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?
1.) Defining "R"-value
During the 1970s, as demand for quality building insulation soared, 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 has been helpful in comparing different brands of the same type of insulating material. However, 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 1 degree Fahrenheit. The thermal conductivity (k) of a material is expressed as the rate of heat flow in BTUs per hour.
Thermal resistance (R) of a material is its resistance to heat flow, and "R"-value is expressed as the reciprocal of the material’s thermal conductivity. Simply put, the greater the "R"-value the better the insulation.
FTC regulations govern home insulation marketing claims, including claims regarding the thermal insulation value provided by the product or material.4The FTC regulations also specify the means by which the R-value is determined. The claimed or rated R-value must be based on actual testing conducted in accordance with one of the test methods specified in the regulations. For products such as insulated siding, the appropriate standard is ASTM C1363 Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus.
In this test, the assembly, including the insulating material, is placed between two instrumented chambers: a “climatic chamber” and a “metering chamber.” The chambers are maintained at a specific temperature difference. The climatic chamber is usually cooler than the metering chamber, representing winter conditions. Wind is directed at the material in the climatic chamber to simulate outdoor winter conditions.
During the test, heat flows through the insulating material from the metering chamber to the climatic chamber, and the amount of energy needed to maintain the temperature in the metering chamber is measured. Calculations are done to convert this measured heat fl ow to the R-value of the assembly. The R-value of any mounting structure or other material that is not part of the insulation product is subtracted, leaving the net R-value of the insulation itself.
Because ASTM C1363 does not include installation details for specific types of products, the VSI Technical Committee has developed the test installation configuration, environmental conditions and procedures that should be used to produce an accurate and consistent R-value for insulated siding. The siding is installed over an 8-foot by 8-foot base wall in the same manner specified by the manufacturer for actual field installation. The installation includes overlap joints between sections of siding and other accessories to replicate normal installation. No artificial sealing of the assembly is done unless specified by the manufacturer for a normal installation. During the test, wind is directed against the surface of the siding, perpendicular to the plane of the wall.
The test is conducted according to the normal procedures specified in ASTM C1363. The base wall is tested by itself (without siding attached). The R-value of the insulated siding is determined by subtracting the measured R-value of the base wall from that of the entire assembly. This procedure ensures that the R-value claimed for an insulated siding product represents the actual thermal insulation value that will be delivered to the home.
As part of the ASTM standard development process, VSI is proposing that a minimum R-value of 2.0 be included in a new ASTM standard specifi cation for insulated siding, as described later. Once this specifi cation has been submitted for ASTM balloting and approved through the ASTM process, an R-value of 2.0 will become the recognized minimum specification for insulated siding. This minimum R-value is consistent with the minimum R-value required for insulated sheathing in the International Energy Conservation Code (IECC). R-values identifi ed through those testing methods can be used to meet the insulation requirements of the IECC. Chapter 6 of this guide provides detailed information on how tested R-values can be used to calculate a wall assembly’s U-factor.