What is the best cold weather sleeping bag for Denali?

Here are some of the most popular sleeping bags for climbing the West Buttress route on Denali:

Expeditions from late-April thru end-of May sleeping bags should be rated to at least -30F (-34C) to -40F:

• Marmot CWM Membrain -40 Degree Sleeping Bag
• Feathered Friends Snow Goose EX -40 Degree Sleeping Bag
• Western Mountaineering Bison GWS -40 Degree Sleeping Bag
• Mountain Hardware Ghost -40 Degree Sleeping Bag
• First Ascent Karakoram -30° Sleeping Bag

Expeditions in June sleeping bags should be rated to at least -20F (-28C):

• Feathered Friends Ptarmigan EX -25 Degree Down Sleeping Bag
• Western Mountaineering Puma Gore WS -25 Degree Down Sleeping Bag
• Mountain Hardwear Wraith -20°F Degree Down Sleeping Bag
• REI Expedition -20 Degree Down Sleeping Bag
• Marmot Col MemBrain -20 Degree Sleeping Bag

A sleeping bag suitable for climbing Denali should be:

• Down Only. Synthetic bags are not acceptable due to their size and weight.
• Sleeping bag needs to be a mummy-style down bag rated between minus -20°F and -40°F.
• Rated to -30F (-34C) for the early and mid-May expeditions; to at least -20F (-28C) for expeditions in June.
• Up until high camp, a -40 bag may be too warm.
• Many people are happy with a -20°F bag, but if you sleep cold you'll want a warmer bag.
• The temperature rating system for sleeping bags is arbitrary and is not a guarantee of warmth.
• Base your selection on how well you do in the cold. If you tend to sleep on the cold side, choose a bag rated on the lower end of the temperature range.
• Unless you are shorter than 5’ 8”, the long version is better to accommodate all the items you need to sleep with: 2 water bottles, liner boots, any thing else that needs to dry out.
• At High Camp, if you need extra insulation to sleep warm, you can wear your overpants and drape your parka over your sleeping bag.
• Using two sleeping bags together is not recommended.

Also you can see the current weather forcast on Denali at various elevations on the mountain at mountain-forecast.com