. Cellulose dust is also irritating, especially during installation. Spray foam requires professional installation with full respirator gear because the catalysts are toxic during application.

Don't block soffit vents. When adding attic insulation, leave a gap or use baffles at the eaves so air can flow from soffit vents to ridge vents. Blocked airflow causes condensation, mold, and ice dams. Most home improvement stores sell pre-formed cardboard baffles for $1-2 each.

Diminishing returns above R-60. Going from R-13 to R-30 cuts heat loss by more than half. Going from R-60 to R-100 only cuts heat loss by another 10 percent — the law of diminishing returns. Most US climates max out the value at R-60 in the attic; beyond that, the cost rarely pays back through energy savings.