Big Agnes Fly Creek UL25 sleeping bag review: A dream for ultralight enthusiasts

Pitched as a premium, packable and minimalist mummy-style sleeping bag, the Big Agnes Fly Creek UL25 sleeping bag makes use of light shell fabric made from recycled fishing nets and high-quality 850-fill power down.

US brand Big Agnes is well known for its ultralight Fly Creek backpacking tents. Now, there’s a sleeping bag to join them: the Fly Creek UL25. With a similar emphasis on superlight, super-efficient performance, it'll help to keep your trail weight to a minimum.

The overall design is similarly tailored for maximum thermal efficiency to boost the bag’s warmth-to-weight ratio. It has a slim and tapered silhouette with unusual diamond quilting on the topside, intended to produce a lighter, more packable bag.

The features have been pared back to virtually the bare minimum, though you still get an ergonomic footbox at one end and a simple but effective hood with an insulated draught collar to lock in heat around your neck. There’s a skinny half zip so you can wriggle in and out too, but otherwise that’s about it.

The benefit of this disciplined design is that you get a bag with an ISO-certified lower limit of -4°C that weighs a scant 700g and has an excellent packed size. Gram counting long-distance walkers and committed fastpackers will doubtless appreciate these qualities.

On the other hand, if you’re looking for plenty of wriggle room and multiple venting options for warmer nights, look elsewhere (maybe at something like the Nemo Disco 15 or the Mammut Comfort Down -5).

But if your priorities tend towards minimal weight and a tiny pack size in a bag that’ll still be warm enough even if overnight temperatures dip to freezing, then the Fly Creek is worthy of serious consideration.

Temperature ratings

As a US manufacturer, Big Agnes expresses the temperature ratings of its sleeping bags and quilts in fahrenheit rather than celsius (centigrade). As its name suggests, the Fly Creek UL 25 has a lab test temperature rating of 25°F, which equates to -4°C.

Bear in mind that this is the EN/ISO certified lower limit (defined as "the temperature at which a standard man can sleep for eight hours in a curled-up position without waking and feeling cold"). The bag’s comfort rating is 2°C (defined as "the temperature at which a standard woman can expect to sleep comfortably in a relaxed position").

Still, depending on how warm/cold you sleep, those figures mean this bag should be suitable for use from late spring through to early autumn, provided overnight temperatures don’t drop too far below zero.

These are still competitive ratings, considering the extremely low 700g weight of the bag. This has been achieved through the use of premium materials including ultralight fabrics and high-grade down fill, as well as a tapered mummy shape and innovative weight-saving construction techniques.

This is reflected in the high £450 ($499.95) price tag though. It’s expensive, albeit actually a little cheaper than other class-leading bags in this category like the Thermarest Hyperion 20 (£470), the Sea to Summit Spark Pro (£500) or Mountain Equipment Xenith III (£550). But it’s also pricier than our 'best in test' winner from 2024, the Rab Neutrino 400 (£420).

If you're looking for a cheaper option, then the Vango Cobra 200 (£225) is worth considering. It’s billed as a two-season bag, with lower grade 700FP down, but actually matches the Fly Creek UL25's temperature ratings. Then there's the Alpkit Pipedream series, which represent another decent, lightweight down option.

Insulation, fill power and fill weight

The Fly Creek UL 25F is filled with very high-quality 850 fill power goose down, which is treated with a Downtek hydrophobic finish to help the down clusters maintain their insulating value even when damp.

The bag comes in two lengths, regular or long, which are suitable for users up to 183cm (6ft) and 198cm (6ft 5in) tall respectively. As you’d expect, the longer length is a bit heavier and has a slightly higher overall fill weight (540g of down compared to 480g for the regular bag).

The topside of the bag has an unusual diamond quilt construction, which is a different approach to the lateral or longitudinal baffles of most lightweight down sleeping bags.

It’s still a stitch-through technique, but it results in fewer lines of stitching overall, to help minimise packed size and weight. The downside is that the pockets of fill are not fully enclosed, which could lead to increased down migration inside the bag.

Big Agnes claims this won’t be an issue, and admittedly I didn’t experience any issues on test – in fact, the bag lofts beautifully, with a very cosy feel – but I reckon you’d be wise to shake this bag out from time to time to help prevent the down fill from clumping in certain areas.

Pack size and weight

The Fly Creek UL 25 tips the scales at precisely 700g in the regular-sized version or 775g in the long length. This is impressively lightweight for a 2°C comfort/-4°C lower limit rating, making this sleeping bag a good weight-saver for long-distance trekking and wild camping adventures.

In fact, it was the second-lightest bag overall in our 2025 three-season sleeping bags group test, beaten only by the more expensive (but also warmer) Mountain Equipment Xenith III.

It also compresses down to a size of 24x20cm when packed inside the included drawstring bag. This is already decently small and packable but with a compression stuff sack it would pack even smaller. Big Agnes recommends using its own ‘Superlight Girdle’ to really cinch it down.

For storage at home, a much bigger mesh storage sack is provided. Using this sack is always a good idea, as it ensures you won’t ruin the insulation’s loft by storing the sleeping bag in a permanently compressed state.

Materials and sustainability

The Fly Creek scores well on the sustainability front. It’s great to see high-end outdoor kit being manufactured with a strong focus on environmental responsibility.

The shell of this sleeping bag is made from superlight 10-denier ripstop nylon NetPlus fabric, which is made from recycled fishing nets. So, as well as avoiding the use of virgin synthetic fibres, it's also reducing ocean plastic.

This material is thin and lightweight but has a nice handle, so it feels soft against the skin. Being ripstop nylon, it also has a good strength to weight ratio. However, this is a bag that should still be treated with care, as it’s significantly thinner and more delicate than most rivals.

The hood also has a low-profile cordlock made with renewable materials from organic waste. Although the natural goose down fill is not recycled, it's certified by the Responsible Down Standard, ensuring that all of the down the brand uses in its sleeping bags and apparel comes from farms that prioritise animal welfare.

It's also treated with a Downtek moisture-resistant finish, using bluesign-approved water-repellent chemistry without intentionally added PFAS solvents.

Size and shape

Most ultralight sleeping bags are heavily tapered for maximum thermal efficiency, and the Fly Creek is no exception. In colder conditions, this ensures it feels very cosy. The footbox is nicely shaped and the hood, though simple in design, features a draught collar and a simple internal drawcord to ensure it pulls in tight around the head.

The form-fitting profile also means there’s very little ‘dead space’ inside the bag to cause cold spots. It warms up quickly and soon feels very cosy indeed, something we appreciated on test in Snowdonia during the frigid months of March and early April, when the highest summits still had a fair bit of winter ice and slush.

On the other hand, some backpackers might find this bag a bit restrictive, particularly if you have broader hips or shoulders. The regular version is suitable for users up to 183cm (6ft) tall. The shoulder girth is 152cm, while the footbox girth is 91cm.

The long version is suitable for people up to 198cm (6ft 5in) tall, and the shoulder and knee girths are 160cm and 99cm respectively. Though the two different sizes on offer will help it fit a wider range of body shapes, it's certainly not the roomiest bag out there.

Features

As an unashamedly minimalist, ultralight down sleeping bag, the Fly Creek UL25 is a little short on features. As noted previously, it has a nicely contoured footbox, a cosy draught collar at the neck and a snug if somewhat simple hood with a single drawcord. However, that’s essentially it.

Most notably, you only get a half zip, and the zipper itself is a slim, low-profile design, with a single puller. The puller is reversible, which means you can at least grab it from inside or outside the bag, but it’s not always the easiest to find, especially in the confines of a darkened tent.

As a result, wriggling in and out of the bag isn’t the easiest, though at least the centre zip placement makes things slightly more practical.

The fact that you only get a half zip also means that ventilation is not a strong suit of the Fly Creek UL25. On warmer nights, you can unzip the bag down to your waist, but unlike some rivals there are no side or footbox vents to help with cooling and airflow.

Verdict

The Big Agnes Fly Creek UL25 is high-spec down bag that's ideal for ultralight backpacking and other weight-critical adventures, but the tapered fit and minimalist feature set won’t appeal to everyone.

For more inspiration, take a look at our list of the best lightweight sleeping bags.

About the author

Matt Jones is a freelance journalist based in the heart of Snowdonia National Park in North Wales. He’s a vastly experienced gear tester and self-confessed outdoor kit geek. Matt’s been one of our main gear testers for the last couple of years and is the first person we call with any complicated kit queries that need in-depth and forensic analysis.