Insulation resists heat flow through the roof.
A thin cotton shirt lets your body heat escape on a cold day. A thick wool sweater holds it in. The sweater keeps you warm because it has more insulating value--greater resistance to heat transmission. That might be all you need to know before deciding dress for the day, but if you are buying insulation for your house, you need more information.
Heat Flow
Heat always flows toward cold.
Heat flowing through any barrier, be it your sweater or the roof of your house, always travels from the warmer side to the cooler. The simplest way to test a material's resistance to heat transmission is to apply a heat source to one side and then measure how much the other side warms up. Test other materials, under the same conditions, and you can begin to assign comparative R (resistance) values.
Standard Test Methods
The American Society for Testing and Materials describes just such a comparative test in ASTM Standard C 518-04. For uniformity, testing laboratories use these guidelines to test materials and certify R-values. The value of standard testing is that it gives consumers a tool to compare products.
R-Values
The greater the thermal resistance of the material sample, the higher the R-value. The R-value indicated on insulation refers to the total R-value of the product as manufactured. For example, 3-inch batt insulation with an R-11 means a 3-inch sample of that insulation tested at an R-11.
The thermal resistance of insulation depends on the material, its density and the thickness of the sample. A thinner sample of high-density fiberglass insulation might have the same R-value as a thicker sample of low-density material. The denser product has a greater R-value per inch.
Component Testing
All building components have some thermal resistance.
Insulation is only one component of the systems that make up a building. Designers and builders responded to this problem by testing the thermal resistance of other materials, such as brick, stone, wood siding, shingles, gypsum board and other components of exterior walls and roofs. Adding together the R-values of the materials produces a total R-value for the wall assembly and a more accurate idea of the thermal efficiency of the building.
System Testing
Whole wall ratings require computer modeling.
One of the limitations of this type of material testing is that it tests specific materials under specific conditions. But according to the U.S. Department of Energy, the effectiveness of building insulation also depends on proper installation. Within the body of ASTM C 518 is a disclaimer that its testing may or may not be representative of actual conditions.
The Oak Ridge National Laboratory is developing whole-wall ratings that take this into account. The process involves physically testing full-scale wall assemblies, including interfaces with other walls, roofs and foundations. This data is used in computer modeling that incorporates as many as possible of the variables in assembling something as complex as an entire house and evaluates the system efficiency.
Test Data
As energy costs continue to rise, the standards and comparative ratings provided by laboratory testing are critical both to the homeowner who just wants to add some insulation to her attic, and to the architect who wants to design the most energy efficient high rise possible.
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