R 2.6 is a term used in denoting the insulative value or Resistance (to heat transfer) of a material. For example, homes in the northeastern U.S. should have an R-25 to 30 value for attic insulation and at least R-15 in the walls.
Thermarest and other reputable mattress makers accurately rate the R value of their products so we have a good idea of their suitability for summer, fall or winter use.
The new RidgeRest Deluxe thicker closed cell foam (CCF) mattress has an R value of 3.6, which is good for most winter camping down to maybe 10 F. Don't forget, snow also has an R value and it's a lot warmer sleeping on a foot of compacted snow than sleeping on cold bare ground.
Thermarest's new Neo-Air tubular mattress has a higher R value than other tubular, non insulated mattresses because Thermarest cleverly used reflective material to reduce radiation heat loss and also V channels which tend to cut down on convective heat loss between the ground and your body.
You lose body heat through:
1. convection (air current cooling such as wind)
2. conduction (direct transfer of heat to a solid object like the ground)
3. radiation (infra red radiation of your body heat to the air)
4. respiration (a combined form of conduction and convection due to breathing out heated air AND heated moisture.)