I'm wondering about the rationale of averaging the R-value of one's sleeping bag and pad. I read one of Richard Nisley's posts where he seemed to say this was possible, that one could mix and match different R-values of pad and bag to achieve the proper level. I Pm'd him, but I thought I'd ask here too. For back sleepers, 35% of one's body is exposed to the pad. To average R-values, I can use the equation 4.9 (R-value of my All-Season pad) * .35 + (6.6 (R-value of my Marmot Helium bag) * .65) = ~6, to determine that my total R value is about 6. My question is whether I can use this value to predict the temps I will be comfortable in, using a graph such as the one found on pg. 30, here: (To find R-value, divide a given M2*K/W by .177)
The all-season is already pretty heavy at 19 oz, but I could supplement it with a 3/4 GG Nightlight, R-value 2.27, 3.6oz or so, and more than match the value of my bag. I'm doing the 50 mile Trans-Zion trek in Zion NP mid-March. I am going to explore and do some running, and complete it in 7 days. Temps could get down to about 10-15 F, but will likely be around 25 F. I have yet to camp below about 27 F, where I was completely comfortable with the All-season, a Montbell UL SS #3, a Montbell UL down parka, and goosefeet down pants.
I will have 14 additional oz of 850-900 down in a jacket and pants (Rab Infinity and goosefeet pants), as well as FF booties with 4 more oz down (I like to sit and explore at night and be warm...), but I know this would be compressed on the pad regardless. Also, I'm 31, fit, with a medium athletic build.