The chest baffles have been specially-designed around the double zips using two long vertical baffles at the sides and short horizontal baffles over the chest, which all work to keep the down in place, maximising loft and warmth.
The outer bag is quite a different design from the inner, with a short central zip and synthetic fill. The central zip means the inner and outer bag zips are offset from each other to minimise heat loss, and avoid confusion in the night if you need to get up. The zip is protected by an insulated flap which folds over the top to prevent freezing from your breath.
We used synthetic fill for the outer bag due to a crucial but little-known fact about sleeping in cold environments. We all lose moisture through our skin when we sleep. If it is cold enough, that moisture can freeze inside the bag fill before it’s had a chance to escape. Over time this can build up as ice and be a real problem on long expeditions. With the Spitsbergen, the ice will still form inside the outer bag fill, but being synthetic will not affect the insulating properties as much, and can dry much faster than down.
To further boost the warmth, we layered the fill in a "shingle” type construction over the top. Shingle construction uses overlapping pieces of fibre blanket instead of a single sheet. This allows for greater freedom of movement and also traps additional warm air between the layers. We have found in EN testing that shingle construction can be up to 15% warmer than a simple blanket fill design. We have selected a recycled synthetic fill that nonetheless meets our demanding standards of thermal efficiency.
The overall shape of the bag is designed to fit over the inner without compressing the fill and the hood is shaped so it can easily be pulled up over the inner hood for warmth or laid flat and filled with clothes to use as a pillow.
We experimented with joining the inner and outer bags together but found it was too easy for the bags to twist up and become tangled inside each other. When the inner bag is free, sleeping is much less restricted.
The outer bag also features a generous internal zipped pocket for valuables with an easy-to-use vertical zip.