Primer

How R-Value Is Calculated

You know what R-value is, but how do they figure it out? And is it really a reliable measure of insulation performance?

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.

Published July 1, 2011