Insulation Calculator
Calculate insulation for walls, attics or floors: square footage, batt count and R-value recommendation for your climate zone — plus a cost estimate.
Wall area or attic floor area
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You might also need:Drywall Calculator·Stud Calculator
Insulation math
bays = (wall length ÷ spacing in ft) — batts come sized to fit 16″ and 24″ framing
Insulation is bought by square footage, not by batt, because bag coverage varies with batt length. The important choices are R-value (set by climate and location), batt width (set by framing spacing) and faced vs unfaced (set by whether you already have a vapor barrier).
Recommended R-values by climate zone
| Location | Warm (1–2) | Mixed (3–4) | Cold (5–7) |
|---|---|---|---|
| Attic | R-30 | R-38 | R-49 to R-60 |
| 2×4 walls | R-13 | R-13 to R-15 | R-15 to R-21 |
| 2×6 walls | R-19 | R-19 to R-21 | R-21 to R-23 |
| Floor over crawlspace | R-13 | R-19 | R-25 to R-30 |
Getting it right
- Air seal before you insulate: insulation slows heat, it doesn't stop drafts. Caulk and foam the gaps first or you'll waste half the benefit.
- Don't compress batts: squashing R-19 into a 2×4 bay gives you far less than R-19. Use the batt sized for the cavity.
- Keep soffit vents clear in attics with baffles — blocked ventilation causes condensation and rot.
- Drywall going over it? Count sheets with the drywall calculator; framing bays come from the stud calculator.
Frequently asked questions
What R-value insulation do I need?+
It depends on climate zone and where it goes. US Department of Energy guidance runs roughly: attics R-30 to R-60, 2×4 walls R-13 to R-15, 2×6 walls R-19 to R-21, and floors over unheated space R-13 to R-30. Colder zones sit at the top of each range.
How many square feet does a bag of insulation cover?+
A bag of R-13 batts for 2×4 walls at 16 inch spacing typically covers 40–100 sq ft depending on batt length. The coverage is printed on every bag — this calculator gives you the total square footage so you can divide by whatever your chosen product covers.
Should insulation be faced or unfaced?+
Faced (kraft paper) batts include a vapor retarder and go in exterior walls with the paper toward the heated side. Unfaced batts are for adding a second layer over existing insulation, interior sound walls, and attics where a separate vapor barrier exists. Never install two vapor barriers — moisture gets trapped between them.
Can I put new insulation over old?+
In attics, yes — lay unfaced batts perpendicular over existing insulation. Never use faced batts as the top layer; the paper traps moisture. In walls you cannot practically add batts to an already-insulated cavity, which is where blown-in or spray foam comes in.
How much does insulation cost?+
Fiberglass batts run $0.50–$1.50 per square foot of wall or ceiling. Blown-in cellulose is $1–$2 installed; closed-cell spray foam $3–$7. Attic insulation is usually the highest-return energy upgrade in an older house.