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)

http://www.mammut.ch/images/Mammut_Sleep_well_pt1_E.pdf


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.

Thanks all!

Short version : for me it is equalising not averaging.

Not sure what Richard meant but to me the important part is to match the value of the sleeping bag and the pad, keeping in mind that the former has to do with air temp and the latter with ground temp.
Maybe not all that obvious but to me it is the same as standing still at 20f wearing shorts and sandals and expecting to be warm because you have a 10f type down jacket on top...
(remember that the infamous "cold feet ? put a hat on" implies that you were not wearing a hat..." )

Franco

Edward,

Both your understanding and calculations are correct. The above assumes that the ground temp tracks the air temp and it generally does. See my post at http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=20434&skip_to_post=163242#163242 that qualifies this statement.
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Thanks Richard and Franco,

I hadn't seen this thread, great. Good to know I'm on the right track! I will test both set-ups before I go as well. Cheers, Ed