As new building and insulation materials have entered the market, confusion over the nature and meaning of R-Value has raised questions among building professionals and consumers alike. What exactly is R-Value? Is it an effective and objective measure of the thermal performance of an insulation product or construction system? And how should the marketplace use it to compare the benefits of one insulation material over another?
During the 1970's demand for quality building insulation soared, when an oil crisis sent heating and cooling costs skyward, With many new products on the market -- and with so many conflicting claims pertaining to the insulating abilities of those products -- the Federal Trade Commission, with the participation and support of the insulation industry, created an objective method for reporting the performance of residential insulation materials. This method is called the R-Value Rule.
The rule provides requirements for product labeling (R-Value) and advertising, and mandates specific ASTM methods for thermal testing. The R-Value rule attempted to create a level-playing field for competing insulation materials. "The R-Value Rule has been helpful in comparing different brands of the same type of insulating material," said Betsy de Campos, executive director of EPSMA, "but as more sophisticated materials and higher technology construction systems are introduced into the building industry we find that the R-Value of a material does not tell the whole story."
R-Value is based on a mathematical term known as R-Factor. The term R-Value was developed to represent the ability of an insulation material to restrict heat flow. It is determined by placing test specimens between two plates in a laboratory apparatus and measuring heat-flow through the insulation. The test specimen usually consists of a square foot of material exactly one inch thick whose surfaces have a temperature differential of 1degree Fahrenheit. The thermal conductivity (k) of a material is expressed as the rate of heat flow in BTUs per hour.
R-Value then, is the R-Factor of an insulation material multiplied by the amount of material used. For instance, if the specified insulation has an R-Factor of 3.8 and you are using 3.5 inches of insulation, the R-Value is 13.3. Thermal resistance (R) of a material is its resistance to heat flow, and R-Value is expressed as the reciprocal of the materials thermal conductivity.
Pretty technical stuff, but the idea that a consumer should be able to compare insulation is essential to ensuring homeowners and building professionals are capable of making informed product decisions. Simply put, the greater the R-Value the better the insulation.
But, there is more to consider when making those decisions today.