Just got it so I haven't had a chance to try it out in sub-zero F weather, but I know it will do just fine. I got it because a friend has one. I've done fine on my old Thermarest with no R rating. But the Klymit rectangular shape has more coverage. The R rating is around 4.5, (some sources say 4.9, others say 4.4, which is what Klymit says.
R rating can be increased on type of pad underneath, anywhere from 1.0 to 3.0 + for a total R rating of 7.4 +. This makes a difference when exhausted/malnourished or have medical issues and need to conserve heat.
NB Side sleeping will reduce surface area and heat retention, reducing R rating.
Klymit has a .5 ounce foam hand pump you can special order now or wait until they have it online in a couple months. Vapor is not optimum posing potential frost layer, reducing R rating and possibly leading to mold. Save your breath, save your strength, get the pump, about $15. Otherwise, it takes about 40-45 breaths on average, more for some, less for others.
This review was written in the old system and had content requirements that are different than reviews written today.