Construction

Insulation R-Value Calculator

Code-minimum insulation R-values come from the IECC (International Energy Conservation Code) and are organized by climate zone (1-7, hot to cold) and assembly type (attic, wall, floor, basement). Hit the minimum and your house meets code; go beyond it and your heating/cooling bills drop. This calculator gives you the IECC-required R-value for your zone and assembly, then tells you exactly how thick the insulation needs to be in fiberglass batt, blown cellulose, mineral wool, open-cell spray foam, closed-cell spray foam, or rigid polyiso — because the R-per-inch varies hugely by material.

R-value and what it actually means

R-value is resistance to heat flow per inch of material. R-13 means about 13 units of resistance. Doubling insulation roughly halves heat loss through that section of the building. R-values are additive: a 6" wall cavity filled with R-19 batt + 1" rigid foam R-5 on the exterior = R-24 wall assembly.

R-per-inch by material:

Fiberglass batt: R-3.2 per inch (R-13 for 3.5" wall cavity, R-19 for 6" cavity, R-30 for 10")
Mineral wool batt: R-3.1-3.7 per inch. Higher density than fiberglass.
Blown cellulose (attic): R-3.7 per inch.
Open-cell spray foam: R-3.6 per inch. Air seals + insulates.
Closed-cell spray foam: R-6.5 per inch. Highest R, also vapor barrier.
Rigid polyiso board: R-5-6 per inch.
EPS (expanded polystyrene): R-3.6 per inch.
XPS (extruded polystyrene): R-5 per inch.

IECC minimums by climate zone

For typical zone 4 (mid-Atlantic, much of US):

Attic: R-60
Exterior wall: R-20 (or R-13 cavity + R-5 continuous insulation outside)
Floor / crawl ceiling: R-19
Basement wall: R-10 continuous

Colder zones bump everything up: zone 6 (upper Midwest, northern New England) calls for R-60 attic, R-21+ wall, R-30 floor. Zone 1 (southern Florida) needs less wall insulation since cooling load dominates.

Worked example: zone 4 attic at R-60, using blown cellulose at R-3.7/inch → 16.2 inches of cellulose blown on top of the joists. That's a deep blown insulation layer — typical settled depth in a code-built house. Hitting the same R-60 with fiberglass batts = 18.75" of stacked batts (R-30 + R-30 batts).

How to use this calculator

Climate zone: look up your county on the IECC zone map (search "IECC climate zone map").
Assembly: attic, exterior wall, floor/crawl, or basement wall.
Insulation material: pick what you'll install.
Output: target R-value (code minimum) and required thickness in that material.

Common scenarios

Pennsylvania attic insulation upgrade, blown cellulose to R-60. Zone 5 minimum R-60. Need 16.2" of cellulose. Settled depth after years of compression: order 18" loose-blown for 16" settled. Material runs $1.50-2.50/sq ft installed.

Vermont new construction wall, R-21 mineral wool in 2x6 cavity. Zone 6 minimum R-20. Mineral wool batts come pre-cut at R-23 for 6" cavity — exceeds spec. Use them; the R-23 vs R-21 cost difference is minimal.

Florida garage ceiling insulation, R-30. Zone 2 minimum R-30. With 12" trusses overhead, 9.5" fiberglass R-30 batt fits perfectly. Cheap, fast, code-compliant.

FAQ

Is closed-cell foam worth the price? +

For tight space constraints (where you can't fit thick fiberglass): yes — you get more R in less depth. For air-sealing a leaky house: also yes (closed-cell foam is the only insulation that's also a vapor barrier and air barrier). For typical attic or open wall cavities: probably not — cellulose or batt insulation at 3-4x the volume gets you the same R for 1/4 the cost.

Open-cell or closed-cell spray foam? +

Open-cell (R-3.6/inch): cheaper, lower density, breathable, fine for wall cavities and ceiling drywall. Closed-cell (R-6.5/inch): twice the R per inch, vapor barrier, structural rigidity, required for spray-on roof underlayment. Closed-cell is roughly 2x the cost per board-foot. Use open-cell where R-3.6/inch is sufficient.

What's the difference between R-13 and R-15 batts in the same cavity? +

R-15 batts are higher-density fiberglass (or mineral wool) in the same 3.5" cavity thickness as R-13. About 15% better R-value for ~20% higher cost. Worth it for new construction; probably not worth tear-out and replacement in existing houses.

Should I add insulation over existing attic insulation? +

Yes, if it's currently below code. Blown cellulose or fiberglass goes right on top of existing insulation — no removal needed. Make sure the existing insulation isn't water-damaged or vermin-soiled (remove if so). Allow proper ventilation at the eaves — don't block soffit vents.

What about air sealing — isn't that more important than R-value? +

Equally important. A house with R-60 attic insulation but leaky drywall ceilings loses 25-35% of its heat through air leaks (cold air pouring into the attic). Air sealing the ceiling penetrations (recessed lights, attic access hatch, plumbing chases) before adding insulation is the right order. Spray foam at every penetration, then cellulose blown over top.

Where do I find my IECC climate zone? +

Search "IECC climate zone map" — the Department of Energy publishes a county-level map for free. Most of the contiguous US sits in zones 3, 4, 5, and 6. Coastal southern states are zones 1-2; northern Maine, ND, MN, MT, WY are zone 6-7.

Does the building department check R-values? +

For new construction: yes, the insulation contractor must certify the installed R-value on the permit paperwork, and inspectors check. For retrofits and DIY: depends on whether you pulled a permit. Most insulation upgrades don't require permits as long as you're staying within code minimums.

Should I do walls or attic first? +

Attic first — it's the highest ROI insulation upgrade. Heat rises, and an uninsulated attic loses 25-40% of total house heat. Air-sealing + R-60 attic insulation is the standard energy-improvement project for older houses. Walls require tearing out drywall or blowing into existing cavities through small holes — more work, smaller savings.