R-values for uninflated air mats?

Pretty much only foam core mattresses will have some r value, though minimal (specifics would require testing data that I don't have and can't acquire).

The reflective coatings really only work for radiant heat so you need an air gap between the film and your body which you mostly lose when the pad goes flat.

Most foam cores also use open cell foam which compresses pretty easily unfortunately, same issue with down and synthetic air mats.

So the values will all be pretty minimal regardless of technology used since every non CCF pad relies on uncompressed elevation from the ground to insulate. If you are truly worried about a flat pad bringing a thinlight (on top of airmat) to boost the r-value and provide backup insulation is the best compromise in my opinion.

EDIT: I should throw out there that actual testing may provide some surprising findings, but looking at the r-values of the minimum height from the latest SOTM for sleep pads seems to confirm this. Most drop to the 1-2 r-value range while still having enough air to keep you 0.5 to 1 inch off the ground. The exceptions are the DAMs and Synmats which werent measured much under 30mm of height (they may have enough structure to keep you from fully bottoming out?)

I've seen 0.7R for an original NeoAir. Floating around these parts, I assume it has some validity and isn't just hearsay.

Pure air mats will have an R value of 0. You will get a Vapor Barrier effect though.

Self-inflating (foam) mats will have an R value equal to their foam thickness.

Down or synthetic fill will be very small (after all, it will be near completely compressed).

From my readings on this site and others, the consensus seems to be that for safety and comfort in the deep cold, most people will bring their inflatable mat and also a CCF mat to go along with it, so that if they pop their inflatable they are not on the ground.
A popular combo seems to be something like a Neo-Air and a 1/4" evazote pad to go on top of it. Of course, there are other options like a Z-Rest or a thicker or thinner evazote pad too. Whatever keeps you comfy and safe. It's some extra weight and bulk, but it affords some dependable insulation if the inflatable goes flat.

Thanks, everyone. Looks like I might want to add a closed-cell foam pad to my list of things to pick up, just for safety's sake.

I think it's too heavy to double up

I have used Thermarest matts. Never popped. 100s of nights.

I have had a slow leak once where I had to re-inflate in the middle of the night, but even then it provided some warmth.

I have had delamination with the most recent Prolite, but that was just cosmetic. Used it for maybe 20 more nights before I finally replaced it. Eventually it would have been more of a problem.