The Updated Mountain Equipment Extreme Light Sleeping Bag Range
Jun 10, 2025
Xenith III vs. Firelite Sleeping Bag – what has changed?
We have loved Mountain Equipment’s Extreme Light range of sleeping bags at UOG for years.
Designed with input from Matt Fuller, Mountain Equipment’s Dr Down (one of the only people on the planet to have a PhD in down) to maximise performance whilst minimising weight, this range just got a significant overhaul for 2025.
Some elements have remained the same, why change things if they already work perfectly, others are totally new. Ultralight Outdoor Gear talked to Matt about exactly what Mountain Equipment have changed on the new Xenith bags versus their predecessors, the Fire range.
To quote Matt, this is where they started with the process–
“We really went shaving every gram we could from the Xenith bags. Me and our sleeping bag designer have never spent longer looking at every possible way to save weight, but we did that without compromising features that really matter.”
Having had plenty of first-hand experience with the outgoing Fire sleeping bags including the Firelite and Fireflash, I was keen to find out more. The Extreme Light bags have always been top performing sleeping bags that kept me warm right down to their quoted limits.
Firstly, the new Xenith bags look stunning!
Fabric
The ultralight 10D fabric looks amazing in the same Merlot colour as Mountain Equipment’s award winning Oreus jacket. It is in fact the same fabric as used for the previous Fire range of bags (and the Oreus jacket), only the colour has changed. This fabric is incredibly lightweight and packable and offers excellent water resistance with it’s DWR treatment (now fluorocarbon free). It worked so well, no reason to change it.
Down
Both the Fire range and the new Xenith range use the same 90-10 European Goose Down with a minimum fill power of 900. The improvements to the baffle design allowed the designers to reduce the amount of down used for no loss in performance. The regular Xenith III uses 63g less down than the Firelite, but has the same EN ratings. In testing, Mountain Equipment did reduce the Good Night's Sleep rating from -9C to -7C to reflect this reduction.
Baffle Design
One very noticeable difference is the change from horizontal baffles to vertical baffles for the torso. The vertical baffles were in fact slightly heavier than the Fire series, but they offer some significant improvements. They give better ‘wrap’ over the user, work better for side sleepers and prevent down from shifting. This concentrates the down over the middle of the chest increasing warmth.
Xenith now has vertical torso baffles.
Xenith & Firelite baffle comparison.
Hood
The Xenith range uses a less complex 5 baffle hood design compared to the 7 baffles on the Firelite and Fireflash. The hoods are the same size and in testing no difference was noted in the way they worked, so less weight for the same performance – about 10g weight saving.
Additionally, the hood cord used on the Xenith was made lighter, changing from 4mm on the Firelite to 2.3mm plus smaller, lighter cord locks.
Zip
No change from the Fire range – still a 140cm full length zip with a Gemini zip baffle.
Xenith 2.3mm hood cord & 4mm hood cord on the Firelite.
New down filled neck collar.
Collar
The other big change between the Xenith III and the Firelite is the introduction of a down filled neck collar. In cold temperatures approaching the sleeping bags limit some people missed a collar on the Firelight and Fireflash bags. Adding one added a fair chunk of weight (around 20-30g) but this is worth it in colder conditions.
Fit
The Xenith range has the same Alpine fit as before but look much more roomy due to the removal of the EXL from inside the bag which ruched it and made it look narrower.
EXL
The Fire bags used something called EXL. The inner fabric of the bag had elastication to keep the lining close to the body and stop pockets of cooler air. For the Xenith, the EXL is now only in the collar. This saved a small amount of weight, and the move to the vertical baffles and improvements in wrap made it unnecessary.
Even though EXL was only on the inside lining before, with such light fabric, it caused the outside of the bag to wrinkle which didn’t look great. Removing it makes the outside of the bag look smoother and frankly much better. After all, we want our sleeping bags to look great as well as keep us warm!
You can see the wrinkling effect on the Firelite in the photos. It also makes the Firelite look a smaller bag because of being ruched up.
Xenith & Firelite shape comparison.
Footbox on the Xenith remains the same.
Xenith III vs. Firelite — Comparison at a Glance
The eagle eyed will notice that the Xenith range doesn’t include a Xenith IV which would be the equivalent of the Fireflash which had a comfort limit of -14°C / GNS Rating -16°C. This is because Mountain Equipment had already introduced the Xeros (Limit -12°C/GNS -14°C) and Kryos (Limit -16°C / GNS -20°C) sleeping bags. For only a little extra weight, these Extreme Alpine bags have a more water-resistant outer fabric, ideal for condensation and melting snow.
Conclusion
The new Xenith III is 65g lighter on our scales compared to its predecessor the Firelite despite gaining several new improvements which make it a better bag.
Matt pointed out that Mountain Equipment could have cut the weight by another 150g with a simpler hood and footbox, a shorter and lighter zip and no collar. But this would have made the Xenith III less practical for most users, and it wouldn’t have worked so well.
In the words of Dr Down, Matt Fuller – “We think with the Xenith we’ve got the right mix of lightweight versus practical usefulness, and we’ve improved on the Firelite, but I’m really not looking forward to having to improve on the Xenith when the time comes!”
The result is a superb sleeping bag and a worthy successor to the Firelite. It’s so good I will be replacing my Firelite after many years of keeping me warm and comfortable with the Xenith III. It really is that good!
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| Danny Pearson |
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Dan is a keen distance walker who loves to geek over the latest gear. In the last few years, Dan has completed a couple of TGO Challenge crossings of Scotland (with another planned for 2024), GR221 in Mallorca, Dales High Route and Hadrian’s Wall plus countless other self-planned routes. A keen mountain biker but finds he doesn’t bounce so well these days when the inevitable happens. |
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We have loved Mountain Equipment’s Extreme Light range of sleeping bags at UOG for years. Designed with input from Matt Fuller, Mountain Equipment’s Dr Down (one of the only people on the planet to have a PhD in down) to maximise performance whilst minimising weight, this range just got a significant overhaul for 2025. Some elements have remained the same, why change things if they already work perfectly, others are totally new.