Sometimes you read something nonhuman R valuein outdoor equipment. The R stands for the thermal resistance and characterized the ratio of thickness to the thermal conductivity of the material of a product. The higher the thermal resistance, the better is the insulation of a layer. In the outdoor area, the isolation property of Thermo carpets and insulation is expressed with a value of R so that these can be made comparable with each other.
The insulation properties of a sleeping bag will depend on various factors. So, the thickest mat not necessarily has the best thermal resistance. Material and thickness as the construction or the surface affect the insulation. The popular (because convenient and easy) inflatable camping mats (like E.g. the Therm-a-Rest NeoAir) often have a pretty good R value, are suitable for temperatures below the freezing point, but only partly because condensation inside the mat can freeze, so that the mat from the inside covered with ice.
Following general guidelines with regard to the R value (source apply to camping sleeping bag):
R value 1 : up to + 7 ° C (3 seasons, warm nights)
Value of R 2 : up to + 2 ° C (3 seasons, moderate nights)
R value 3 : up to-5 ° C (3-4 seasons, cooler nights)
R value 4 : to-11 ° C (4 seasons, winterized)
R value of 5 : to-17 ° C (high-altitude winter bivouacs)
R value 6 : to-24 ° C (extended winter trips)
Note You should be that there is no uniform methods for determining the value of R. So for example the outdoor outfitter GlobeTrotter at all does not specify: “In the Guide and on the website we not specify R values, because so far no uniform methods for calculating these values exist.”.
You should make depends on the choice of a suitable sleeping bag not only by pure numbers like the R value. The sleep comfort is very important to me. My Therm-a-Rest NeoAir offers a thickness of more than 6 cm and weighing just 410 g. The R value is here “only” 2.5. Should I really out there stay in the depths of winter, a combination of NeoAir and a classic lightweight foam mat would be the easiest solution here. R values can indeed add up!
There is a very detailed comparison of mattresses and sleeping bags in their R value and weight when visiting on Pimasleepingbags.