How This Calculation Works
The insulation calculator determines how much insulation you need based on the area to be insulated, the current R-value, and your target R-value. The R-value difference tells you how much insulation to add.
Different insulation types have different R-values per inch: fiberglass batts (R-3.2/inch), cellulose (R-3.7/inch), spray foam open-cell (R-3.6/inch), and spray foam closed-cell (R-6.5/inch). The calculator divides the needed R-value by the material's R-per-inch to find the required thickness.
Common Mistakes to Avoid
- Compressing batts. Compressing fiberglass reduces its R-value. Use the correct width for your stud spacing.
- Ignoring vapor barriers. In cold climates, a vapor barrier on the warm side prevents moisture buildup and mold.
- Leaving gaps. Even small gaps can reduce insulation effectiveness by 25% or more due to air convection.
Worked Example
Scenario: An attic with 100 m² of area, current R-13, target R-38, using cellulose (R-3.7/inch).
Step 1 — R-value needed: R-38 − R-13 = R-25.
Step 2 — Thickness: 25 ÷ 3.7 = 6.76 inches (≈ 172 mm).
Step 3 — Volume: 100 m² × 0.172 m = 17.2 m³ of cellulose.