Which statement correctly describes how total R-value is determined for a material?

• A. Total R-value is thickness divided by the material's thermal conductivity.
• B. Total R-value equals the product of thickness and thermal conductivity.
• C. Total R-value is determined by the area and temperature difference.
• D. Total R-value is simply the material's nominal R value.

Answer: (A)

Heat flow through a material is resisted by its thickness and how well it conducts heat. For a single, uniform material, that resistance is determined by dividing the thickness by the thermal conductivity: R = L / k. So thicker layers raise the R-value, while materials with higher thermal conductivity lower it. This is why the total R-value of a single material is thickness divided by its thermal conductivity.

Note: if you’re dealing with a multi-layer system, you’d add up the R-values of each layer to get the total. The other ideas—relying on area or temperature difference, or using a nominal value—don’t describe this intrinsic resistance of the material itself.