Leaving aside the fancy fabrics and technology just now, the main thing to say about the Plasma is that it works very well. The bag is rated to –9 degrees Celsius (which is 15 degree Fahrenheit, accounting for the 15 part of its name). I have slept in the Plasma at –9 without a tent or bivvy bag on a breezy night and was perfectly comfortable. I'm not a particularly warm sleeper – I'm normally fine when I get into bed but often feel the cold by morning time as my metabolism slows through the night. In the Plasma 15 I was still warm first thing in the morning when the temperature was at its lowest, and that gives me lots of faith in the rating. Of course, temperature ratings on bags can only be a guide. Everyone is different; women feel the cold much more than men (and this is reflected in a warmer suggested minimum temperature for female users), but for me the rating seemed spot on. I was only wearing my basic winter 'pyjamas', a merino baselayer, woolly hat and very light down filled socks, so by just adding a fleece and second pair of long johns I expect I could sleep comfortably at temperatures a bit lower than the Plasma 15's –9 rating.
"...There aren't many bags with 500 grams of filling that weigh so little...."
The second major point is that the bag is light. The quoted weight is 864 grams for the regular (although according to my postal scales mine is a whole 3 grams lighter than that!), the stuff bag weighs and additional tiny 27 grams. There aren't many bags with 500 grams of filling that weigh so little. But lack of weight doesn't mean lack of warmth, there are few other 500 gram-fill bags with a temperature rating as low as the Plasma 15's. Comparisons of these type have traditionally been opening a can of worms due to different rating systems, but now Marmot along with some other well known sleeping bag manufacturers are having their bags tested against the EN 13537 standard, meaning that these comparisons are more solid. This means when we look at the competition to the Plasma, a bag like the well-respected Mountain Equipment Xero 550 is both a little heavier and isn't as warm, according to the ratings coming out of the test; although to be fair the Plasma is a more expensive bag.
So how have Marmot managed to get so much insulation out of so little weight?
Firstly the down is 900 fill power and that is by the higher European standard. This is really as good as down gets and means that even with less filling than some other bags, you get more insulation. The insulation comes from the amount of air trapped by the down, and this depends on the loft of the down. Hence the higher the down quality, the more loft you get for each gram of down and the more insulation it provides.
Secondly, the bag has a novel design using vertical baffles (the tubes that hold the down in place). They run up and down the body, rather than the more normal across. This stops the down from migrating towards the sides of the bag and creating cold spots over your chest. To stop the down migrating toward the feet in the vertical baffles, Marmot have put in internal baffles to keep the down in place within the longer tubes. Marmot calls these 'flow gates' and say that if needed you can actually move down through them manually – if cold spots occur. Personally, I trust Marmot's designers to know where the down should be distributed and can't see myself trying to move it around inside the bag, but it is nice to know you can if over the years you find cold spots. The vertical baffles are also said to involve less sewing than the normal horizontal ones, again minimizing weight. Thirdly the bag is made with ultra silky and light Pertex Quantum nylon. This is very light, very compact but also pleasant against the skin.
Beyond being plenty warm the bag is also very usable.
The first time I used it in a pitch black tent it felt very intuitive, the neck baffle poppered shut easily and the both the baffle and hood drawcord were easy to find and not excessively long – always a bête noire of mine, leaving you with the probably irrational fear that you might end up strangling yourself in your sleep. Everything around the hood/zip-end is so well designed and cut that it does up nicely without needing Velcro tabs; both light and faff-free. I think the hood itself is great. A well fitting hood is central to getting the maximum performance out of a bag, and this one fits well, isn't at all constricting and creates a great seal meaning only your face is exposed to the elements. The full zip-baffle also does its job well.
Dampness is the enemy of down, and the Pertex has a DWR coating that, whilst not being as water resistant as some heavier materials, still offers some resistant. If you splash water onto it you can see it bead up rather than soak in. Sleeping in the bag (indeed any bag) in the open at well below zero I find that I get a build up of moisture and ice on the body of the bag just below my chin. This is the moisture from my breath freezing on the first cold surface it encounters. I've not worked out a way of stopping this, wearing something like a Buff over your mouth and nose helps but isn't particularly comfortable. But, importantly, despite this happening each time I've bivied in the bag, the DWR on the Pertex seems to handle it well – the moisture or ice is all on the outside, it dries away very quickly on bringing the bag inside and doesn't seem to lead to the down underneath getting damp.
The only thing I found against the Plasma 15 is that the silky Pertex has an annoying tendency to jam in the zip, despite there being zip stiffeners. The material feels so light you have to be very careful un-jamming it. It would also be nice if the bag cost less, but that's not really a fair complaint when you understand what you are paying for!
- Certified 900+ fill power goose down
- EN tested
- Flow gate construction allows user to manually shift down to preferred locations
- Vertical baffles for better thermal value and increased weight reduction
- Ultralight and durable 10 denier Pertex Quantum fabric
- Wrap around foot box construction incorporates flow design with Marmot's classic trapezoid shape
- Full length zipper
- Insulated draft tube
- Hood draw cord
- Insulated collar with easy access cord
- Velcro-free face muff
- Two way zipper sliders
- Two hang loops
- Weight reg: 864g, long: 919g
- Fill weight reg: 500g, long: 555g
- Size reg: (183cm), long:(198cm)
- Insulation: 900+ fill power goose down
- Material: 100% nylon micro ripstop dwr 0.8 oz/yd
- Lining: 100% nylon micro ripstop dwr 0.8 oz/yd
- EN rating: comfort: -1.70c, lower limit: -7.80c extreme: -25.90c
So overall the Plasma 15 is a beautifully constructed, top of the range bag for the serious backpacker or alpinist going to cold places. For the British user it will see you through all but the most exceptionally cold of central Highland nights, but still packs small and weighs very little in your rucksack. When I first started backpacking as a teenager I used my dad's Blacks of Greenock down bag that he had bought in the early 1960s and it still worked fine 30 years on. If you look after down it lasts fantastically, so although the Plasma 15 is expensive, you can look on it as an investment that should give you years – possibly decades – of good service.
About Toby ArcherToby Archer is based in Finland.
He describes himself as an "international politics think-tanker, perennial PhD student, hopeless but enthusiastic climber, part-time gear reviewer, often angry cyclist, idealist, cynic."
Climbing keeps him from getting too depressed about politics. He blogs about both at: