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Insulation Calculator

Insulation thickness needed to hit a target R-value for any material and wall area.

Written by Golam Rabbani, Founder & Lead Engineer

Units:
Units

R-13 (2x4 wall) / R-19 (2x6 wall) / R-30+ (attic)

How to use this insulation calculator

  1. Toggle imperial (ft) or metric (m) for the wall dimensions.
  2. Enter the wall length and wall height — the area is calculated automatically.
  3. Set your target R-value (R-13 for a 2×4 wall, R-19 for a 2×6, R-30+ for an attic).
  4. Pick the insulation material from the dropdown.
  5. Press Calculate to see the thickness needed and the wall area.

About this insulation calculator

The insulation calculator finds the thickness needed to hit a target R-value for nine common insulation materials. R-value measures thermal resistance: higher means better. The formula is thickness (in inches) = target R ÷ R-per-inch of the chosen material. R-per-inch values are published by the US Department of Energy: fiberglass batt 3.2, mineral wool batt 3.7, cellulose loose-fill 3.5, EPS rigid foam 3.8, XPS rigid 5.0, polyiso rigid 6.0, open-cell spray foam 3.7, and closed-cell spray foam 6.5.

Worked example: a 40 ft × 8 ft wall needs to hit R-19 (a typical 2×6 cavity target). With fiberglass batt at R-3.2 per inch, thickness needed = 19 ÷ 3.2 = 5.94 inches — perfect for a 5.5″ 2×6 cavity. Using closed-cell spray foam at R-6.5/in, you only need 2.93 inches, leaving room for additional fill or ventilation. Wall area is 320 ft², so you would buy enough material to cover that area at the chosen thickness.

Match thickness to your cavity. A 2×4 wall (3.5″ actual) maxes out around R-13 batt or R-21 closed-cell foam. A 2×6 (5.5″) reaches R-19 batt or R-35 closed-cell. Attics typically target R-38 to R-60 in cold climates; check the US DOE recommended R-value map for your IECC climate zone before settling on a target.

FAQ

What R-value do I need?
US DOE recommends R-13 to R-21 for walls and R-30 to R-60 for attics depending on climate zone. Cold climates use the higher numbers, mild climates the lower. Always check current local code.
Which insulation has the highest R per inch?
Closed-cell spray foam (~R-6.5/in) and polyiso rigid board (~R-6.0/in) lead. Fiberglass batt is ~R-3.2/in; cellulose loose-fill ~R-3.5/in.
Why does material choice matter beyond R-value?
Spray foam doubles as an air seal; batts do not. Rigid foam can be installed outside the framing to break thermal bridging. Cellulose dampens sound better than fiberglass. Each material has trade-offs in cost, install complexity, fire rating, and vapor permeability.
Can I exceed my cavity depth?
Only with a furred-out wall or by adding rigid foam on the exterior. Compressing batts below the rated thickness reduces R-value — never stuff R-19 batts into a 2×4 cavity.
Are these R-values code-compliant?
They are the published DOE/ASHRAE typical values, but local codes (IECC, IRC) and energy programs (ENERGY STAR, LEED) may require different minimums. Always confirm with your building department.
Is this calculator free?
Yes — free, runs in your browser, no signup.