Field Guide
How Much Insulation Do I Need? Walls, Attics & Floors
Insulation is sold by coverage, and coverage changes with the R-value and the brand. So the job is: measure the area, pick the R-value your framing and climate call for, and divide by the coverage on the package you're buying.
1. How to Calculate Insulation (The Math Explained)
Here's the exact process this calculator uses, with 120 ft of 8 ft exterior wall, 2 doors and 6 windows as the example.
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1
Find the area and subtract openings
Walls = 120 ft × 8 ft = 960 sq ft Openings = 2 × 21 + 6 × 15 = 132 sq ft Area = 960 − 132 = 828 sq ft -
2
Add waste and divide by the package coverage
Every package lists the square feet it covers; it varies by brand and R-value. If your R-13 package says 40 sq ft:
With 10% waste = 828 × 1.10 = 910.8 sq ft Packages = 910.8 ÷ 40 = 22.8 → 23 packages
2. Choosing an R-Value
R-value measures resistance to heat flow: higher is better. In walls and floors the framing depth limits what fits; in attics there's room for more, and climate decides how much is worth it.
| Where | Typical R-value | Notes |
|---|---|---|
| 2×4 walls | R-13 to R-15 | Fills a 3½ in cavity without compressing. |
| 2×6 walls | R-19 to R-21 | Fills a 5½ in cavity. |
| Attics | R-30 to R-60 | The U.S. Department of Energy's range; colder climates at the high end. |
| Floors over unheated space | R-19 to R-30 | Crawlspaces and floors over garages; hold batts up with supports. |
3. Blown-In Depth
Loose-fill insulation is bought by the bag, and each bag's coverage depends on how deep you blow it. Divide the target R-value by the product's R-value per inch to find the depth, then use the bag's chart for the coverage at that R-value.
Depth = Target R ÷ R per inch
R-38 fiberglass: 38 ÷ 2.5 ≈ 15 in
R-38 cellulose: 38 ÷ 3.5 ≈ 11 inPro tip: seal air leaks first
Gaps around light fixtures, plumbing stacks and the attic hatch let warm air bypass the insulation. Seal them with foam or caulk before you add insulation, and keep vents and recessed lights clear as the manufacturer requires.
4. Frequently Asked Questions
How do I calculate how much insulation I need?
Measure the area: for walls, the total length times the height, minus about 21 sq ft per door and 15 sq ft per window; for attics and floors, length times width. Add 5–10% for trimming, then divide by the coverage printed on the package and round up.
What R-value do I need for walls?
Match the batt to the framing depth: 2×4 walls take R-13 or R-15 batts and 2×6 walls take R-19 or R-21. Squeezing a thicker batt into a shallow cavity compresses it and lowers its R-value, so it isn't worth it.
What R-value do I need in my attic?
The U.S. Department of Energy recommends roughly R-30 to R-60 in attics, depending on your climate; colder areas are at the high end. Many homes in cold climates aim for R-49 to R-60.
How deep should blown-in insulation be?
Divide your target R-value by the product's R-value per inch. Loose-fill fiberglass is roughly R-2.5 per inch and cellulose roughly R-3.5, so R-38 takes about 15 in of fiberglass or 11 in of cellulose. The coverage chart on the bag gives the exact settled depth and coverage per bag.
Should I subtract windows and doors?
Yes, for walls. This calculator subtracts 21 sq ft for each door and 15 sq ft for each window. The waste allowance covers the small pieces you cut to fit around them.
Batts or blown-in for an attic?
Batts and rolls are easy to lay between open joists in a clear attic. Blown-in fills around wiring, bracing and odd spaces, settles into an even blanket, and goes in quickly over large areas; you'll need a blowing machine, which many home centers lend with a bag purchase.
Do I need faced or unfaced insulation?
Kraft-faced batts include a vapor retarder and are common in exterior walls, with the facing toward the heated side in cold climates. Unfaced batts are used where no vapor retarder is wanted, such as when adding a layer over existing attic insulation. Local code and climate decide what's required.