The R-value of a substance is its direct measure of its resistance to transferring energy or heat; R Values are expressed using the metric units (m2.K/W). Basically the higher the figure the better it is at resisting energy transfer, so the easier it is to maintain a difference in temperatures across it for a longer time.

In the metric system, the R value measures per meter squared the amount of degrees kelvin temperature difference required to transfer one watt of energy. So an R value of 1 means per meter squared a single degree difference will transfer one watt of energy. So an R value of 2 will transfer half a watt of energy for a degree of difference.

Usually the R value is given for a certain type and thickness of material as installed (often known as the 'added R value'); i.e. a low density glasswool batts would need to be 130mm installed to achieve an R of 2.5, but only 100mm thick of medium density. Note: We say 'as installed', taking a low density batt that is designed to work in 130mm as installed and squashing it to fit in 100mm will not be the same as using a medium density batt in the first place.