It is common knowledge in the building science industry that all insulation systems perform at different levels depending on environmental conditions.  R-value performance can actually change as much as 50% or more.  Because Vermont, with its cold winters, is at the extreme end of this variation in thermal performance, the impact of climate cannot be ignored.  While all insulation materials have published R-values determined by laboratory testing, many of the materials we are using in our buildings have published R-values that apply to other climates, not Vermont. For example: if a particular insulation material or system performs at R=19 in a moderate southern climate, but at 9.5 in a cold Vermont climate, twice as much of that type of insulation should be used in Vermont if the code requires a real-world performance equaling R-19.

The test results from the U.S. Department of Energy verify that Fiberglas R-values are compromised when a 25° Fahrenheit temperature difference exists between the inside (70°) and the outside   (45°). When the test conditions reached an outside temperature of  -18°,  the R-values of the fiberglass insulation (both blown in and batts) decreased to less than 50% of the manufacturer’s stated R-value.

Cellulose R-values, on the other hand, went up. The R-value of Cellulose insulation began to rise when a 25° Fahrenheit temperature difference existed between the inside (70°) and the outside (45°).  When the test conditions reached the outside temperature of -18°, the R-values of the Cellulose Insulation continued to rise above the manufacturer’s stated installed R-value. It was so determined that cellulose insulation improves in R-value any time a temperature difference of 25° or more exists

U.S. Department of Energy Test Results

Loose Fill Fiberglass
Metering Chamber (F) Climate Temperature (F) R-Value
70 44.6 17.8
70 32 16.1
70 26.8 14.1
70 8.6 12
70 -4.0 10.6
70 -18.4 9.2
Loose Fill Cellulose (Settled)
Metering Chamber (F) Climate Temperature (F) R-Value
70 40 18
70 20 18.8
70 8 19.4
70 -5 19.6
70 -18 20.3

While this study only looks at two of the many types of insulation on the market, it is clear from this and a preponderance of other test data that changes in temperature can dramatically change the R-value of different insulation products in very different ways.