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What Does R-Value Mean?

An R-Value is a measure of the resistance to heat transfer of a given material. R-Values allow consumers to compare the insulating properties of products used in residential and commercial buildings.


What Does R-Value Mean?

Roll insulation such as this has an R-Value of 3.25 per inch

R-Values are usually stated without any units, such as R-13 or R-24.  The units are actually hr-ft²-°F/Btu, which measures how long it takes a unit of heat to move through a specified area of the substance.


What Do Different R-Values Mean?

A larger R-Value number means it takes longer for the same amount of heat to move through a substance, so something labeled R-24 is a better insulator than something labeled R-11.  R-Values measure only the material's resistance to heat transfer, so the quality of the seal against the insulation's surroundings is critical.


Typical R-Values

The R-Value of insulation depends on the material and its thickness.  For instance, a "fiberglass" roll or batt insulation has an average R-Value of 325 per inch.

Expanded polystyrene slabs used for insulation have an R-Value of 40 per inch.  Other building materials are poor insulators: an 8-inch concrete block's R-Value is 104, and a single-pane window has an R-Value of 113.

About the Author

Kelvin O'Donahue has been writing since 1979, with work published in the "Arizona Geological Society Digest" and "Bulletin of the American Association of Petroleum Geologists," as well as online. O'Donahue holds a Master of Science in geology from the University of Arizona, and has worked in the oil industry since 1982.

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