The R-value is a modern fairy tale. It's a fairy tale that has been so touted to the American consumer that it now has a chiseled in-stone status. But the saddest part of this fairy tale is that the R-value by itself is almost a worthless number.
It is impossible to define an insulation with a single number. To do so, we must know more. So why do we allow the R-value fairy tale to perpetuate? I don't know. I don't know if anybody knows. What we do know is that the R-value fairy tale obviously favors fiber insulation.
Consider the R-value of an insulation after it has been submersed in water or as a 20 mile-per-hour wind blows through it. In either of these scenarios, the R-value of fiber insulations goes to zero. But those same conditions barely affect solid insulations. That's why I believe that R-value numbers are misleading, meaningless numbers unless we know other characteristics.
In all probability, no one would ever buy a piece of property knowing only one of its dimensions. Suppose someone offered a property for $10,000 dollars and told you it was a seven. You would instantly wonder what that number referred to: Seven acres? Seven square feet? Seven miles square? What? You would also want to know the property's location: In a swamp? On a mountain? In downtown Dallas? In other words, one number cannot accurately describe anything, and that includes the value of an insulation.
Nevertheless we have Code bodies mandating R-values of 20s or 30s or 40s. But a fiber insulation with an R-value of 25 placed in an improperly sealed house will allow wind to blow through it as if there were no insulation. Maybe the R-value is accurate when the material is lab tested. But a lab environment may not even remotely duplicate conditions in the real world.
Consequently, we must start asking for some additional dimensions to our insulation. We need to know its resistance to air penetration, to free water, and to vapor drive. We must begin demanding the R-value of an insulating material after it is subjected to real world conditions.
As it is currently used, an R-value is a number that is supposed to indicate a material's ability to resist heat loss. It is derived by taking the k-value of a product and dividing it into the number one. The k-value is the actual measurement of heat transferred through a specific material.
Test to Determine the R-Value
The test used to produce the k-value 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.
Nor does the test 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. 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 a 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? Obviously, it changes from summer to winter — even from day to night. In a wintry 20 F below zero environment, the inside of an occupied house will certainly be the warm side. But during sun-shiny summer months, the outside will be the warm side.
Sometimes a novice owner or builder will put vapor barriers on both sides of the insulation. Vapor barriers so placed generally prove to be disastrous. It seems the vapor barriers stop most of the moisture but not all. Consequently, small amounts of moisture move into the fiber insulation, between the two vapor barriers and become trapped. The moisture accumulates as the temperature swings back and forth. This accumulation can become a huge problem. It can eventually total buckets of water that saturate the fiberglass. We have re-insulated a number of potato storages that originally were insulated with fiberglass and a vapor barrier on both sides. Fiber insulation needs ventilation on one side; therefore, the vapor barrier should go on the side where it will do the most good.
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