A sleeping pad's R-value measures the pad's ability to "resist" (that's what the "R" stands for) heat from your body passing through the pad as the air inside tries to come to temperature equilibrium with the ground. A higher R-value pad is more insulating than a lower R-value pad. Most sleeping pads range from an R-Value of 2 or less, which is very minimally insulated, to an R-value of 5 or more, which is well-insulated. A sleeping pad works with your sleeping bag to create a comfortable system for a good night's rest. When sleeping bags are assessed for temperature rating, that assessment is done with an R-value 5 pad. Layer a closed cell foam pad under any air mattress, and the R-values add up, increasing warmth.