R-value is a measure of the insulation factor of a given product. To protect you from the cold from the ground, the sleeping mat serves as a protective layer. The degree to this protection is based on the thickness and insulation of the mat. The mats’ effectiveness against the cold is denoted by R-value. The higher the R-value the higher the insulation level. As a rule, a “summer” sleeping mat has an R-value from 1-3, 3-Season requires a value from 3-5 and winter is 5+ depending on the extremities of the cold. Always check the specs. Thickness does not directly correspond to a higher R-value.