Condensation when sleeping with all layers in cold temps…

For mountaineering, you have to tweak your expectations of UL. It’s just too cold and exposed on high summits like Rainier to get away with sub 10 lb base weights IMHO. You’ll want warmer footwear, warmer insulation layers, shelter that can hold up to high wind, and a warmer more bomb-proof sleep system.

A 10 to 15F bag would probably be fine on Rainier in June combined with a down parka and other insulation layers, especially if you carry a VBL too. But a 0 degree bag is probably not overkill, especially if you sleep cold, or plan to do lots of winter trips in the future. A bag with a good water repellant treatment is helpful for the inevitable frost falling all you from the inside of the tent walls.

When sleeping on high peaks where temps can easily go below freezing, I always carry a light VBL to make sure the down stays dry in my bag – especially on extended trips. The down parka gets draped over the feet at night and I wear long undie tops and bottoms with a fleece jacket for the upper body.

It’s one thing to push the limits of temp ratings in 3 season backpacking vs. mountaineering.  I would make sure my bag could handle the lowest expected conditions and still have the down parka/pants for emergencies.  It can get cold very quickly in the mountains.

Make sure that  bag is one that is Expedition cut then, they need to be big enough so that you actually can wear that warm clothing inside it if needed. Naturally a VB liner won’t work in that situation so VB clothing perhaps.

The alternative is to use a double bag, your existing sleeping bag inside an oversized synthetic insulated overbag.

Lester is correct, the summits are no places for shortcuts or extreme UL even in summer. As an old climber I always slept in my down gear and used a WPB bivvy cover on my sleeping bag at all times, Nikwax made a huge difference to the damp down problem

I took an alpinism course on Baker about 10 years ago, then climbed Rainier unguided shortly after that, We were on Baker in late June, Rainier in early July. We used “0” degree bags that are probably really 20 degree bags after years of use.

If I did it again I would use my down quilt instead with my jacket for extra insulation. Obviously you want to check the weather forecasts before the trip but I really think if you can sleep down to 20 in your setup you’ll be fine. Most likely the temps will be way warmer than that anyway. It’s not like you are sleeping on the summit, you will be making camps then summiting from there.

By the way, the class was great, I think I went through American Alpine Institute but I imagine Alpine Ascents is equally good. My only advice is if you have to jump in a crevasse as part of the course, wear a lot of clothing. I got incredibly cold while waiting to get hauled out and I was kind of done for the day after that.

Thank you everyone for your comments…

I usually use quilts for everything, but for my new mountaineering hobby I’ve purchased both 10-degree and 0-degree Feathered Friends mummy bags.  The 10-degree bag (Lark 10 UL) is made of 10-denier Pertex Endurance and packs down nice and small.  The 0-degree bag (Snowbunting) is Pertex Shield and is a thicker denier – the fabric is FAR less packable, but I gather that the water/frost resistance is much better on the Snowbunting.

One of my biggest reasons for wanting to go with the 10-degree bag was just so I had less volume (I’ve got lots of stuff to carry on this trip).  But maybe it’s better to play it safe with the heavier bag until I’ve got some more mountaineering trips under my belt.  I have a sub-10 pound base weight for my usual backpacking due to years of figuring out what works and doesn’t… I should probably start on the heavier side and pare down as I gain experience.

If you truly get into mountaineering you will be amazed at how much weight you will be willing to shed on other gear so you can carry more on the rack, and how cold and miserable you will be prepared to sleep so you have to carry less weight. I recommend reading Andy Kirkpatrick now before you get totally disillusioned and reading Mark Twight on training and clothing; especially where belay parkas and staying just on the edge of cool are concerned.

Edit to add a few links

https://books.google.com.au/books?id=DHYSEvCp2e8C&pg=PA84&lpg=PA84&dq=Dwight+the+climber+and+the+belay+parka&source=bl&ots=OrQDp-uX6U&sig=KJ9XipBN0tiAAKDrlxk67SPFssw&hl=en&sa=X&ved=2ahUKEwjChtK7rendAhXM7mEKHcATA3UQ6AEwEHoECAoQAQ#v=onepage&q=Dwight%20the%20climber%20and%20the%20belay%20parka&f=false

https://andy-kirkpatrick.com/articles/view/the_belay_jacket

https://andy-kirkpatrick.com/blog/view/belay-jackets-revisited