Mountain Equipment Xenith I sleeping bag review: A minimalist marvel

The Mountain Equipment Xenith I is a hyper-specialised, ultralight sleeping bag built for gram-counters tackling warm-weather adventures.

Weighing just 379g (on our scales) and packing down tiny, it’s a minimalist marvel with premium materials – including 900 fill power European goose down and a featherlight 10-denier Plasma shell.

It delivers an impressive warmth-to-weight ratio, though the fill weight is low (148g), and it’s rated for a “Good Night’s Sleep” at 5C – making it a spring/summer sleeping bag.

The lack of an EN/ISO temperature rating is a shame, and the hoodless design will divide opinion, but the long 110cm zip adds practical usability. Sustainability is solid thanks to Mountain Equipment’s industry-leading Down Codex traceability scheme too.

Overall, it’s a first-rate and top-tier offering, particularly for adventures where every gram counts. It’s not the toastiest overall, due to the lightweight build, so if you need extra warmth opt for the Xenith II or Xenith III.

Temperature ratings

Unlike many sleeping bags on the market, the Mountain Equipment Xenith I hasn’t been tested against the usual EN/ISO temperature rating standards, meaning you don’t get the standard trio of comfort, comfort limit and extreme figures. That’s a little disappointing, as it makes like-for-like comparisons with other bags more difficult.

However, Mountain Equipment do quote an internal “Good Night’s Sleep Temperature” of 5C (41F), which feels fair and refreshingly realistic. I tend to feel the cold quite badly, and while I might not feel toastily warm at 5C, I reckon I’d be just about comfortable – which is more than can be said for some brands’ wildly optimistic claims. Rab’s Mythic Ultra 120, for example, has a stated Rab Sleep Limit of 0C, which seemed overly ambitious in my experience.

The Xenith I is clearly designed for warmer nights in spring and summer only, making it a niche product with a narrow usage window. It’s best suited to ultralight adventurers who are counting every gram and know they won’t be camping in cold conditions. That specialisation means it won’t suit everyone – and in places like the UK, where summer nights can still dip close to freezing, it does carry a degree of risk.

But for the weight, I’d describe the warmth as impressive and certainly ultralighters will love this sleeping bag.

Insulation type, fill power and fill weight

The Mountain Equipment Xenith I is insulated with 90-10 European Goose Down, boasting a minimum fill power of 900 – a top-tier grade that delivers exceptional loft, fluffiness and warmth-to-weight performance.

This is seriously high-quality down, with a thick, plush structure that traps air brilliantly and offers impressive thermal efficiency. However, while the quality is superb, the quantity is low – the bag contains just 148g of down.

That’s significantly less than many other bags on the market, which often pack in 400g or more. It’s this low fill weight, combined with the ultralight shell materials, that keeps the Xenith I so featherlight overall.

Although it’s slightly surprising not to see hydrophobic down used at this premium level, the bag’s smart construction and the PFC-free DWR-treated Plasma shell do help to offset moisture concerns to some extent.

If you’re looking for extra warmth beyond the Mountain Equipment Xenith I, the Xenith II and Xenith III are excellent options that maintain the ultralight ethos while extending the temperature range.

The Xenith II (£430) offers an EN/ISO comfort rating of 8C. It features 197g of European Goose Down (900 fill power) and weights 470g in total. The 3-season Xenith III (£550), which received a 4.5/5 star rating in our full review, has an EN/ISO rating of -2C. It contains 373g of the same high-quality down and has a total weight of 690g.

Pack size and weight

The Mountain Equipment Xenith I performs exceptionally well in the pack size and weight department – a true ultralighter’s dream and a gram-saving revelation.

With an official weight of just 360g (regular) and packing down to a minuscule 27cmx15cmx13cm in its included rolltop stuff sack, it’s both impressively lightweight and super compact.

The stuff sack itself feels sturdy and well-designed, with a rolltop closure that adds reassurance for wet-weather trips. On my own scales, I measured the sleeping bag at 379g (a curious 19g over the official figure), with the rolltop storage bag weighing 36g and the included mesh storage cube – which is ideal for storing the bag uncompressed at home to preserve down loft – coming in at 68g.

Materials and sustainability

The Mountain Equipment Xenith I is built with premium, high-end materials that reflect its ultralight pedigree.

It’s filled with 900 fill power goose down for maximum warmth-to-weight performance, while the outer shell is made from Mountain Equipment’s 10-denier Plasma fabric – a superlight, downproof ripstop nylon that keeps weight low without compromising durability.

On the sustainability front, the credentials are strong, if not flawless. Unlike Mountain Equipment’s Earthrise Down Jacket, which uses recycled down, the Xenith I uses virgin down – but it’s among the most ethically sourced in the industry.

Mountain Equipment has led the way since 2009 with its Down Codex programme, a rigorous auditing system that ensures traceability, animal welfare and environmental responsibility across its down supply chains.

Practices like live-plucking, force-feeding and bill-trimming are strictly banned. Each bag also includes a unique ‘Trace My Down’ code, allowing buyers to look up the exact origins and ethical standards of the down inside their product.

Size and shape

The Mountain Equipment Xenith I features what the brand describes as an “alpine fit” – a fairly tapered mummy shape designed to minimise bulk and maximise thermal efficiency. While some campers might find this style a little tight or restrictive, I personally found it very comfortable.

I’m quite slim, and the fit felt well-proportioned – snug enough to retain warmth without creating cold spots, but with just enough room to shuffle around and adjust position during the night.

It never felt claustrophobic or overly loose, striking a well-judged balance between comfort and performance. The Xenith I is available in both regular (£350) and long (£400) versions, with the long designed to suit anyone over 6ft tall.

Features

The Mountain Equipment Xenith I keeps things minimalist, but there are a couple of standout features worth highlighting. First is the hoodless design – a decision that may divide opinion.

Some might find the absence of a hood disappointing, especially when it comes to maximising warmth and comfort on chilly nights. Others won’t bat an eyelid, simply pulling on a beanie and cracking on. Which side of the fence you fall on will depend on your personal preferences and cold tolerance.

Second is the generously long 110cm zipper – a practical touch that makes getting in and out of the bag much easier than many ultralight alternatives, which often skimp on zipper length to save grams.

Other technical details include vertical box wall chest baffles and slant main body baffles for optimal down distribution, a stitch-through baffle back, ground-level side seams to reduce heat loss, a shaped and offset foot-box for natural positioning, and a full-length Gemini zip baffle with an integrated neck collar to help seal in warmth.

Verdict

The Mountain Equipment Xenith I is an ultralight sleeping bag featuring high-quality 900 fill power down. It’s a niche offering, best suited to gram-counting adventures on warmer spring/summer nights.

Looking for something a little warmer? Check out our top-rated 3 season sleeping bags of 2025.

About the author

James Forrest writes regular features and route guides for Trail and LFTO, and has been one of our main gear testers for the last few years. James is based on the edge of the Lake District so when he isn’t off on his latest crazy adventure or challenge, he’s walking in his local fells.