Re: Sleeping bag advice for spike camping
I'm going through the process of "field testing" in my back yard right now, so I'll throw in my 2 cents...
First, you must have an appropriate pad or you can throw everything else out the window. In my ignorance I suffered through the Colorado 1st Rifle Season on a Thermarest Ridgerest and that will not happen again. This winter I tested the new Thermarest Neoair Xtherm with an "R value" of 5.7 in temps of about 12 F. From a warmth perspective, I believe this pad could handle anything the Colorado Rockies could dish out. Even the largest version is unbelievably light at just a shade over a pound. Unfortunately I broke my back a few years ago and the loft of this pad (2.5") just wasn't quite enough for me and I made the switch to the considerably heavier but thicker and warmer Exped DownMat 9. It's rated to something like -38 F so you could consider it overkill, but it's the only one they have with 3.5" of loft and my back insisted.
As for the bag, I will agree with the folks that say spend the money on a good down bag. The one I have right now is a Marmot Couloir, rated at 0 F, but "EN Tested" at something like -2 F, meaning that it should actually be comfortable at that temp, not just "you will survive at that temp". I've had it down to about 10 F on top of the Exped and I have no complaints from above. I didn't have the pad tightly inflated so I did have some cold spots underneath while sleeping on my side--another lesson learned.
Something to consider with down bags is that you perspire 24x7 whether you realize it or not and eventually, unless you manaage to dry your bag out well every day or two, that perspiration will condense in your down and make it less efficient. I'll be heading out for about a week without any guarantee I'll be able to dry my bag out each day, so I recently ordered a set of vapor barrier clothes from Warmlite. This will do a few things...1) Keep my bag dry so I get maximum insulation each night, 2) VB clothes actually add 5-10 deg just like a bag liner someone else mentioned, and 3) if it gets REALLY nasty, I can wear my down jacket, heavy socks, etc to bed without all the perspiration effects on those clothes. So then you have a little dual-purpose insurance policy if the weather really turns bad. The down jacket when you first wake up or while sitting around making dinner at night can also make all the difference between shivering uncontrollably and being comfortable. It also weighs next to nothing.
In the end, everyone has to work out a system that works for them, but there are some points to consider. The biggest thing I would say--wait for the mercury to drop and test it out in your back yard. It's helped me immensely.