Thanks, but I don't think I was confused:
The ridge rest is 3/4 inch from a flat surface above to a flat surface below (ie total thickness = ridges + foam thickness). (IIRC, the foam thickness is ~3/8 inch.) The primary insulation is in the foam, not the airspaces of the ridges so its insulation value is much closer to the insulation value of a 3/8 inch flat pad than the insulation value of a 3/4 inch flat pad.
The ridges actually reduce the insulation value (by increasing the area for heat loss) but the airspaces created by the ridges are claimed to increase the insulation value. (Of course, the ridges are flattened under body weight which would decrease their value.) One would need the R-value of a flat pad using the same amount of foam to determine whether the ridges actually help or are just a gimmick (and both R-values should be measured with the pads compressed by body weight).
Currently listed on the REI website:
* RR Regular: 5/8 inch, R-value 2.6, 20x72 inches, 14 oz (regular)
* RR Deluxe: 3/4 inch, R-value 3.1, 20x72 inches, 19 oz (regular)
(Don't know if these R-values were measured under a simulated body weight--my guess is that they were not.)
* REI standard blue foam (3/8 inch), R-value ?, 20x72 inches, 8 oz. (Sold as 24x75 inches, 10oz. The above weight is for a pad cut down to 20x72 to match the RRs.)
Thus, 2 REI blue foams would be lighter than, most likely give more insulation than, and would probably pack larger than a 3/4 inch RR. (And, of course, you could carry just one blue foam in warm weather.)