GEAR REVIEW: Big Agnes Flume UL 30 Sleeping Bag

The Big Agnes Flume UL 30 is a cutting edge ultralight down bag designed for warmth. Its pricing is similar to other bags in its class, but the design features and details are a level above.

By Will Rietveld

Because key sleeping bag specifications are often lacking, it’s “baffling” for a consumer to compare them and make an informed purchase decision. In this review I share some of my wisdom from testing numerous ultralight sleeping bags, and present my evaluation of the new Big Agnes Flume UL 30 mummy bag.

Introduced in spring 2017, the Flume UL 30 (and its sibling the Hitchens UL 20) is a truly ultralight down sleeping bag from Big Agnes. Prior to this, Big Agnes offered literally dozens of high quality, utilitarian sleeping bag models for most any outdoor activity, except ultralight backpacking.

A good ultralight mummy style sleeping bag has thin shell fabric, a minimal zipper, high loft down (850 fill-power or greater), a good fitting hood, and enough shoulder girth to enable wearing camp clothing inside the bag to extend its warmth. So, how does the Flume UL 30 perform in relation to these criteria?

The Big Agnes Flume UL 30 sleeping bag features 850+ fill-power water-resistant down, ultralight shell fabric and  Insotect Flow construction weighs 24.8 ounces in size Long, and costs $500.

Specifications and Features

Manufacturer	Big Agnes (www.bigagnes.com)
Model	Flume UL 30 (size Long tested)
Materials	Downtek water-resistant down, ultralight high-tenacity ripstop nylon outer shell (with DWR) and lining, 40 in YKK #5 zipper
Weight	Measured weight 24.8 oz; mfr specification 24 oz
Features	850 fill-power Downtek water-resistant down, Insotect Flow™ construction, 3-D anti-snag draft tube, vertically structured side walls integrating contoured baffles, contoured hood baffles, oversized interior anti draft collar, ergonomic foot box, ultralight high tenacity shell fabric, low profile cordlock lets you un-cinch the hood with one hand, minimal 40” zipper, zipper garage, interior fabric loops for sleeping bag liners, exterior loops for hang drying or storage, mesh storage sack, nylon stuff sack
MSRP	Size Regular $469.95; size Long $499.95

Technique

We are not a society of string beans; far from it. Yet numerous ultralight mummy bags are simply too slim (shoulder girth of 59 to 60 inches) for the average hiker. I’m 6 feet tall, 165 pounds, and normally wear a size Large shirt – which is fairly average – but I can hardly close the zipper on a bag with the above girth if I am wearing an ultralight down jacket. If you are smaller than me, then you should be okay with a trim bag. If you are my size or larger, read on.

Ultralight backpackers like to carry an ultralight down jacket to wear in camp for warmth, and then wear it inside their sleeping system to extend its warmth. That’s an important technique to save weight and increase efficiency. To do that, an average or larger person needs 61 to 62 inches of shoulder girth, and that extra inch or two makes a big difference. Using this technique, I can sleep warm in a 30F rated sleeping bag when the temperature drops into the 20s, assuming an adequate shelter to minimize convective heat losses.

Description

I chose the size Long Flume UL 30 bag for it 62-inch shoulder girth (size Regular has 60 inches of girth). The extra length also makes it easier to enter/exit the bag and pull the hood over my head.

The Flume UL 30 contains 12 ounces of 850 fill-Power down in size Regular and 14 ounces in Size Long. The fill weight is a good way to compare sleeping bags, and 12 oz (Regular) or 14 oz (Long) is pretty decent for a 30F rated bag.

Insotect Flow construction claims the following advantages: 1) it holds down in place with fewer, smaller baffles; 2) it allows vertical baffles that contour to the body, holding insulation closer to the sleeper, and 3) it distributes body heat through the length of the bag. Using this construction method, the bag can be “body mapped” to place more down where it is needed (like the topside) and less down where it isn’t (like the bottomside). I have tested bags with “continuous baffles” that allow the user to shift the down as desired, which is good, but requires constant attention to avoid cold spots.

The Flume UL 30 has vertical baffles using Insotect Flow construction.

For an ultralight sleeping bag, the Big Agnes Flume UL 30 is feature rich (see specifications above), which perhaps adds 2 to 3 ounces to bag weight compared to a more minimalist bag. However, in my opinion, those details translate into more warmth, reliability, and convenience.

Big Agnes does not provide detailed specifications on the bag’s shell and lining fabric other than “ultralight high-tenacity ripstop nylon”. It appears to be equivalent to Pertex Quantum GL in terms of denier and downproofness.

Many manufacturers opt to put a full-length zipper on an ultralight bag, bowing to the notion that buyers insist on it, but Big Agnes seems to focus more on warmth and functionality. Rather than a long #3 zipper, Big Agnes uses a shorter more robust #5 zipper with a well-defined zipper track. The result is adequate zipper length for convenient entry/exit, more warmth, and little zipper snagging. The downside is the bag can’t be used as a blanket as easily on warmer nights.

Testing

I tested the Flume UL 30 bag on several summer mountain backpacking trips and one fall desert trip. Most nights the nighttime temperatures neared the bag’s temperature rating. The photo above was taken after a very damp night caused by an evening rain followed by a clear night and large temperature drop. The outer shell of the bag was coated with heavy condensation, but the bag’s DWR and water-resistant down kept it perfectly dry inside.

Ergonomically, the bag suits me just fine. I did not find any inconvenience with the shorter zipper; I entered the bag by sitting near the hood, bending my knees to slip my legs in, then sliding to my sleeping position.

The bag’s 62-inch girth provides plenty of room to wear camp clothing inside the bag to extend it s warmth, which is a must-have for me.

I had few problems with zipper snagging, owing to the bag’s roominess and zipper track design. While a tight-fitting bag puts stress on the zipper, causing it to lock or snag, a roomier bag allows the zipper to slide unimpeded. It does help to straighten the zipper channel with one hand while zipping with the other. One negative is the zipper will separate if it is unzipped all the way to the end, and it is frustrating to restart the zipper in the dark. I learned to stop zipping 1 to 2 inches before the end.

Sleeping bag warmth is very difficult to assess because it’s the composite of numerous variables. Wearing long johns, wool socks, and an ultralight down jacket inside, I found the Flume UL 30 to be warm down to its claimed 30F rating, which is actually a favorable evaluation, because most sleeping bags are optimistically rated. For that reason, some outdoor writers recommend getting a 20F bag if you want to be warm at 30F, or getting a 10F bag if you want to be warm at 20F. My preference, instead of going to a heavier 20F bag, is to wear my camp clothing inside the bag. That’s more versatile, because I wear the insulated clothing to stay warm in camp, and wear it in my bag to extend its warmth, and eliminate the need for a bag liner.

Assessment

An ultralight sleeping bag is a key component of an ultralight gear kit, and it’s expensive, so it’s very important to do thorough research before you buy. This is something you want to get right the first time; otherwise it’s expensive buyer’s remorse.

Researching down sleeping bags is more difficult than other gear items because information is often lacking and it’s hard to compare them on a uniform basis. Comparing them solely on the basis of bag weight is not a good idea, because you can end up with a cold, tight fitting bag. More important are fill weight (the amount of down in the bag), fill-power (the volume one ounce of down expands to, in cubic inches), and shoulder girth. Since down fill-power varies, for comparing bags it helps to multiply fill weight by fill power to get total fill volume. The shoulder girth issue is explained above.

I found the Big Agnes Flume UL 30 to be a cutting edge ultralight down bag designed for warmth. Its pricing is similar to other bags in its class, but the design features and details are a level above. It’s warm to its specified temperature if you wear a few clothes inside, which is something you probably want to do anyway. The only way to get a warmer bag in this weight class is to order a custom bag with overfill.

The weight of the Flume UL 30 is a bit higher than other ultralight 30F bags, but in its defense I note that I have tested 20F bags of the same weight that are not any warmer. A smidgeon of extra weight is okay, if that weight is put into features that enhance warmth and functionality. In that context, Big Agnes gets it right with the Flume UL 30.