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C-value denotes a material’s thermal conductance. The C-value of a material is equal to the k-value divided by its thickness. The lower the C-value of a material, the better it will perform as an insulator.

For example: If the C-value of one inch of fiberglass insulation is 0.24 Btu/hroft2o°F, the C-value of two inches of the same material will be 0.12.

U-value denotes a construction’s thermal transmittance, or its overall heat transfer coefficient. This is similar to the C-value but is generally used in denoting the thermal conductance of a construction comprised of different materials, such as in a typical building envelope. It also includes the air film resistances on both sides of the construction.

The lower the U-value of a construction component, the better it will perform as a thermal insulator.

R-value denotes a material’s thermal resistance, or how well it is able to retard heat flow. The higher the R-value the better the material will perform as an insulator.

Insulation products are typically made with materials designed and produced specifically to resist heat flow. They work by creating tiny pockets of trapped air. These pockets resist the transfer of heat through the material. Animals do much the same thing when they use their fur to trap pockets of body-heated air.

R-value is also useful in determining the total thermal transmittance of a construction component, such as a specific building envelope assembly. In this case, the R-values of all of the materials included in the construction, plus the inside and outside air film resistance values are added. The reciprocal of this sum is the construction’s U-value (R = 1/C).

Among the listed thermal performance measures, R-value is the one most commonly seen on insulation products. The higher the R-value the better the insulating performance will be. A well-insulated wall will typically incorporate R 13-R 19 insulation. Well-insulated attics and cathedral ceilings may have R 30-R 38.