double sleeping bag system

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.

double sleeping bag temp ratings

double sleeping bag fill

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.

double sleeping bag loft comparison

down (green/dark grey) & synthetic (light grey/black/red) loft side by side

a big key with making this system work is sleeping only in a baselayer. wearing insulation inside the bag keeps body heat from fully warming the inner down layer, which means any moisture present will still condense somewhere within the down. its somewhat counter-intuitive, but its actually warmer this way, and easily felt if you feel the temperature between your insulated jacket and the inside of the sleeping bag, which will be quite cool. sleeping in a baselayer puts that cool air space between the inner and outer bags instead.

another important factor is how you can regulate sleeping temperature by unzipping both or either of the layers. opening the down bag allows more body heat to reach the synthetic layer and further push moisture outwards – good for if youve bought a lot of moisture in with you (damp boots, snow, water bottle leaks etc).

theres a lot to be said about using 2 sleeping bags combined, and im just starting to really work it out. the more i tweak the bag system the more im also seeing how the mat system beneath me and the shelter around me affects things as well – not to mention the calories within me as well. easily enough material to devote a few thousand words to, before even introducing a third down or synthetic bag. andy kirkpatrick, the crash-test-dummy of british climbing, gives his more developed and tested thoughts here on the matter.