The Test to Determine the R-Value

The test used to produce the k-value (and thus the R-Value of a material) is an ASTM (American Society for Testing and Materials) test. This ASTM test was designed by a committee to give us measurement values that -- they hoped -- would be meaningful.

Unfortunately, the test was designed with a flaw or bias. Because of the way it's designed, the test favors fiber insulations: fiberglass, rock wool and cellulose fiber. Very little input went into the test for solid insulations, such as foam glass, cork, expanded polystyrene or urethane foam.

The test does not account for air movement (wind) or any amount of moisture (water vapor). In other words, the test used to create the R-value is a test in non-real-world conditions.

For instance, fiberglass is generally assigned an R-value of approximately 3.5 per inch. It will only achieve that R-value if tested in an absolute zero wind and zero moisture environment. Zero wind and zero moisture are not real-world. Our houses leak air, all our buildings leak air, and they often leak water. Water vapor from the atmosphere, showers, cooking, breathing, etc. constantly moves back and forth through walls and ceilings. If an attic is not properly ventilated, water vapor from inside a house will very quickly semi-saturate the insulation above the ceilings. Even small amounts of moisture will cause a dramatic drop in fiber insulation's R-value — as much as 50 percent or more.

Vapor Barriers

We are told, with very good reason, that insulation should have a vapor barrier on the warm side.

Which is the warm side of the wall of a house?