"best" winter bag temp rating?

My choice is my 25f WM Puma. Have not pushed it much below -5f yet since getting it. I do have 40f synthetic quilt to pair it up with.

5 degree WM Antelope here.  Even my winter trips rarely get below 10 or 15 (without windchill).

Any colder than that is way too expensive, specialized, and rarely used for me; I’d just as soon rent than own a sub zero bag.

And same as above; I’d pair it with an overbag/quilt if I expected lower than 5.

For me it is a Mountain Hardwear Phantom 0*.  If it is any colder than that, I don’t have the knowledge or gear to stay comfortable.  I don’t even get to use the 0*F bag very often.

I find manufacturers temperature ratings to be somewhat optimistic. Instead, the following chart from the Mammut Sleep Well guide is a quick and simple reference that is reasonably accurate:

When I bought my WM Puma 10 years ago it was rated to -15F.  Now the same bag is rated to -25F although it has the same exact specs and weight as the older model.  I think WM fudged the numbers some.  For real world use it works down to about -10F for me in the comfort range.  I can’t imagine it working at the advertised -25F—for me.  Subjective and personal numbers.

The beauty of an overkill winter bag is it can be used as an unzipped open quilt 90% of the time until temps reach 10F and then I have the option to get mummified and fully zipped.  Quilts can’t perform this function.

All bags should be rated 15 degrees lower than anticipated temps no matter the brand.  Why?  Because no down bag will be as dry and as lofted on a long winter trip as it is at home fully fluffed and ready to be packed on Day 1 to the trailhead.  Not to say a down bag will accumulate moisture, but high air humidity and in-tent condensation occasionally causes the best down bag to lose loft and warmth.

Maybe on Day 12 of your trip you’ll leave the overcast days and the sleetstorms and the high humidity and hit a stretch of bone dry wind, inwhich case the bag returns to high loft, dry shell and incredible warmth.

You can always tell this by how the bag packs into its stuff sack.  At home it’s a struggle and when bone dry in camp, when slightly moist it packs easier and stuffs smaller.

I have often used my Puma at 30f all zipped up and did not overheat at all.

T. Walter,

You’re right, down winter bags absolutely will accumulate moisture over a multi day winter trip. That is precisely why I like DWR treated down, like the -20 F. Bag I just ordered from Bean’s. They will accumulate some moisture yes, but not nearly as much as untreated down.

I’ve seen my -20 MH synthetic bag shell soaking wet in a poorly ventilated tent in the morning. From the day on I’ve always sprayed a good DWR like Grangers on my bags.

Eric,

At temps below 0, I’ve always use two bags. The outer bag takes the hit for any rogue condensed vapor and the inner stays perfectly warm and dry.

My warm bag is an old TNF “Foxfire DL”, rated to “-5”. I couple that with a 50/40d quilt, and I’ve never had any issues whatsoever. When I switched to a water resistant down quilt a few years back, I’ve felt less clammy and it’s never taken on any moisture.

Like you said Eric “I realize this is a question with many variables like: expected low temps, cold or hot sleeper, synthetic or down fill, VBL or no, type of mattress, etc.”

For a shortish trip (one, two, three, four nights) in the Sierra Nevada below tree line, I think a zero degree down bag (MH Phantom) has served me just just fine. I am usually sleeping inside a Water Resistant Bivy (an ancient BPL Vapor made by Oware) under a MLD Duomid.  If I expect the temperature to drop below 10 degrees F, I might go to bed with 2 or 3 Nalgene hot water bottles. These will keep be toasty and warm until about 300 or 400am and be ready for breakfast at sunrise. If I expect the temperature to go lower than 10 degrees, I would dig a snow trench. If a really big storm was forecast, I would bring the double walled tent or dig a snow trench. The main issue I have to manage is building my sleeping platform flat enough so I do not roll off of it. When I wake up cold, I have usually rolled off or slid off the sleeping pads. One time I did try out an eVent bivy I found it way too warm, crawled out of it and slept on top of it.