Insulation Calculator
Calculate how many batts or rolls of insulation to buy.
Check the coverage printed on the package \u2014 a typical R-13 roll covers about 40 sq ft. 5% waste included.
Insulation is one of the few home projects that literally pays you back — but only the parts you actually cover. Running out two joist bays from the wall on a Sunday evening is how attics end up with cold spots for years. Our insulation calculator above turns your area and your product’s package coverage into an exact package count, with 5% waste included.
The Calculation
Packages = (Area to insulate × 1.05) ÷ Coverage per package, rounded up
The formula is simple because manufacturers do the hard part for you: every bundle of batts and every roll states its coverage in square feet on the label. A typical R-13 kraft-faced roll covers about 40 sq ft; an R-30 batt bundle often covers 50–60 sq ft. Put the real number from your chosen product into the calculator — coverage varies widely between R-values and brands, because higher R-value means thicker material and less of it per package.
Measuring Your Area
Attic floors: length × width of the attic footprint. Don’t subtract for framing — the overlap is your waste margin.
Walls: wall length × height per wall, minus large openings like windows and doors. For a whole exterior, perimeter × wall height minus openings gets you close.
Floors and crawl spaces: the same footprint math as attics.
For oddly shaped spaces, split the area into rectangles and add them up. Round up on every measurement — insulation compresses into slightly small spaces far better than it stretches into gaps.
Choosing an R-Value
R-value measures resistance to heat flow — higher is warmer. Recommendations vary by climate zone, but common US targets are:
- Attics: R-38 to R-60 (cold climates need the top of that range)
- Exterior walls (2×4): R-13 to R-15
- Exterior walls (2×6): R-19 to R-21
- Floors over unheated spaces: R-19 to R-30
If your attic already has some insulation, you can add unfaced batts or rolls on top, laid perpendicular to the joists. Never stack a vapor-barrier (kraft/foil) face in the middle of an insulation sandwich — trapped moisture is how mold starts. Facing belongs against the warm-in-winter side, and only in the first layer.
Batt Width Matters
Batts come sized to standard framing: 15-inch widths for 16-inch on-center framing, 23-inch widths for 24-inch on-center. Measure your joist or stud spacing before buying — the wrong width either leaves gaps (kills performance) or requires cutting every batt lengthwise (kills your weekend). Insulation performs at its rated R-value only when it fills the cavity snugly without being compressed.
Installation Tips
- Wear protection: long sleeves, gloves, goggles, and a dust mask. Fiberglass itch is real and lasts days.
- Cut with a utility knife against a board, compressing the batt with a straightedge.
- Don’t block soffit vents in attics — use baffles to keep the airflow path open.
- Split batts around wires and pipes rather than stuffing them behind; compression creates cold spots.
- Air-seal first: caulk and foam the gaps around penetrations before insulating. Insulation slows heat; it doesn’t stop drafts.
Frequently Asked Questions
How many rolls for a 500 sq ft attic? With 40 sq ft rolls: 500 × 1.05 ÷ 40 = 14 packages. With 60 sq ft R-30 bundles: 9 packages.
Is it OK to compress insulation to fit? No — R-value drops when compressed. An R-19 batt squeezed into a 2×4 wall performs around R-13 anyway; buy the batt made for the cavity depth.
Faced or unfaced? Faced for first layers against living space in most climates; unfaced for top-up layers and sound walls. Local practice varies with climate — when in doubt, ask your building department.
Estimates are for planning purposes. Check coverage on your specific product’s label, and consult local code for required R-values.