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I'm planning an extended winter trip into the White's, and am having issues with a temperature rating... I'm debating between -25 and a 0 degree bad. I did see the other recent thread on the topic, and it did have a lot of good points, but I'm really interested in suggestions based off of this region. Being out for a few weeks, the temperature can easily get down to the -20's at any point so that is why I'm really leaning towards a warmer bag. However with those really cold nights only happening once and a while, could I get away with a 0 bag and all my layers? I'm looking at Feathered Friends bags, making it a pretty big investment so I'm hoping to get the most out of the bag. Any suggestions are a big help!

When I buy a piece of gear, I try and buy it for what it will be used for 99% of the time. So if you are going to be in mostly 0 degree temperature ranges (and above) I would go with the zero bag and put on the hat and down jacket, and sleep with a warm nalgene bottle if the temperatures drop below that. It will also be in the teens at night, a -25 bag might be too warm. It seems like you will get more use out of the zero bag. I use the mountain hardwear phantom 0 on Mount Washington and it has treated me well. I hope this helps. Cheers.

Get the 0 degree bag.

However, you need to make sure you get the bag large enough to wear with your layers.

I regularly sleep in sub zero temperatures with a 0 degree bag, but I use bags that are cut a bit wider so they can accommodate my puffy jacket.

Temperature ratings are tricky. They don't really mean very much except that for a given manufacturer in a given model year, their -25 bag will keep you warmer than their 0 degree bag. Whether you're at all comfortable at the rated temperature depends on your metabolism and a host of other factors. I tend to think of the temperature rating as a survival rating, and find that I'm uncomfortably cold at least 10 degrees above the rating, sometimes 20, depending what temperatures I've been accustomed to in the previous nights.

You can certainly expect temps around -10, sometimes lower, but you'll often see +10 or higher, in which case a -25 bag is probably too warm even for skinny cold sleepers like me. I love a warm bag, so I'd go with a -10 if that was an option (and still plan to wear my jacket on the coldest nights). If choosing between a -25 and a 0 for a long trip, I'd hesitate a long time but I think I'd choose the 0 after considering the savings in cost, weight, and space. Make sure you have room inside with all your layers on, and bring a hot water bottle and an extra quilt. Also make sure your tent is weathertight and your sleeping pad is adequate.

What Nartreb said. I'll just add that I think my winter bag claims to be -20, and it's a might puffy bag, but at -10 it's hard to get any sleep and at -20, forget it. On the other hand, I don't find that I suffer from the "too hot" issue at any temps below 40 above--it's really easy to adjust for a little extra warmth.

Consider bringing a vapor barrier liner. They weigh almost nothing and add a good 10 degrees to a bag on a really cold night. You would only use the vapor barrier when you are pushing the lower limit of the bag. Here's an example, although it's a little heavier than mine: http://www.campmor.com/vapor-barrier-liner-regular.shtml

+2 to what Nartreb said. I sleep cold, so I tend to bring a warmer bag than some of my tent mates.
A couple of things to think about are:
1) like Nartreb suggested a compromise like a -10 bag.
2) Eat a bunch of chocolate and peanut butter before bed on cold nights in a warmer bag
3) some bags (I'm not familiar with weather Feather Friends does this or not) use continuous baffles so you can adjust the down from top to bottom and vice versa. I don't know how well this actually works but I know it's often suggested with Western Mountaineering bags. I keep meaning to try it with mine, but I always forget/get impatient.