The Xtherm isn't that great for below zero use ….. here's way and a better way.
The Therm-a-Rest NeoAir Xtherm certainly is the warmest, lightest, most comfortable pad I own (of about a dozen in my family), and perhaps the toughest too, but I won't use it again for below zero F camping. Why? The R-value certainly is high enough… but its a different technology…. and thus cold.
The Xtherm is an air mattress, meaning air can easily flow from cold to warm areas as you roll, shift, or even breath. In very cold places, this is enough to chill you in a warm bag. With a foam pad, air doesn't move from warm to cold.
I've tested this a few times in the backyard when temps dropped to -10F. My bags were quite warm (usually didn't even need to close the inner bag) and below I had an EPS accordion (2" expanded polystyrene rigid foam board, R-10, 6' long, 2' wide). In between I've tested a variety of pads, mostly to make the rigid foam comfortable on these old bones. (If you search around, you'll see the R-10 foam is appropriate for -40F ish.) Air pads certainly make the board comfortable but they were cold, especially when laying my side — which is when it should be the warmest b/c the fluffy bags wrap downward. It was old because the cold air had a way to get in, along the channels of the Xtherm. Using a decade-old self-inflating Therm-a-Rest or even a cheap foam pad isn't as comfy but certainly is warmer.
In summary, Xtherm's are great above 10F-ish when you can use it alone. Below zero, go for non-air-mattress technology.