There’s a simple answer to this question. R-value is the numerical representation of what insulative factors different types of insulation contain. But just like a number that describes what place you have waiting in line, it leaves out the most of the important parts about who you are. R-value is used to describe the insulative effects of insulation, but it leaves out the most important parts of the process like air penetration, condensation and other natural effects that occur in the real world. That’s why R-values are often a misleading number.

Fiber Insulation

When R-values were first conceived, the majority of insulation materials were made from cellulose, Rockwool or fiberglass—fiber-based materials. Defined by the American Society for Testing and Materials (ATSM), R-values often are biased towards fiber insulation and give very little effort into the effects the a solid based insulation like spray foam have on a home in a real world environment. For instance, if a fiber-based insulation with an R-value of 20 gets wet, its R-value is reduced to zero. R-values are determined in a lab and aren’t subjected to real world conditions like air penetration or moisture, ensuring a misleading number is applied to the insulative material.

Air Loss

One of the biggest problems with misleading R-values is air penetration or loss. Every structure in the world losses or gains air as the structure ventilates. Outside air temperatures and inside temperature differences ensure that air movement through walls, floors and ceilings takes place. This makes fiber materials that aren’t sealed properly have a lower R-value than described by the ATSM. Sealed insulations like closed cell polyurethane foam have the highest true R-value because they seal out air penetration and prevent conditioned air from escaping the structure. These materials work especially well in attics and flooring because air movement occurs more actively vertically than horizontally.