How much insulation do I need?
Three numbers decide the whole order: the area you're covering, the target R-value you're hitting, and the material you're using. Nail those and the quantity falls out of simple arithmetic — no guessing at the store.
Start with the three numbers
Every insulation job comes down to the same chain: area → target R → material → quantity. Measure the surface in square feet, decide what R-value you're building to (your climate zone sets that — see what R-value do I need), then pick the material, which fixes the R-per-inch and therefore the depth. From there the quantity is a one-line formula for whatever product you're buying.
Measure the area first
- Attic / floor: length × width of the space. Don't deduct for joists — you insulate over and between them.
- Walls: perimeter × height, then subtract windows and doors. A rough net is fine; order a little over.
- Cathedral / rafter bays: measure along the slope, not the flat footprint, or you'll come up short.
Turn depth into a target R (or the other way around)
The single identity behind all of this is R = thickness (in) × R-per-inch. Flip it to find the depth you need: required thickness = target R ÷ R-per-inch. Do that in the insulation thickness calculator, and check what a layered assembly actually delivers in the R-value calculator.
Worked example: a 1,000 ft² attic to R-49
Say you're bringing a 1,000 ft² attic up to R-49. Here's the same job in four materials:
| Material | R/inch | Depth for R-49 | You order |
|---|---|---|---|
| Blown fiberglass | ~2.5 | ~19.6 in | ~36 bags (coverage chart at R-49) |
| Cellulose | ~3.5 | ~14.0 in | ~32 bags (board-feet method) |
| Fiberglass batt | ~3.2 | ~15.3 in | R-49 batts, ~25 bundles at 40 ft²/bundle |
| Closed-cell foam | ~6.5 | ~7.5 in | ~7,500 board-feet |
Same R, wildly different depth and package count — that's why you fix the material before you count. Run your own numbers in the blown-in bag calculator, the batt bundle calculator or the spray-foam board-feet calculator.
The quantity formulas, one per product
- Batts: bundles = ceil(area ÷ coverage-per-bundle). Coverage falls as R rises (R-13 ~88 ft²/bundle, R-30 ~58).
- Blown-in: bags = ceil(area ÷ coverage-at-R), or ceil(area × depth ÷ bag yield in board-feet).
- Spray foam: board-feet = area × thickness; sets = ceil(board-feet ÷ kit yield).
- Rigid board: sheets = ceil(area ÷ 32) for standard 4×8 sheets.
Order extra — the part DIYers skip
The math gives you the theoretical minimum. On a real job you cut around framing, lose material in gaps and (with loose-fill) watch it settle. Add roughly 5–10% and round up to whole packages. Blown-in especially: blow it a little deep so it settles to your target, not below it. Buying one bundle short means a second trip and a color/lot mismatch; buying one over costs a few dollars.
Common mistakes
- Using a batt's nominal R without checking it fits the cavity — compressing an R-19 into a 3.5" bay gives you ~R-13, not R-19.
- Reading the wrong coverage line on the bag — coverage is quoted at a target R, not per bag flat.
- Forgetting the sloped area on cathedral ceilings.
- Ignoring air-sealing — insulation slows conduction, but a leaky attic loses heat by air movement no R-value fixes.
Worked example: a whole small house
Say you're insulating a 1,200 ft² single-story in a cold zone. Rough it out: attic 1,200 ft² to R-49, exterior walls ~1,000 ft² net to R-21, and the floor over the crawlspace 1,200 ft² to R-30. In blown fiberglass the attic is ~43 bags; the walls in R-21 batts (~58 ft²/bundle) are ceil(1,000 ÷ 58) × 1.10 ≈ 19 bundles with waste; the floor in R-30 batts (~58 ft²/bundle) is ceil(1,200 ÷ 58) × 1.10 ≈ 23 bundles. That's one shopping list off three one-line formulas — and it shows why you attack the attic first (biggest area, highest R, cheapest per R).
Mixed assemblies and odd shapes
Real jobs aren't rectangles. Break an L-shaped attic into two rectangles and add the areas. For a gable-end wall, area = base × height ÷ 2 for the triangle on top. For a stepped or tray ceiling, measure each plane on the slope. When you're stacking materials to reach a target — say cavity batt plus continuous exterior foam — remember the R-values add (see how to calculate the R-value of a wall), so R-13 cavity + 1" polyiso (~R-6) gives you a nominal R-19 assembly and kills the stud thermal bridge at the same time.
Working in metric or from a bill of materials
Most US product is labeled in ft², inches and R (hr·ft²·°F/BTU). If a spec sheet gives you SI (RSI, m², mm), convert before you count: 1 RSI ≈ R-5.68, 1 m² ≈ 10.76 ft², 1 inch = 25.4 mm. And if a contractor hands you a bag/bundle count instead of an area, back into the area with area = bags × coverage-at-R so you can sanity-check the quantity against your own measurement — a padded material line is the easiest place for a quote to drift.
Quick numbers to leave with
- The chain: area → target R → material → quantity. Everything else is one formula.
- Depth: required thickness = target R ÷ R/inch. R-49 is ~14" cellulose, ~19.6" blown fiberglass, ~7.5" closed-cell.
- Batts: bundles = ceil(area ÷ coverage). Blown: bags = ceil(area ÷ coverage-at-R). Foam: board-feet = area × thickness. Rigid: sheets = ceil(area ÷ 32).
- Always: add ~5–10% and round up to whole packages; air-seal before you insulate.
Bottom line: measure the area, look up your target R, pick the material, then let the formula — or the calculators — give you the package count. Add 5–10% and you'll leave the store with the right pile of material once.