inside: 750gm down bag (400gms of fill, rated to -1c comfort). outside: 750gm synthetic bag (2 x 60gms/m2 supermix fill, rated to +5c comfort).
*note: all the ‘rating’ stuff that follows is what the good pedigree companies nominate for these bags. make of it what you will in regards to the environment temperatures given by the japan weather stations. note too: all this refers to sleeping in a baselayer, well fed, in a variety of tents and bivvys.
for such an obviously great idea theres very little written about using 2 sleeping bags in combination, and even tho the term ‘sleeping system’ is thrown about a lot, its not until you use 2 bags that it really means much.
most winter climbers are very aware of how they apply down and synthetic layers to their bodies, tweaking things in all sorts of ways to get what works. and yet what they sleep in is usually an unadapted and off-the-rack gamble. matching up multiple sleeping bags is where sleeping starts getting smart. down/synthetic bag systems work because of how they allow you to manage moisture – and after about day 3 of a winter trip thats what its all about. not only do you need to stay dry enough to stay warm, but on a trip somewhere remote you want the confidence in your gear.
the bag combination ive experimented with this winter over 30 nights has been a thin synthetic (rated to +5) over a lightweight alpine down (rated to 0c) in temperatures down to -18 or so. together both bags pack down to and weighs the same as an 800gm fill down bag rated to -15 or so, or a synthetic bag rated to -10. what the combined system has as its advantage is adaptability – something at the core of winter climbing. with the 2 bag system in the photo above, i will happily take it to -20c and lower.
the basic concept is the inner down bag specializes in the warmth, and the outer synthetic bag takes on the moisture, with the air space between forming an important transition zone that lets both layers function at optimum. in a single bag, moisture come from inside the bag and meets cold air from outside somewhere in the middle, which then condenses to gradually wet out the insulation. in the double bag system the outer synthetic both protects from environmental moisture (snow, condensation etc) but traps a layer of air over the inner down layer which allows both the down to remain moisture free as its fully warm, as the synthetic takes on the moisture with its less affected insulation. at first it appears its simply all about the warmth of 2 bags combined, but it soon becomes apparent its actually the moisture management of the 2 insulations that is providing much of the extra warmth.
for this to work, the outer synthetic layer need not be very thick – just enough to warm the down bag right thru so any moisture moves beyond it rather than condensing inside. this is as much a matter of trapping radiation as cutting convection. this is where the synthetic bag is much more than a simple bivvy sack, which really only cuts the convection from wind/snow.
down (green/dark grey) & synthetic (light grey/black/red) loft side by side