The term R-value as used in relation to sleeping pads is a measurement of the ability of the mattress to retain warmth. The higher the R-value, the warmer the sleeping pad.
A major consideration when considering a sleeping pad purchase is its temperature rating; the seasonal use for which it is designed. Some manufacturers will provide a true temperature rating while others refer to an R-value. That R-value references a pads ability to retain the warmth of your body during the night. Put simply, the higher an R-Value the warmer a sleeping pad will be and the warmer you’ll stay throughout the night.
The materials, construction, and design of a sleeping bag or sleeping pad directly impact that R-value. For example, an inflatable, uninsulated sleeping pad is designed for summer use and probably has an R-value somewhere between 1.0 and 0.7; low values that are perfectly fine for summer or warm weather usage. A 4-season pad with foam insulation might have an R-value as high as 9.0 or even 10.0, which you’ll need if you’re looking for winter usage. For a true 4-season pad, search for a pad in the R-value range of 3.5 to 4.0.
If you’ve got a 3-season pad that you’re fond of and prefer to sleep on year round you may want to consider combining it with a low-profile, closed-cell foam pad, which will go a long way to increasing your R-value and keeping you warm enough for 4-season cold weather usage.