Insulation estimating has two layers: how much area you are covering, and the R-value that area needs to hit code for your climate zone. Batts, blown-in, and spray foam are each quantified differently — bags, square feet, board-feet — so this guide covers R-value targets, area, coverage by type, and cost.
Before any quantity, decide the target R-value — the measure of thermal resistance. Code minimums are set by the building's climate zone and the assembly being insulated. Attics need the most; walls and floors less. Higher R-value means thicker batts or more blown-in depth, which directly changes how much material you buy.
| Assembly | Common target |
|---|---|
| Attic / ceiling | R-38 to R-60 |
| 2×4 exterior wall | R-13 to R-15 |
| 2×6 exterior wall | R-19 to R-21 |
| Floors over unconditioned space | R-19 to R-30 |
| Basement / crawl walls | R-10 to R-15 |
R-value minimums vary by climate zone and are periodically updated. The table is a planning guide — confirm the current requirement for your jurisdiction before ordering.
The area to insulate is the wall, ceiling, or floor area of the cavities — calculated the same way as drywall or siding: perimeter times height for walls, length times width for ceilings and floors. For tight estimates you can subtract the framing fraction (studs and plates occupy roughly 15% of a wall's face), but most estimators leave it in as part of the buffer.
Batts and rolls are pre-cut fiberglass or mineral wool sized to stud and joist spacing — 15 in wide for 16-in on-center framing, 23 in for 24-in. Buy by the package, each of which states the square feet it covers at a given R-value. Divide cavity area by coverage per package and add waste for cuts at the top and bottom of walls.
Loose-fill cellulose or fiberglass is blown in, mostly in attics. It is sold by the bag, and the key subtlety is that coverage per bag falls as the target R-value rises — a deeper layer covers fewer square feet. Every bag has a printed coverage chart listing square feet per bag at each R-value; read it at your target.
1,200 ÷ 40 = 30 → order 30 bags. At a lower target the same bag might cover 60 sq ft, needing only 20 bags — always read the chart at your actual R-value, not the maximum coverage number on the front.
Spray foam is quantified in board-feet: one board-foot is one square foot sprayed one inch thick. Multiply the area by the thickness in inches to get board-feet, then size the kit or quote against that. The two types differ sharply in R-value per inch and price:
Depending on assembly and climate you may need a separate vapor retarder (faced batts include a kraft or foil facing) and an air barrier. Faced batts must be installed facing the conditioned side; getting this wrong traps moisture. Closed-cell spray foam is its own vapor and air barrier, which is part of why it costs more.
| Type | Typical figure |
|---|---|
| Fiberglass batts | $0.50–$1.50 / sq ft |
| Blown-in cellulose / fiberglass | $1–$2 / sq ft |
| Open-cell spray foam | $0.45–$0.75 / board-ft |
| Closed-cell spray foam | $1–$1.75 / board-ft |
Use the insulation calculator to convert area and R-value into batts or bags with a regional cost, then verify the target R-value against current local code.