The R Value Myth
R Value is the capacity of a building material to resist heat flow. Americathe building codes use R-values to determine minimum levels of insulation allowed in exterior systems. The R value is calculated by the following formula:
R=l/Ŷ where l is the thickness of material in meters andŶ is the thermal conductivity in W/mK. (Google r value for symbols, these are not right, formula should be larger font, different color)
However, the R value is not the most accurate measure of an insulation material. The actual R value of an insulation material does not take into account air or moisture movement through the material. As we know fiberglass batt insulation is not an air or vapor barrier and actual R Value can drop drastically (up to 50%) with an introduction of even a small amount of either.
The R value only takes into account conduction where as the U value takes into account conduction, convection and radiation. The U value of an insulation material is the inverof the total thermal resistance to that element. The U value is calculated by the following formula:
U=(1/(R-Value))+Convevtion Heat Losses + Radiation Heat Losses . (Google r value for symbols, these are not right, formula should be larger font, different color)
Because the U value takes into account 3 different forms of heat loss instead the the R value’s 1 form of heat loss, the U Value is a much accurate measurement of heat loss in an insulation material. Why do we use the R Value then? Fiberglass batt insulation has a much higher R Value rating than U Value. The R Value of open cell foam insulation, cellulose and fiberglass are pretty similar. However, the actual performance of these materials in insulating a building are very different.
It is commonly believed that the fiberglass batt industry was very effective at regularly lobbying and contributing to keep the R value as the common standard of measurement in the building industry.
As the owner of Caddis Insulation I strongly believe that we should move to the U Value system of measurement or the R Values required in the building systems should reflect the performance of the insulation material. For example, an R-30 of Closed Cell foam insulation or an R-38 of Open Cell foam would drastically outperform an R-49 of Fiberglass Batt insulation in a roof system.