Insulation / R-Value Calculator
Batt or roll square footage and the R-value you need.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Insulation is bought by the square foot of coverage but specified by R-value, the measure of how well it resists heat flow. This calculator gives you both — how much material to buy and the R-value to target for your space.
How it works
Area times a 10% waste allowance for the square footage; the target R-value comes from your climate zone (R-13 walls, R-30 to R-49 attics).
Coverage is the wall or ceiling area plus a 10% waste allowance for trimming around studs, joists, and obstructions. The target R-value comes from where the insulation goes: roughly R-13 for walls, R-30 for a mild-climate attic, and R-49 for a cold-climate attic.
R-value is additive, so you reach a target by layering batts — two R-19 layers stack to R-38, for example. The calculator helps you size the material; your climate zone sets how high the R-value needs to go.
Square footage to buy = area × 1.1 (10% waste); target R-value = your climate-zone goal.
Worked examples
A 40 ft long × 8 ft tall wall run, R-13. → 352 sq ft of R-13 insulation.
40 × 8 = 320 sq ft × 1.1 = 352 sq ft to buy, sized for an R-13 wall target.
A 30 ft × 40 ft cold-climate attic, R-49. → 1,320 sq ft of R-49 insulation.
30 × 40 = 1,200 sq ft × 1.1 = 1,320 sq ft, targeting R-49 for a cold attic.
Tips & gotchas
- R-value is additive — if a single batt won't reach your target, layer a second perpendicular run on top, especially in attics.
- Don't compress batts to fit; squashing insulation into a tight cavity destroys much of its R-value.
- Air-seal gaps, top plates, and penetrations before insulating — air leaks bypass even thick insulation.
- Match the batt width to your stud or joist spacing (16 or 24 inches on center) so it fits snugly without gaps.
FAQ
What R-value do I need?
It depends on the location and climate: roughly R-13 to R-21 for walls, and R-30 to R-60 for attics, with colder zones at the high end. Your local code sets the minimum.
Can I add new insulation over old?
Yes in attics — laying new unfaced batts or blown-in insulation over existing material adds R-value. Use unfaced products on top so you don't trap moisture between two vapor barriers.
Does thicker insulation always mean better?
Up to a point — R-value has diminishing returns, and over-insulating one surface while ignoring air leaks elsewhere wastes money. Hit your zone's target, then spend on air-sealing.
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