Each type of insulation has its own advantages and disadvantages based on its thermal properties. These properties, along with established insulation ratings, help builders and engineers determine which type of insulation would be most effective in a given situation. The insulation rating indicates how well a material insulates. Insulation is rated in three different ways:

The U-value rating indicates the overall heat flow between air on the warm side and air on the cold side of a wall, floor or ceiling, which is insulated with a given material. The lower the U-value, the more insulated a unit is against heat transmission.

The R-value measures the thermal resistance of the material. It indicates the total resistance of a material to the passage of heat or cold. The higher the R-value, the more effective its insulation properties. The R-value is equal to the U-value divided into 1 (R = 1/U).

The K-value is the measure of heat conductivity of a material. It is the equivalent of the U-value per square inch of thickness of the material. That is, the measure of the amount of heat, in BTUs per hour, that will be transmitted through one square foot of material that is one inch thick to cause a temperature change of one degree Fahrenheit from one side of the material to the other. As with the U-value, the lower the K-value for a material, the better it insulates. If the K-value of the material is known, the R-value per inch can be determined by diving 1 by the K-value (R-value per inch = 1/K value).

Building codes set minimum R-values for insulation in various regions, or thermal zones, in the U.S.

A good resource for learning more about insulation types, alternatives, and ratings is the Owens Corning Web Site