The R value you tells you how warm the mat will be to sleep in. The main use for a camping mat is to keep you insulated from the ground – which even in summer is pretty chilly and can conduct heat away from your body. The higher the R-value, more insulating it’ll be. Broadly, an R value below 2.0 is for summer use, 2.0 to 4.0 will suit 3-season use, and if you're heading out in winter, look for 4.0 or above.