Test results at 75°F are adequate for controlled comparisons when choosing materials, but most insulation materials have a higher R – value at lower temperatures. The variation in value is caused by changes in the conductivity of air within the insulation and by changes in radiant heat transfer.

In some cases, this variation can be significant. For example, in win­ter, the outside temperature may be 0°F and the inside temperature 70°F, resulting in a mean temperature of 35°F. Alternatively, for sum­mer conditions, particularly in southern climates, mean temperatures of 90 to 110°F can be experienced. The variation in R-value between these two extremes can be as much as 27 percent.4