Big Agnes Fly Creek UL 25° review: the ultralight sleeping bag that redefines pack size and warmth

Welcome to our review of the Big Agnes Fly Creek UL 25°, a premium down sleeping bag designed for backpacking and mountaineering in sub-zero temperatures of around 25˚F (-3.8˚C) and above.

While tents, torches, chairs and stoves tend to grab the most attention when it comes to online camping-related searches, it’s the quality of your sleep that ultimately determines how much you enjoy your time outdoors, and this begins with the best sleeping bag.

The three-season Fly Creek UL 25° is one such contender that pushes the technological envelope for warmth, weightlessness and supreme packability. Let's dive in.

Big Agnes Fly Creek UL 25˚ review

Price and availability

In the UK, the Big Agnes Fly Creek UL 25° retails between £270 and £405 and is available from Alpin Store (£272), EkoSport (£326), AlpineTrek (£360) and UltraLight Outdoor Gear (£405). If shopping in the EU, visit the Big Agnes EU store to purchase this bag for €500.

If you live in the US, try Big Agnes, CampSaver or Jenson USA, where the FlyCreek UL 25˚ sells for around $500.

What's the difference between down and synthetic insulation?

There are two main types of insulation filling in modern sleeping bags: down and synthetic. Each material offers distinct strengths and compromises suited to different types of camping, whether it's backpacking, family car camping, or those seeking warmth on a budget.

Down sleeping bags are cherished for their exceptional warmth-to-weight ratio, as the natural loft of down traps heat extremely efficiently, allowing bags to be lighter, warmer, and far more compressible than almost all synthetic alternatives.

This makes down an ideal insulation for backpackers, long-haul trekkers and alpinists where space and weight are absolutely critical. Down also tends to last longer, maintaining its loft over many years if stored and cared for correctly.

Granted, down sleeping bags usually cost more to buy (quite a lot more in some instances), but that's the price one pays for being cocooned in the ultra-fine insulating plumage found beneath the feathers of geese or ducks.

On the downside, for want of a better noun, down can be very vulnerable to moisture unless pre-treated by the manufacturer – when wet, untreated down loses much of its insulating ability and drying it in the field can be extremely challenging, as anyone who has tried to dry out a washed down puffer jacket will know only too well.

Synthetic sleeping bags, on the other hand, prioritise reliability in damp conditions. This is because 'man-made' synthetic insulation continues to insulate even when damp. It also dries quickly, making this type of insulation perfectly well-suited to wet climates, high humidity and less predictable weather.

Synthetic sleeping bags are also more affordable and easier to wash and store, making them more appealing to beginners and regular car campers. However, synthetic bags are typically heavier, much bulkier, and they lose loft faster over time.

Now you know the ins and outs of down versus synthetic, let's look at the down-equipped Big Agnes Fly Creek UL 25° in more detail.

Design and features

Colorado-based Big Agnes has been in the camping business for a couple of decades and has a strong reputation for pairing high-quality sustainable materials with attentive, minimalist design that just works. This premium three-season sleeping bag is a case in point.

Granted, the Big Agnes Fly Creek UL 25° is one of the most expensive down-filled bags currently on the market, but then it's also one of the best choices for backpackers, bikepackers, long-haul hikers and hardy mountain campers who place a premium on low weight, compact pack size and reliable three-season warmth.

Sitting firmly in the ultralight category, the Fly Creek UL 25° delivers exceptional thermal performance without the bulk and weight of comparable synthetic models. In fact, at just 785g in weight and a tiny 25 x 20cm when stuffed into its moisture-resistant sack, this is by far the lightest and smallest pack size I've ever seen in a sleeping bag.

What's more, with some deft squashing, it'll compress into an even smaller package than the one stated above, making it a shoo-in for smaller backpacks or for those who need more space in their pack for the tent.

The Big Agnes Fly Creek UL 25° is available in two sizes, each with a maximum user height of 6 feet (183cm). Regular has a shoulder girth of 60" (152cm), a hip girth of 54" (137cm) and a foot girth of 36" (91.5cm) while the Long version measures in at 63" (160cm) x 57" (145cm) x 39" (99cm).

The Regular variant I received uses 17 ounces (481g) of 850-fill-power PFC-free goose down, which is about as fluffy, light, and insulating as it gets. Moreover, the down itself is coated with DownTek, a moisture-repellent nanotech treatment that resists moisture while allowing the down to stay lofted for longer, dry faster and maintain warmth, even in damp conditions.

For its diamond quilted outer shell, Big Agnes has opted for ultralight 10D recycled NetPlus ripstop nylon cast from recycled fishing nets. This extremely lightweight fabric slices off many grams, but I would be doubly mindful of keeping it away from sharp objects – and especially naked flames – because it is an extraordinarily thin material.

Thermally, the Fly Creek UL 25°'s ISO comfort rating of 25°F (-3.8°C) to 35°F (1.6°C) makes it suitable for typical three-season backpacking from early spring to late autumn. While the highly-tapered mummy shape might not appeal to everyone, it does minimise internal volume for quicker heating-up time and better warmth retention, especially around the feet. Likewise, the generous hood and draft collar work very well at reducing cold spots around the neck, head and face.

Unlike most sleeping bags that come with a full-length side-mounted zip, this model features a centrally located half zip to help shave off some extra grams. While a half zip isn't as effective at ventilation control, it does improve overall insulation when the temperature drops and, for some users, is easier to use when fully ensconced.

To finish, this bag comes with a large breathable mesh sack for storage at home and a featherweight, moisture-resistant stuff sack for supreme packability when hitting the trail.

Comfort and performance

Having spent a good amount of time wrapped in the Big Agnes Fly Creek UL 25° (in both the MSR Hubba Hubba NX and the Hilleberg Soulo), it became perfectly clear that this bag is built for performance-first backpacking.

I was blown away by the tiny Tardis-like size of its pack, and when I started pulling it out, it just kept on coming. Even more impressively, when laid out and given a quick shake, the insulation immediately began to expand, and after five minutes, it was about five times as deep. Very encouraging.

Since this sleeping bag is so light and lofty, I'll admit it felt a bit odd when climbing in for the first time. In fact, it felt almost worryingly inadequate and akin to lying under a single silk sheet with almost zero weight above me.

But after a few minutes, the 850-fill down started doing what it's designed to do, and I was soon as snug as a bug from head to toes. I will add that I used it atop a soon-to-be-reviewed Big Agnes Rapide SL Insulated 3-season air-chamber sleeping pad.

While I personally prefer the position of a centrally located, weight-saving half-length zip, I'll admit it's less convenient than a full zip for complete temperature regulation in warmer weather. Also, getting into a bag with a half zip isn't quite as easy as one with a full-length side zip (with this model, you need to climb in rather than slide in).

But on the plus side, a half-zip offers slightly better insulation when the temperature drops, and I found it easier to access and use when snuggling in. Encouragingly, I also noticed that condensation resistance was better than I expected, with the treated down coping well in the higher humidity conditions of early November in the UK.

I'm of very slim build and 5'5" in height, and the Fly Creek UL 25° has been a perfect fit for me. While I'm generally not a big fan of tightly tapered mummy bags like this, I can certainly appreciate the extra warmth this cut style provides to the leg and foot areas. I have also been impressed by the puffy hood and internal drawstring, which effectively cocoon the face and neck against the chill of the night.

Overall, I’d rate this high-performing, cleverly designed bag as an excellent – albeit pricey – choice for three-season backpacking where weight and pack size are of the utmost importance. In fact, I'm frankly amazed that something so light can achieve such excellent thermal efficiency, and I thank my lucky stars I wasn't born 50 years earlier, when this type of technology simply didn't exist.

Verdict

The Big Agnes Fly Creek UL 25° is an excellent down-filled choice for backpackers who prioritise low weight, compact pack size and reliable three-season warmth. This light-as-a-feather model delivers impressive thermal efficiency, lofts quickly and performs consistently well in cool and even damp conditions. While the half-length zip may limit versatility for some, its overall performance, high-end materials and minimalist construction make it a top choice for weight-conscious adventurers.