How can R-20 of spray foam insulation outperform R-38 of fiberglass?

R-Value is one of the most misused and misunderstood measurements in the world. The test results are determined in a 75ºF lab with no wind load and ideal humidity. In such conditions the two insulation types are identical with respect to heat transfer. Unfortunately, insulation isn’t needed in those conditions. However, once the insulation is installed in a home in real world conditions, the heat loss/gain of fiberglass and cellulose are outrageous since cold and warm air freely move about them through convection. Spray foam insulation is an air barrier AND it is physically adhered to the wood framing members of a structure thus minimizing air infiltration. Think of fiberglass as a screen and spray foam insulation as a window. Coincidentally, most furnace filters are made from fiberglass. These filters are obviously designed to allow the free movement of air through them.