Insulation R-Value Calculator
Determine the total thermal resistance (R-value) of an insulation layer by entering its thickness and material type. Use it when planning wall, attic, or floor insulation to meet energy code requirements.
About this calculator
R-value measures a material's resistance to heat flow — the higher the R-value, the better the insulation performance. Each insulation material has a characteristic R-value per inch of thickness, which varies by type: fiberglass batt (~R-3.1/in), spray foam (~R-6.5/in), and mineral wool (~R-3.7/in) are common examples. The formula is: Total R-Value = Thickness (inches) × R-Value per Inch. This linear relationship means doubling the insulation thickness doubles the thermal resistance. Building codes specify minimum R-values by climate zone and building assembly (walls, roofs, floors), so this calculator helps confirm compliance. Proper R-value planning reduces heating and cooling costs by limiting unwanted heat transfer through the building envelope.
How to use
Suppose you are insulating an attic with 10 inches of blown fiberglass, which has an R-value per inch of 2.9. Apply the formula: Total R-Value = 10 × 2.9 = R-29. If your local energy code requires R-38 for attics, you would need additional thickness: 38 / 2.9 ≈ 13.1 inches. Adding approximately 3 more inches of the same material would bring you to R-38 and meet code. This helps you order the correct quantity of insulation and avoid a failed inspection.
Frequently asked questions
What R-value do I need for attic insulation in a cold climate?
In cold climate zones (IECC zones 5–8, such as Minnesota or Maine), the U.S. Department of Energy recommends attic insulation of R-49 to R-60 for most homes. Warmer zones (1–4) typically require R-30 to R-49. Your local building code will specify the minimum, but exceeding the minimum often pays back in energy savings within a few years. Always check the current IECC edition adopted by your jurisdiction, as requirements are updated periodically.
How does R-value per inch differ between insulation materials?
R-value per inch varies significantly by material due to differences in density, fiber structure, and air-trapping ability. Open-cell spray foam provides about R-3.7 per inch, while closed-cell spray foam delivers roughly R-6.5 per inch — nearly double. Fiberglass batts average R-2.9 to R-3.8 per inch, and rigid foam boards range from R-3.8 (EPS) to R-6.5 (polyisocyanurate). Choosing a higher R-per-inch material lets you achieve required performance with less physical thickness, which is critical in space-constrained assemblies like 2×4 stud walls.
Does adding more insulation layers increase R-value proportionally?
Yes — R-values of individual insulation layers are additive when stacked in series. If you install 3.5 inches of fiberglass batt (R-11) inside a stud bay and then add 1 inch of rigid foam (R-5) on the exterior, your combined wall assembly R-value is R-16 (before accounting for framing and air films). This additive property makes it straightforward to plan a multi-layer assembly to hit a target R-value. Keep in mind that framing members have lower R-values than insulation and slightly reduce the whole-wall effective R-value.