Sleep systems

Denali: System 3

Wearing your full Expedition Double Suit (the warmer version) inside the Hispar 600 moves the bag’s performance rating right up to -50°C (-44°C with the standard version Double Suit) (the bag in the system is the Wide version to make sure that the suit can loft fully). Using just the inner down suit inside the Hispar 600 gives a full system sleep rating of -33°C. The Hispar bag weighs around 800g (28.2oz) less than a standard bag rated to -48°C and packs down to about two-thirds the volume. The “K-Series” Hispar 600 has even more impressive stats.
Typical Operating Temperature
Guideline minimum temperature. Sleeping Bag TOT: you should get a night’s sleep, under normal circumstances. Clothing TOT: you should feel warm enough, assuming low/moderate activity. Sleep System TOT: relates to full system. For more details, see our Technical Briefing.
-50°C / -58°F
Schematic representation of Sleep System. Actual components differ.
"On the summit, I was ample warm and came down MUCH MORE comfortable than anyone else."


  • System Products:
    • ​Main Sleeping Bag: Hispar 600 (Wide) (Sleep Rating: -21°C / -5.8°F)
    • Plus Items:
      • Expedition Double Suit (-50°C)
      • Xero Down Socks
  • Full System Sleep Rating (using full Expedition Double Suit): -50°C (-58°F)
  • Full System Sleep Rating (using inner suit only): -33°C (-27.4°F)
  • Sleeping Bag Weights:
    • Hispar 600 Down Sleeping Bag (Wide): 1154g (40.7oz)
    • Standard lightweight Sleeping Bag to achieve Full System warmth: 1760g (62.1oz)


This system is similar to one already used successfully on Broad Peak (see Case Study), but it uses a much warmer sleeping bag and the full versatility of the Expedition Double Suit. In theory, a Hispar 400 would do (the 400 has a similar rating to the Hispar Overbag used on Broad Peak), but the Hispar 600 only weighs a little over 200g (7.1oz) more and gives a greatly increased margin for mishap or bad weather.

The Expedition Double Suit can cope exceptionally well with the extremes of low and high temperatures experienced in high-altitude mountaineering. What’s more, the double suit has significant advantages as part of a Sleep System. The inner suit can be used on its own in a Hispar 600 sleeping bag to provide a full-system sleep-rating of -33°C (-27.4°F). Adding the outer suit higher up the mountain shifts the performance to -50°C (-58°F). This bag should be warm enough for lower camps by itself, but combined with the Expedition Double Suit—first the inner and then the full suit—it provides enough warmth for comfort and safety at each of the different camps and altitudes.

K Series: The Hispar 600K Down Sleeping Bag (Wide) is available in the K Series. The exceptional loft of 1000 down reduces the weight to just 982g (34.6oz), creating a unique sleeping bag that still fulfils its -21°C rating at under a kilo pack weight.

This review relates to Hispar 600 Down Sleeping Bag

"This bag has an unmatched warmth/weight ratio ... The draft tubes undoubtedly provide the best insulation behind the zip of all the bags tested – not a trivial detail if you face harsh conditions."
from Carnets d'Aventures Magazine's test of 18 sleeping bags for cold conditions.

This review relates to the Expedition Double Suit

"On the summit, I was ample warm with just the inner suit and came down MUCH MORE comfortable than anyone else. Overheating was a major factor for my teammates - so much so that one almost didn't get down due to a combination of fatigue and heat-exhaustion. I was chipper on the other hand.

The beauty of the inner suit system was its softness and flexibility which allowed me to move a lot more freely than in a more conventional, bulkier suit."

Rob Casserley using the PHD Expedition Double Suit on Cho Oyu (8,188m)

This review relates to Xero Down Socks

"The Xero booties were great for sleeping in and I could also wear them inside my Sportivas for mooching around camp. All in all, perfect."
Nick Lewis, Antarctic Logistics & Expeditions.

Sleep system components

Works well with
From £2,366.00 1465g -50℃
An expedition double suit made of 2 layers: a one-piece Inner Down Suit & a two-piece Outer (down Jacket & Trousers). Designed to cope with the extreme temp. range in high altitude mountaineering.
From £214.00 170g
High-altitude expedition socks. Light, compressible and comfortable. For warmth in high camps.
From £900.00 1040g -21℃
Mountain lightweight for low temperatures and high peaks. Ideal for fast ascents and quick travel.