Most of the advice I see on sleeping pads is along the lines of "for cold weather, get a sleeping pad with an R-value of 5+." I've also read that R-value is additive—that is, if you stack two sleeping pads, you can add their R-values together.
I have a Big Agnes Insulated Air Core Ultra Sleeping Pad with an R-value of 4.5. It's an air mattress made up of 3.25" air-filled tubes. I also have a bunch of Ridgerest closed-cell foam sleeping pads with an R-value of 2.
My Big Agnes pad is super comfortable, and looking at the R-value I was excited to take it winter camping. It was probably around 0ºF. I put down the Big Agnes on top of a Ridgerest for a theoretical R-value of 6.5 … and I froze my butt off all night.
FWIW, I'm sleeping in a Western Mountaineering sleeping bag rated to -30ºF or so. And I don't think it was down to other factors. I've been camping for many years in much colder temperatures, and I know how to sleep warm.
I figured R-value must mean something different when it comes to an air mattress, so I went back to using two Ridgerest pads (R-value of 4), which has kept me reliably cozy down to -35ºF. Or a Ridgerest and a more traditional Therm-A-Rest–style sleeping pad (i.e., about 1" thick).
A year or so later I took my cousin camping. He has the Klymit Insulated Static V Luxe SL sleeping pad with an R-value of 6.5. It's also a tube-style air mattress. I tried to talk him out of it, but he was sure he knew better. It was around -25ºF that night, and he froze his butt off. He was sleeping in a 20ºF sleeping bag inside a 5ºF sleeping bag, which are probably warmer together than my bag.
But I was just browsing REI's website and noticed this in the description of the Big Agnes pad:
This pad has an R-value of 4.5, which makes it best for adventures in cold weather; it provides considerable insulation from ground temperature
Most sleeping pads we sell range from 1.0 (good for warm weather) to 5.5+ (for use in extreme cold); the higher the R-value, the more insulated the pad
And I just can't square that with my experience. It seems to me that something about sleeping pads with lots of air in them makes the R-value less reliable. My hypothesis is that at temperatures substantially below freezing the air in your mattress is going to be cold. And cold air next to your body means you're going to feel it. So the less air between you and the ground, the better.
Can anyone explain this better?