The R-value of all materials – insulation or not – changes when the mean temperature to which they are exposed changes. Temperature is a measure of the velocity of the atoms or molecules in a given material. Higher temperatures mean a higher molecular velocity (in a solid this would be in the form of more rapid vibrations). Higher temperatures and the corresponding higher velocities (vibrations) transmit heat energy through the material more rapidly, resulting in a lower R-value. Conversely, lower temperatures and correspondingly lower velocities result in higher R-values.
Why does R-Value increase when temperature decreases?