Lightweight Down Sleeping Bags

© Pegs Bailey
Over this spring and summer Dan Bailey has taken three different top-end lightweight down bags up hill and down dale in an unscientific attempt to work out which one's best: the Rab Neutrino 200, Mountain Equipment Xero 250 or the Mammut / Ajungilak Sphere Spring.


Unless your pockets (and gear cupboards) are bottomless it's perfectly possible to own too many sleeping bags; I should know. But really you can get away with a mere two - something beefy for winter and other snow-related mountain trips, and a lighter model for the rest of the year.

For the sake of brevity let's call this second niche two-season-plus. This is a broad category, covering quite a variety of conditions and as many possible activities – a chilly spring bivvy on the Cuillin Ridge; hut-to-hut-ing through a Pyrenean heatwave; camping in rainy autumn gales in Wales. While a cold weather bag has a fairly defined temperature remit (ie. proper brass monkeys) your 'rest of the year' choice should be more a jack of all trades, suited to temperatures ranging from the low single digits to, say, the low twenties. One of the joys of backpacking and mountaineering in the (relatively) warmer months is the freedom to travel without the encumbrance of all the winter weight stuff, so to capitalise on that the ideal summer sleeping bag is going to be the lightest one capable of doing the job. To me that means a down bag, and specifically goose down.

An early spring camp - above freezing, but not by much  © Dan Bailey
An early spring camp - above freezing, but not by much
© Dan Bailey

Despite the best efforts of the proverbial white coated boffins, good old fashioned down still beats synthetic fillings for warmth-to-weight. It's been in development for millions of years after all, and when have you seen geese shivering? Though they're heavier for an equivalent insulation synthetic fibres are sometimes preferred over down for their superior performance when saturated. In our damp climate this has obvious advantages for things that might conceivably get wet, like jackets, but with sleeping bags I'm less convinced. When wet even a synthetic bag is going to give you a very unpleasant night, but how often do you manage to soak a sleeping bag in the first place? I camp in the rain a lot – it's inevitable in the UK. Still I remember only two occasions in the last couple of decades when my sleeping bag got wet enough to compromise its insulation - and actually neither took place in Britain.

'Two-season-plus is a broad category, covering quite a variety of conditions and as many possible activities. It should be a jack of all trades'

To my mind bird-fluff has only one down side versus nylon – it generally costs more. None of the three sleeping bags on review here is a budget option, but while cheaper down bags are available (see Alpkit for instance) I've yet to find a lower priced model that matches the sort of weight saving offered by these three. These are among the lightest and best quality summer down bags widely available, and if looked after properly should give years of service to justify the initial outlay.

All pack away to a fairly similar size; 1L water bottle for scale  © Dan Bailey
All pack away to a fairly similar size; 1L water bottle for scale
© Dan Bailey
Xero 250, Sphere Spring and Neutrino 200  © Dan Bailey
Xero 250, Sphere Spring and Neutrino 200
© Dan Bailey

'Having spent several nights in each of the three sleeping bags and lent them to the odd friend the best I can offer here are my own unscientific, unsystematic observations.'

As far as possible this review is a comparison of like-for-like offerings from three manufacturers, each being the lightest conventional mummy-shaped down-filled sleeping bag they produce. But still each is slightly different, with varying weights and qualities of filling. Outside a laboratory a genuine objective comparison is impossible in any case. First and foremost this is because every body is unique, with differences in things like metabolic rate and gender affecting whether or not you tend to sleep 'warm' or 'cold'. It's thought for instance that an average woman (if there is such a person) will feel the cold more at night than a typical man. The theory is borne out in my experience; that is, I've experienced the moaning of female tent buddies on perfectly balmy evenings. Then of course there's nutrition; a sleeper with calories to burn is likely to be more comfy than someone running on empty. And what sleeping mat will you be using? Insulation from the ground has a huge part to play. Finally let's not forget the weather, which is of course different every time you go out.

Remember all these variables when reading this review. Having spent several nights in each of the three sleeping bags and lent them to the odd friend the best I can offer here are my own unscientific, unsystematic observations. All three have been used in a variety of conditions from early spring through summer, up hill and down dale. Each has been out both in warm weather and on fairly chilly nights, though nothing too hostile; the lowest temperature I recorded on a night out was 7 degrees though doubtless it got colder than that on several camps while I snoozed in the early hours. I usually sleep pretty warm (perhaps it's the body fat), so bear in mind that where I've felt comfortable others might not - and vice versa.



Neutrino 200  © Rab
Neutrino 200
© Rab

Rab Neutrino 200

  • Price £210
  • Weight 610g (actual weight)
  • One size – max user height 190cm
  • Quoted comfort limit 2C

More info at Rab website

This is the slightest of Rab's Neutrino range, a series of bags designed specifically for lightweight uses such as backpacking and (for the warmer bags in the series at least) quick-moving mountaineering in less extreme conditions. The Neutrino 200 is ideal for the former, outside of the winter months.

Unsurprisingly the number refers to the weight of the bag's filling, with the Neutrino 200 having 200g of European goose down with a fill power of 800 and a down/feather ratio of 90/10. For those that don't know I'll explain a bit about this jargon. The higher the quality of down used, the greater its lofting ability, measured under test conditions as 'fill power'. The bigger this number the less down is needed for an equivalent insulation, so though the Neutrino 200 (fill power 800) has 50g less down than the Mountain Equipment Xero (fill power 750) it is not ridiculous for Rab to suggest the bag has a lower temperature comfort limit. 100% pure down is not attainable as there will always be some feather in the mix; Rab reckon that 90/10 down/feather ratios are the best consistently available while the ME and Mammut bags both have slightly better ratios than this – make of that what you will.

As with the other bags on test here Rab's offering is built in China, but the Neutrino is at least hand-stuffed in the UK - a major plus for anyone concerned by the state of Britain's balance sheet. According to Rab 'European down is generally better quality than Chinese down due to the birds being older and having better developed down plumage when the down is harvested.' Rab use down sourced as a by-product from the meat industry, and they do not support live plucking of mature birds to yield loft values of 900 fill power.

Rab Neutrino 200  © Pegs Bailey
Rab Neutrino 200
© Pegs Bailey

Of course no down is much use if it migrates around the bag and clumps up, so to keep it where it's needed, consistently distributed across the bag, the Neutrino uses trapezoidal chambers. The baffle walls between these are of a lightweight mesh that allows air circulation, helping keep things dry and making the bag lighter and more compressible. Bits of down also tend to grip onto the mesh walls, which helps hold them in place.

Warmth-wise I have yet to knowingly test the Neutrino's quoted comfort limit of 2C. To date though it has seen me right on a breezy bivvy on Ben Hope in early spring when the early morning temperature can't have been far off that, and a mid-height hill camp since, also in temperatures well below double figures. Having been kept in Scotland all summer it's not had to cope with anything that most people in other countries would call hot weather, but we did have what passes for a heatwave in April/May, and valley camping during that period was comfortable enough. Unlike the other two bags on test there is however only a ½-length zip on the Neutrino 200 (heavier models in the range have ¾), so if things really were sticky you'd have more chance of getting too hot. While I'm talking zips, the Neutrino 200's has two YKK zippers, the top one being a locking model but the bottom one not (this saves weight, as only the top zipper is likely to come undone by mistake). There's also an anti-snag strip on the internal draft baffle, and it's worth noting that this baffle is stuffed with Primaloft rather than down since this artificial insulation resists crushing well.

'The inside of the Neutrino has the nicest next-to-skin feel of all the bags. The hood and collar are the best-thought-out too, while the stuff sack is the best in test'

The shell fabric is highly packable Pertex Quantum, a 15 denier nylon that's the lightest Pertex available at 30g/m², yet still comparatively strong; look closely and you'll see its mini ripstop pattern. Its dense weave makes this both highly windproof and virtually downproof; it's also very breathable, and comes with a water-shedding DWR treatment to help keep moisture out (do not put this to the test if you can help it). It is silky-soft to the touch, and so light that it only minimally impedes the down's natural loft. Obviously it's less abrasion resistant than heavier materials, and without poking holes in all three it does give less of an impression of toughness compared to the fabric used on the Xero and Sphere Spring. I've been fairly (not ridiculously) careful with the sample Rab sent me and so far it still looks new. The Neutrino 200's outer is a striking yellow colour, a bit garish for my taste though that matters a lot less with a sleeping bag than a jacket. The inside of the Neutrino is Pertex Quantum too, giving it the nicest next-to-skin feel of all the bags on review. These things count.

The bag's cut is longer on top than beneath, resulting in an angled footbox that promotes the most comfortable foot position - toes forward of heels - when you're lying on your back; it's a subtle touch, but I like it. All three bags on review boast something similar. At the top end of the bag though I think the Neutrino beats its two competitors by a nose. For me at least the hood is marginally the best-fitting of all three, with the snuggest-fitting drawcord around the face. Unlike the other bags there's also a second drawcord in an internal round-the-neck collar which noticeably keeps drafts out and heat in. The drawcords are elastic rather than string, and I find the slight give they offer makes for a more comfortable fit when the bag is cinched tight. A little zipped pocket inside the neck baffle (for your watch, say) completes a really well thought-out head area. The Neutrino's robust stuff sack also wins best-in-test prize as it's a full-on roll-top dry bag (what was I saying about not soaking your down?). For storage there's a bigger cotton bag.



Xero 250  © Mountain Equipment
Xero 250
© Mountain Equipment

Mountain Equipment Xero 250

  • Price £230
  • Two sizes:
  • Std: Max user height 185cm; 600g (manufacturer's value)
  • XL: Max user height 200cm; 680g (manufacturer's value)
  • Quoted comfort limit 4C

More info at Mountain Equipment website

Mountain Equipment are celebrating their 50th anniversary this year. That's a long and respectable heritage, and they are still turning out quality products even in this disposable age. The Xero is no exception, being well designed and equally well built. It's also a little different, and insofar as any mere sleeping bag could be called clever this one can. Let's start with the most important component - its innards. These comprise 250g (300g size XL) of 93/7 Hungarian goose down with a minimum 750 fill power. Though this is the lowest value of the three bags it does not seem to me to result in any discernible difference in loft, and in use the Xero 250 certainly gives good warmth for its weight.

According to independent tests the Xero is recommended for a temperature range of 0-25 degrees, figures correlating to EN13537 (a European standard for comparing like-for-like bags). That's quite a span. I'd say 0 is pushing it, while at 25 even a sheet would feel excessive to me, and at rather less than this indoors on a friend's couch I had to keep the Xero 250 fully unzipped all night. By ME's own reckoning the absolute bottom comfort limit for a 'standard man' lying in a curled up foetal position is 4 degrees, while below -10 you risk hypothermia. I won't be hurrying to put this extreme rating to the test, but for me at least the 4 degree comfort limit sounds about right. During a very blustery high camp at 850m in Knoydart in early summer I resorted to wearing a fleece and hat to bed. My thermometer read 7 degrees, but in fairness my tent was rather too well vented for the conditions and had it been less draughty I imagine I'd have been perfectly happy without the extra layers even in a rather lower ambient temperature.

Throughout the body down is held in place with box-wall baffles, a common low-weight construction method. At the toe things get more interesting, with a 5-baffle bullseye style arrangement that's said to maximise loft. As with the other bags on review the 'Sharks Toe Foot' is slightly offset, allowing your feet to adopt a natural position when lying on your back; this definitely feels comfy, and is also said to reduce compression of down. At the top end is a head-shaped 'Anatomical Hood' (I'm not sure this needs a name - surely all hoods should be head-shaped?), with a simple smooth-running single drawstring.

Xero 250  © Dan Bailey
Xero 250
© Dan Bailey

The Xero's weight is kept down in part thanks to ME's use of the not-particularly-memorably-named 'He30.rs', a shell fabric from their exclusive Helium range that's said to offer 'the lightest weights and breathability around' at a respectable 30g/m². If you're lying stationary it's hard to test for breathability, but I can say that I've never felt sweaty inside the Xero, for what that's worth. He30.rs is certainly light and almost tissue-thin, but it also seems relatively tough, giving the impression at least of being the hardest wearing shell fabric of all three bags; only time and further abuse will tell. Both the fabric and the seams are feather proof, so there's been none of that slow leaking of stuffing that you sometimes get with down bags. It's a fetching electric blue colour with matching zip. My one gripe with the shell is the slight crinkly noise that the fabric makes when you move, like the furtive rustling of crisp packets at the cinema. The lining material is similarly light but softer to the touch, and I don't find it clammy-feeling on warm nights. If I had to criticise the lining I'd bemoan the place where it is not used – inside the hood, where ME have reverted to that crinkly shell material; right by your ears, not ideal.

'The Xero is well designed and equally well built. It's also a little different, and insofar as any mere sleeping bag could be called clever this one can. The elasticated lining is like being gently hugged - mmm, lovely'

Inside the Xero is cut quite narrow, its snug fit accentuated by ME's use of a patented 'EXL' elasticated lining that pulls the walls close to the body and according to them improves the bag's thermal efficiency by a full 2 degrees. This is the clever touch I was referring to. It's like being gently hugged, without having your movement restricted – mmm, lovely. If you're buying online watch out for the sizing: my fairly average 6-foot dimensions are almost too much for the Xero in Standard size, for which the maximum user height is quoted at 185cm. For anyone between 185-200cm tall an XL size is available at an extra 80g in weight. Now seems a good time to mention a quibble with weight. Mountain Equipment give the standard size Xero a respectable 600g, but I make it 650g. In use a difference of 50g between stated and actual weight is not one I'd generally notice, but with companies engaged in a featherweight arms race the discrepancy is worth pointing out.

In common with the other bags on test the zip is not full length. Here it's ¾ length, which saves a little weight at the possible expense of hot feet on summer nights. As with the other two bags, the top one of the two YKK zippers is self-locking to prevent inadvertent night-time unzipping. An anti-snag strip on the inside draft-excluding baffle is welcome, while I still manage to catch that thin outer shell fabric in the zip occasionally (but then I'm all thumbs). With its taped seams the tough-but-light stuff sack provided would be fully waterproof if not for the fact that its closure is just a drawstring. In really wet conditions I'd want the extra security of a fully sealing roll-top dry bag. At home the Xero lives in a large mesh storage bag.



Sphere Spring  © Mammut / Ajungilak
Sphere Spring
© Mammut / Ajungilak

Mammut / Ajungilak Sphere Spring

  • Two sizes:
  • 180cm £305; 530g (manufacturer's value)
  • 195cm £320; 550g (manufacturer's value)
  • Quoted comfort limit 0C

More info at Mammut website


With a stuffing of 850 fill power European goose down in a 95/5 ratio the Mammut/Agjungilak Sphere Spring has on paper the best quality insulation of all three bags, and in use the loft certainly feels excellent. There's 240g of filling in the 180cm model and 260g in the 195cm version. Having put this bag through a range of weather from goose bump-inducing starry spring nights in the Highlands and Hebrides to muggy valley conditions in southern Germany I would rate the Sphere Spring as marginally the warmest of the three, though not by a lot perhaps. The quoted men's 0C comfort limit would seem to bear out my impression of greater warmth, but however good its down there's still less than 300g to go round and for this reason rather than any negative personal experience to date I'd take Mammut's lower limit with a wee pinch of salt unless you sleep unusually warm. Mammut suggest 5C as the comfort cut-off for women.

The internal construction uses wave-shaped baffled chambers, and while I struggle to picture exactly what they look like they certainly do a good job of keeping down in place and minimising cold spots. The outer is something called prolightTX, a 15-denier nylon that's basically another variation on the superlight-but-strong shell fabrics that all three of these bags boast. This is highly compressible, water repellent, reasonably wind proof and said to be very breathable – though again it's hard to test that in a sleeping bag. In my highly accurate pinch test it's not quite got the tissue-thin feel of Rab's Pertex Quantum but we are getting into hair splitting territory here. As far as I can make out the inner fabric is also prolightTX, but whatever it's made from I have two negative comments to make of the lining: a) There's a minor tendency to leak the odd feather. You'd forgive that in a cheaper bag but perhaps it's not ideal at this price. b) The bag (or rather its user) can suffer from a slightly sticky-feeling lining against a sweaty and unwashed body - a common complaint if you go to the hills with me.

'By my reckoning rather than Mammut's the Sphere Spring ties for first place in the lightness race, while on paper it's the warmest of the three. My experience would seem to confirm this'

While I'm being picky I've got to raise the spectre of weight again. Is this bag the lightest of the three? My sample is a 195cm model, for which Mammut claim an impressive 550g; but by my measure it's actually 610g, precisely on a par with the Rab Neutrino 200. Again I'm not one for counting every gram, but for anyone going all-out for the lightest possible bag this weight discrepancy is worth noting.

Mammut / Ajungilak Sphere Spring  © Dan Bailey
Mammut / Ajungilak Sphere Spring
© Dan Bailey

The ¾-length zip has the seemingly obligatory YKK zipper combination – locking zipper at the top and simpler zipper at the bottom. It's quite possible to snag the lightweight shell in the zip, but the material seems robust enough to handle this; meanwhile the draft-excluding inside baffle is protected with a wide anti-snag strip. A little fabric hood stops the top of the zip from poking your chin. As mentioned with the other two bags, an offset foot section lets the feet relax in a natural position and helps reduce down compression.

At the top end the bag is cut quite broad around the shoulders, which I like, then tapers quite sharply into a nice simple hood that draws in snugly around the face. There's a tiny almost jacket-esque peak around the top of the hood opening, which I guess might help keep some weather out if you're bivvying. However in detail the slightly odd two-part drawcord arrangement is not well thought out. A short piece at the neck is made of webbing strip, not very smooth-running in the toggle, while the elasticated section running around the head is so skinny that it easily pulls through the adjustment toggle, even when the button is not depressed. On a bag of this price you might expect something more effective.

Still in nitpicking mode, there's the stuff sack, a traditional compression sack with floating lid and adjustable webbing straps. This is not waterproof, and neither is it robust; I managed to rip the stitching holding one strap to the sack on the bag's first outing. The mesh-and-nylon home storage bag is fine though.

By my reckoning rather than Mammut's the Sphere Spring ties for first place in the lightness race, while on paper it's the warmest of the three. My experience would seem to confirm this, though again I'd stress the unscientific nature of the test conditions. Bar the stuff sack the Sphere Spring is as well made as the other two bags. So we come to the bottom line; the most notable difference between the Sphere Spring and its rivals is the price. Spend this price difference on a decent jacket and you'd more than make up any possible thermal shortcomings, and have a jacket as well as a sleeping bag to show for your money. Perhaps there's been a shift in the Swiss Franc / Pound Sterling exchange rate, to the detriment of Swiss companies?

Summary

So cut to the chase, which is best? I hear you cry. Well much as some questions simply have no answer, some races don't have an overall winner. In terms of performance you could barely slide a sheet of ultralight nylon between the three, so like the judge at a primary school sports day I'm going to award prizes all round. The Rab Neutrino 200 wins out on value and (just) on build quality; Mountain Equipment's Xero 250 has the toughest-feeling shell fabric and that nifty performance-boosting elastication; the Mammut Sphere Spring is (marginally) the warmest, with the highest fill power down. You pays your money (quite a bit in some cases) and takes your choice.




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