How R-Value Is Calculated

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R-value, which measures resistance to heat transfer in insulation, is one of the most popular building metrics—but what it does and doesn’t tell us is often misunderstood.

There are two test methods that are important to measuring R-value. The first, ASTM C518, is relevant to single materials. In this test, a sample of the material is placed inside a heat flow meter apparatus, between a cold plate and a hot plate. Heat flows from the hot plate to the cold plate through the insulation as the testing device measures how much heat is flowing.

Heat Flow Meter


Illustration: Peter Harris

Heat flow moves in three ways—conduction, convection, and radiation (see “How Insulation Works,” EBN June 2011)—and the test for R-value measures all three. If you were testing a rigid foam board, heat would be moving through the foam via conduction, and through the air bubbles within the foam via radiation. There wouldn’t be convection through such an airtight material, but convective loops forming within the air bubbles would speed the transfer of heat through the material and would affect the heat transfer measurements.

By capturing the effect of all three modes of heat transfer through materials, R-value gives us a great way to compare insulation products. In fact, the R-value measurement was created and popularized because it easily communicates relative insulation values.

Everett Shuman, a researcher at Penn State University, proposed the R-value measure in 1945. Prior to that, the primary measure for insulating value was U-factor, which measures heat flow. U-factor is the inverse of R-value (U = 1/R, and R = 1/U), which measures resistance to heat flow. While “good” insulation has low U-factors—an R-13 fiberglass batt has a U-factor of 0.08—R-value caught on because people apparently find it easier to understand that higher numbers are “better.” Round numbers also don’t hurt.

When we build enclosures—otherwise known as foundations, walls, and ceilings—the R-value of the individual materials becomes just one of several things to watch. ASTM C1363 is a test of the performance of a wall assembly or a ceiling assembly, and it uses a bigger test apparatus called a guarded hot box. (The “hot box” is the part where the heat is flowing. It is “guarded” by a layer used to ensure that the interior portion of the device sees a steady temperature.)

This testing, and calculations used in the design process, can capture other subtleties. For example, the thin layer of static air found on the exterior and interior surfaces of vertical walls has measurable R-value—R-0.17 for the exterior film and R-0.68 for the interior film. A one-inch air space (R-1) within an assembly adjacent to foil-faced insulation provides an R-3 layer due to foil’s ability to block radiant heat transfer. More conductive components, like studs, significantly reduce the nominal R-value of a wall assembly through thermal bridging.

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July 1, 2011