Question 1

What is R-value?

Accepted Answer

R-value measures thermal resistance — how well an insulating material blocks heat flow. A higher R-value means better insulation. Total R-value of an assembly equals the sum of the R-values of each layer, so adding more insulation increases the R-value linearly.

Question 2

What is the difference between R-value and U-value?

Accepted Answer

R-value measures thermal resistance (higher is better for insulation), while U-value (or U-factor) measures thermal transmittance (lower is better). They are reciprocals: U = 1/R. Windows are usually rated by U-value; insulation is rated by R-value.

Question 3

What R-value do I need for my attic?

Accepted Answer

The U.S. Department of Energy recommends R-49 to R-60 for attics in most cold climates, R-30 to R-49 in mixed climates, and R-30 in warm climates. Walls typically need R-13 to R-21 and floors R-25 to R-30. Local building codes specify minimums for your zone.

Question 4

How is R-value calculated from thickness?

Accepted Answer

R-value equals thickness divided by thermal conductivity (k): R = thickness / k. Each material has a known R-value per inch. This calculator multiplies the per-inch rating by the layer thickness and sums all layers to find the assembly's total R-value.

Common insulation R-value per inch