Mountain Equipment Firefly Sleeping Bag Review

© Richard Prideaux

The Firefly is the lightest and 'most efficient' sleeping bag Mountain Equipment have yet made, they say, offering 'reliable warmth for minimum weight'. So how did Richard Prideaux get on with it?

I don't want much from a sleeping bag. I want it to keep me warm, to be not too baggy but also not too tight, for it to be easy to get in and out of, not to be too fragile or sensitive to difficult conditions and - perhaps most of all - to be lightweight and packable enough not to resemble a Space Hopper down at the bottom of a pack.

ME have done a great job on the fit  © Richard Prideaux
ME have done a great job on the fit
© Richard Prideaux

The Mountain Equipment Firefly sleeping bag – in a rather fetching Ombre Blue – has been my doss bag for the past few weeks in the mountains and forests of North Wales. And it ticks the most important of my boxes.

Straight out of - and into - the bag

As is often the case with review items that UKHillwalking/UKClimbing throw at me, the Firefly arrived in the post with no supporting documents other than the tags that dangle off the zip. Having had a particularly annoying day in the office I shut down the computer, threw some stuff in a bag and sodded off into the lower edges of Y Carneddau for a night away from people (and out of phone signal).

First - this thing packs down very well. There is a drybag-style stuff sack with a vent hole towards the edge of the reinforced opening, and it's easy to get all the air out before turning the top over and sealing it down. Once everything is wrapped up and secure the resulting bundle is smaller than a child's head (or one of those weird half-loaves of bread). I could probably get away with shoving it into the lid pocket of most of my 40L+ packs. A separate, cube shaped, bag is supplied for uncompressed storage at home.

The drybag style stuff sack is a nice touch  © Richard Prideaux
The drybag style stuff sack is a nice touch
© Richard Prideaux

The Firefly packs down very small...  © Richard Prideaux
The Firefly packs down very small...
© Richard Prideaux

So the Firefly scores extremely well on packability. It is also very easy on the shoulders. My review bag (the longer version, Regular length also available) weighs only 631g with the drybag.

So that fulfils a good portion of that critical requirement for me – a small packed size, low weight and not having to store the stuffsack inside another drybag to keep it safe. Whilst grumbling away at the world I was able to plod up the slope from the A5 towards Pen Yr Ole Wen without my sleeping arrangements holding me back.

"Like an oak tree had mated with a bear…"

The next thing any good mountain sleeping bag needs to do is fit. In the drive to cut weight and reduce packed size a lot of manufacturers have developed a habit of making techy equipment only fit for those with hips and shoulders narrower than the result of that referendum we had a few years ago. As anyone who has met me or read any of my other reviews will know, I do not fit the Racing Snake profile. I am 6'2" and with a chest circumference a full 12" more than that of my waist. I am an odd shape, and I fully expected to have problems with this bag. However, it seems Mountain Equipment have really done something nifty here: I can get into it and zip everything up AND still wriggle around inside.

It's an excellent lightweight down bag for summer use  © Richard Prideaux
It's an excellent lightweight down bag for summer use
© Richard Prideaux

I think it's partly down to the Plasma 10D fabric used for both the outer and inner. It's slippery enough not to cling on the shoulders and hips, whilst still being very resilient – after about a dozen nights so far it looks like new.

There is also an elasticated banding arrangement that runs along the inner edge of the baffles, which hugs the bag against the body and gets rid of those pockets of cold air that can build up behind your back or against the stomach if sleeping on your side. ME call it the EXL System, and describe the bag as having an 'Alpine Fit' – well it works for me, and I am able to rotate inside the bag without feeling constricted.

Nicely sculpted hood...  © Richard Prideaux
Nicely sculpted hood...
© Richard Prideaux

...and foot box  © Richard Prideaux
...and foot box
© Richard Prideaux

The zip is backed by a generous baffle that has a slight stiffening to it. This helps prevent snags and ensures the baffle does sit right behind the zip when in use, but it isn't noticeable or uncomfortable against the body. That zip only goes about three-quarters of the way down the bag, but it does separate enough for it to be opened out and used as a top-quilt-with-a-foot-box.

In a world where we are increasingly reminded about the hidden environmental, ecological and moral cost of our spending, I like this level of traceability in the down


The down that fills this bag (274g of it) is from Russian sources, all traceable and 'sustainable' as rated by the Mountain Equipment Down Codex. If you go along to the Down Codex website you can enter the 12-digit code found on a label inside the bag. Mine is from the Rostov region of Western Russia. I'm told that:

"This relatively small supply chain consisting of just 3 farms and a single slaughterhouse was last visited in the Autumn of 2018. Our inspection showed that this down is collected from birds that are reared for their meat. Included within this supply chain is both a hatchery and parent farm. The birds live predominantly free-range lives with access to plenty of space and water. The slaughterhouse is located only a short distance from the farms and the raw material is then shipped to Germany for final processing."

There is also an embedded map and some information about the type of down and fill. In a world where we are increasingly reminded about the hidden environmental, ecological and moral cost of our spending, I like this level of traceability.

The Firefly is listed as having a minimum down fill power of 800, although according to the Down Codex site my particular down was more like 905; so it's high quality stuff, highly compressible and high-lofting. This helps make the Firefly warm for its weight.

In an age when hydrophobic-treated down is becoming more common, it is worth mentioning that the down used in this bag isn't treated with anything, so the traditonal precautions of keeping it dry and regularly airing really must be adhered to.

The 'EXL system' and 'alpine fit' work for me, making for a close but unrestricted feel  © Richard Prideaux
The 'EXL system' and 'alpine fit' work for me, making for a close but unrestricted feel
© Richard Prideaux


The interior construction and design of this bag falls in line with the rest of the Mountain Equipment range – slanted box-wall baffles that eliminate cold spots along those baffle seams, and a higher density of down fill in the places it will be most useful. The footbox area is contoured to encapsulate the feet without adding unnecessary fabric, and the hood wraps around the head and shoulders again without restricting movement.

A drawstring and cordlock allows the hood to be cinched down around the face. There is no flap-style neck baffle, just a slightly different box-wall baffle on the neck area. Mountain Equipment say that this is a "significantly lighter yet highly thermally efficient replacement for the conventional neck collar".

A different way of measuring comfort

A couple of years ago Mountain Equipment introduced their own temperature rating system for sleeping bags, which they present alongside figures derived from the industry standard EN13537 test. The reason for the extra rating is that the standard test doesn't tell you everything about a bag - namely what it's like to actually sleep in. As a result they have redefined what 'comfort' means, introducing their own 'Good Nights Sleep' rating that attempts to put a number on this subjective experience based on extensive real world use.

For the Firefly the Good Night's Sleep Temperature is listed as being -1°C. This is arguably a more practical overall guide to the bag's capabilities than the official ratings (Comfort: 4°C, Comfort Limit: -1°C and Extreme: -16°C - this latter figure can always be ignored unless you enjoy suffering and don't mind risking hypothermia). Confused? See here for an explanation of sleeping bag temperature ratings:

For my first use of the bag, merely hours after I had taken possession of it, I set it up on an insulated sleeping mat and allowed it to expand and decompress for a couple of hours in the tent as darkness fell. The outside temperature was hovering somewhere just above freezing, and a digital thermometer inside the tent inner showed about 4-5°C. So about right for the claimed comfort ratings for this bag.

I slept through the night, and was warm – but only just.

It's light, well-cut and has a sensible weight of full for spring and summer... value for money at this price  © Richard Prideaux
It's light, well-cut and has a sensible weight of full for spring and summer... value for money at this price
© Richard Prideaux

The problem with personal sleep ratings is that they are very subjective, and that so many factors can affect how cold one 'feels'. Fatigue, feeding and fidgeting will all have an impact on your perception of how warm or cold you think you are. This makes reviewing sleeping bags a little tricky, as you can only measure it against your other experiences.

On this first night in the Firefly I think I made an error with my body positioning. I climbed into the bag, wriggled around to check the fit and freedom of movement, cinched the hood drawstring down over my face… and then turned onto my side, bringing the whole bag with me. If the bag is optimised for insulation when lying back-downmost then I cocked it all up by rolling onto my side, exposing my back and bum to the cold air. What I should have done – what I have since learned to do – was to roll onto my side IN the bag and keep the orientation of those baffles and elastication in the direction the designers intended.

The breathability/air permeability of that Plasma 10D fabric may also have had something to do it – it was a particularly windy night and there was certainly airflow within the tent inner.

Subsequent nights in the Firefly, ranging from -1°C to +12°C, have been much more successful, and I have slept through without any cold patches at all. At the time of writing I am at least a dozen trips into the lifetime of this bag, and I will probably be choosing this one over my other down bags for the rest of the summer and well into the Welsh autumn. I'd say the 'Good Night's Sleep Temperature' is about right then.

The Firefly has a 3/4 length zip, and ultralight fabric inside and out  © Richard Prideaux
The Firefly has a 3/4 length zip, and ultralight fabric inside and out
© Richard Prideaux


At only 10 denier, and 26g/m2, the Plasma fabric used throughout is extremely light - but it still feels tough enough for regular use, and (crucially) it's down-proof too. For a lightweight, low-bulk bag intended for alpine use the Firefly feels remarkably rugged, and I would have confidence in taking this on a longer trip. Being highly breathable, this fabric should help the contents dry quickly - which is what you want in a down bag.

Limitations of lightweight?

When 'being light' is at the top of the design spec list there is always going to be some compromise. The Firefly loses features that have become common on similar bags (no interior pocket for headtorch or earplugs, no neck baffle, no hanging/drying tabs) in favour of cutting down weight and bulk. It is (nearly) the epitome of Colin Chapman's "simplify, then add lightness". But there can't be many sleeping bags on the market that are both warmer and lighter - especially at this sort of price. The cut and general quality of the Firefly are top notch, too.


The Firefly is a very light, well-made and cleverly designed down bag that's arguably at its best for summer mountain use or relatively mild weather in spring and autumn. Take it down below the 'Good Night's Sleep Temperature' of -1C and you'll probably soon know about it. For the sake of weight saving it may sacrifice a few features commonly found on down bags, but then it is very light and compact when packed. The drybag stuffsack is a nice touch, too. Overall I've found it very comfortable, and accommodating of even the lumpiest of users. The traceability of the down in the bag and the attention to detail in the stitching, fabric choice, zip layout, footbox and other areas does go a long way to justify the price tag of £360. You could certainly spend less on a summer down bag, but you'd be unlikely to get something cheaper with a warmth:weight to match the Firefly. Likewise you could spend more - but you wouldn't necessarily get a lot more performance for your outlay.

Mountain Equipment say:

The Firefly is the lightest, most efficient bag we have ever developed. If you want reliable warmth for minimum weight, look no further. The advanced design which features our 7-baffle anatomically shaped hood and four-baffle foot-box coupled with ultralight 10 denier fabric throughout have helped open up a new standard in superlight bag construction.

  • Sizes: Regular or Long
  • Weight: 560g (Reg)
  • Comfort: 4°C Comfort Limit: -1°C Extreme: -16°C
  • Good Night's Sleep Temperature: -1°C
  • Plasma™ 10D outer shell is incredibly light and exceptionally breathable
  • 274g of 90-10 Russian Goose Down with a minimum fill power of 800
  • Alpine fit with EXL® system improves loft and maximises thermal efficiency
  • Slanted box-wall baffles throughout
  • Ground level side seams
  • 7 baffle low volume anatomically shaped hood
  • 4 baffle anatomically shaped and offset foot-box
  • Full length Gemini™ zip baffle
  • Plasma™ 10D inner throughout
  • Supplied with waterproof roll-top stuff-sack and storage cube

For more info see

Richard Prideaux head shot  © Richard Prideaux

About Richard Prideaux

Richard Prideaux is the owner of established North Wales outdoor skills training and activity business Original Outdoors. He spends on average one night per week sleeping in a forest, up a mountain or on a beach somewhere in the UK and further afield and the rest of the time teaching navigation, foraging, tracking and other wilderness skills.

For more info see

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22 May, 2019

Just basing it on the photos can we assume it doesn't fit in a tent and is totally waterproof?

I know they say that 'apicture paints a thousand words', but they also say not to 'judge a book by its cover', so might I suggest reading the words - or watching the video - that Richard has so very kindly written on the subject before coming to a bleak conclusion.

Taking photos of sleeping bags a tricky thing to get right, especially within the confines of a tent. Having tried this before (back when I reviewed the ME Helium a few years ago), you often end up with highly cramped + awkward pictures which don't really tell you a great deal. It's even harder if you are out on your own. As such, pictures of bivvying are generally best, as they give a far better impression of how the bag actually looks/lofts, plus how it is length-wise.

That said, whilst nice images in reviews are clearly a benefit, it's ultimately the words, experience, and analysis that is what you're really after (unless I'm much mistaken and having been getting it wrong all these years?!). The fact that Richard has gone to the efforts of creating a three minute review video is to be applauded too, as not everyone has the time to read the full review.

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