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R-value and U-value

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• R-value and U-value

R-value

The widely used "R-value" measurement simply refers to thermal "resistance value" - that is, a measure of insulation's ability to impede heat flow, thereby keeping hot or cold air outside. A higher R-value means that insulation works more efficiently. Increasing the thickness of an insulation layer increases R-value.

Rigid polyurethane and polyiso foam insulation products have high R-values. The specific R-value for a product can be found in the product literature; however, here's a few examples of typical R-values (per inch of insulation) found in different polyurethane and polyiso foam products:

Regulation

The U.S. Federal Trade Commission (FTC) governs claims about R-values to protect consumers against deceptive marketing claims. The regulation (16CFR460) covers home insulation labels, fact sheets, ads, and other promotional materials. R-values provided in promotional materials must be based on test done under methods developed by the American Society of Testing and Materials (ASTM).  These tests include:

• (ASTM C 518) Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. This test method covers the measurement of steady state thermal transmission through flat slab specimens using a heat flow meter apparatus.
  
  

• (ASTM C 177) Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus. This test method covers the achievement and measurement of steady-state heat flux through flat-slab specimens using a guarded-hot-plate apparatus. The method encompasses both the single-sided and the double-sided mode of measurement. Both distributed and line source guarded heating plate designs are included, in principle, in this test method. The reader should consult the standard practices on the single-sided mode of operation and on the line source apparatus for further details on these variations of the method.

For more information, visit the FTC and read about Labeling and Advertising of Home Insulation.

R-value Comparison of Common Insulation Materials

Polyurethane is a highly efficient insulator. For every two inches of polyurethane foam, this much of another insulating material would be needed for equivalent insulatin performance:

U-value

"U-value" (or U-factor) measures heat transfer through insulation or a building component over time, under standardized conditions. Since it's an inverse measurement to R-value (R = 1/U), a low U-value number indicates a superior insulator.

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