…I learned in architecture school. R-value measures a material's resistance to heat flow. The formula to find r-value is area (ft^2) x temperature difference (*F)/heat generated by the insulated source (btu/hour), or ft^2 x *F/ (btu/hr).
Here's how it might work:
Say you want to keep a nice, comfy layer of 70 F air next to your skin while you sleep, but the ground you are sleeping on is only 30 F. The temperature difference between the temp you want and the ground is 40 F. Assume an insulation surface area of ~8 ft^2 (that's the area of my sleeping pad). A sleeping person generates about 13 btu/hr per ft^2 of body surface–again, we can assume ~8 ft^2 for body surface area. Actually, since the surface generating heat and the surface needing insulation are the same, we can drop both and just divide 40 F by 13 btu/hr, for a required r-value of about 3.
Pretty close to the numbers above, esp. since clothing, squished sleeping bag and tent floor all contribute a bit of r-value. Plus, different areas of the body lose heat at different rates, and ground temps are somewhat warmer at night than air temp (at least in winter). So R 3 is a ballpark number, not a buying guide, but suggests R 1 won't work on freezing ground.
My Prolite 3 is 1" thick with an r-value of 2.3–I find it both comfortable and warm for all the 3-season conditions I've encountered. But R 2.3/1" is actually pretty mediocre insulation, by building standards: some rigid foams achieve R 7/1". Of course, sleeping on a slab of foil-faced foam would be uncomfortable, and you'd look a little silly walking thru the woods with it.
