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Big Agnes Mystic UL 15 Sleeping Bag Review

Big Agnes Mystic UL 15 Sleeping Bag Review

The Big Agnes Mystic UL 15 is a lightweight 850 fill power mummy-shaped down sleeping bag that can be used for camping and backpacking. Weighing 32 oz, it is a cross between a backless quilt and a hooded mummy sleeping bag because it relies on your sleeping pad to provide proper back insulation. You still have to have an appropriately rated sleeping pad for the combination to keep you warm at night, but it saves on gear weight and gear volume, while completely eliminating any drafts between the pad and the sleeping bag.

Big Agnes Mystic UL 15 Sleeping Bag


UL Big Agnes System Bag

The Big Agnes Mystic UL packs down small, it's less confining than a regular mummy bag, and is a comfortable sleep system for a side sleepers.

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Specs at a Glance

  • Temperature rating: 15F
  • Gender: Unisex
  • Bag Weight: Small: 1lb 13oz / Regular: 2lb / Long: 2lb 5oz (bag used in this review)
  • Fill Weight: Small: 13oz / 369g / Regular: 14oz / 397g / Long: 16oz / 454g
  • Draft Collar: Yes
  • Draft Tube: Yes
  • Zipper: Full length
  • Insulation: DownTek 850 fill power
  • RDS-Certified: Yes
  • Shell Fabric: Nylon ripstop with a water-repellent finish
  • Lengths Available: Small -fits up to 5’5”, Regular -fits up to 5’10”, Long -fits up to 6’6”
  • Dimensions: Shoulder S-63”, R-69”, L-72”/ Hip: S-57”, R-60”, L’63” / Feet: S-43”, R-49”, L-52”
  • Mesh Storage sack: included, along with polyester ripstop stuff sack
  • Recommended Stuff Sack Size: 10L

Hybrid Sleep System Design

The sleeping bag sleeve on the Mystic UL 15 is the latest version of the “Big Agnes System”, first introduced in 2001. The Mystic UL 15 sleeping bag uses a modified version of the original Big Agnes System with a Flex Pad Sleeve, that marries the sleeping bag to an inflatable sleeping pad, and is easier to use than the original design.

The core idea behind the Big Agnes System is simple: down or synthetic sleeping bag insulation will not trap air and retain warmth if you lie on top of it. As long as you sleep on an appropriately insulated sleeping pad, (in the case of the Mystic UL 15 this is a pad with an R-Value of at least 3.5) the sleeping pad will provide sufficient insulation to keep you warm instead.

The Flex Pad Sleeve tightly couples the sleeping bag to the sleeping pad.
The Flex Pad Sleeve integrates the sleeping bag and the sleeping pad.

The key to making the Big Agnes System work is having a pad sleeve that’s attached to the sleeping bag, to ensure that you stay on top of the pad all night long and can’t roll or slide off at night.

With the Mystic UL 15, the pad is held in place by an elasticated pad sleeve that locks the corners in place and wraps around the sides of a sleeping pad. The pad sleeve can accommodate inflatable pads that are 20-25″ inches in width, however Big Agnes recommends using a rectangular pad rather than a mummy-shaped one for a more secure fit.

For example, I pair my Mystic UL 15 (long) with a Big Agnes Q-Core SLX 78″ x 20″ pad, which fits perfectly into the bag’s pad sleeve. Since the two are attached, I can shift around, sleep on my side, or roll over at night and not get all twisted up inside the sleeping bag. As a side sleeper, I often toss and turn at night, so I enjoy the extra freedom of movement that the Mystic provides me.

In a more traditional mummy bag setup, I usually wake up every few hours, having to re-adjust things when a shoulder or hip has slid off my sleeping bag pad. The pad sleeve eliminates that problem for me and I sleep much better.

Sleeping Bag Features

The Mystic UL 15 has all the premium features you associate with a cold-weather mummy sleeping bag, except that it’s very lightly insulated on the bottom where it lies on top of the sleeping pad. It has a beefy draft collar that wraps over the shoulders and around your neck to block drafts, a full-length zipper with a thick draft tube, a two-way snag-free zipper, and a well-insulated hood.

The Mystic is insulated with 850 fill power waterproof down and has generous internal dimensions. I feel that the waterproof DownTek treatment is a huge benefit, especially if wet weather is in the forecast. It helps maintain the down’s thermal properties even in humid weather and dries faster when the down gets wet.

The mesh cell phone pocket keeps your phone and other essentials secure inside the bag
The mesh cell phone pocket keeps your phone and other essentials secure inside the bag..

Standout Features

The Mystic UL 15 has two features that I have come to really appreciate and want to call out:

First, there is a cell phone pocket on the inside of the sleeping bag. I didn’t think this was a necessary feature, but I am aware that I have degenerated into a sort of dependence on my smartphone. This handy pocket helps avoid “cell phone panic” in the middle of the night when I want to either check the time or need a quick flashlight to find a boot to throw at a snoring tent-mate. Additionally, my phone battery stays nice and charged during cold temperatures thanks to it being warm inside my bag.

The jacket-style hood opening contours to the shape of your face for a comfortable fit when fully zipped up.
The jacket-style hood opening contours to the shape of your face for a comfortable fit when fully zipped up.

I also like the mummy hood on the Mystic UL 15 which has a jacket-style opening and a pillow pocket. The jacket-style opening conforms to the shape of your face but can be tightened to reduce the size of the front hole. I like the fact that it moves with you if you sit up or roll onto your side. The hood also has a pillow pocket that keeps your camp pillow in place while sleeping. I’ve found that a low-profile pillow works best and often just stuff a polar fleece jacket into the pocket to get the right height.

Temperature Rating

I purchased this bag last spring and have used it on a variety of outings in the Uinta mountains throughout the summer and hunting trips into the late fall. You may ask why use a 15-degree bag in the summer? Well, at 10,500 feet, and with a late summer cold snap, a warmer bag was needed to sleep comfortably.

However, because of the aforementioned lack of insulation on the bottom of the bag and the need to tailor the R-value of your pad to the season that you sleep in, I have heard lots of opinions and had a jaundiced eye of skepticism cast my way by others as I am putting the system together in the campground. The fact is that the 15-degree mark suggested by the company is just that: it is not an official EN 13537 rating.

The hood gives you the freedom to lift your head a few inches off the pad and the jacket-style hood opening contours to the shape of your face
The hood gives you the freedom to lift your head a few inches off the pad and the jacket-style hood opening contours to the shape of your face.

Due to the configuration of these system bags, they are not able to be tested for an EN rating, so the 15-degree rating is more of a guestimation by Big Agnes than anything. You will not find an EN rating posted for any of their system bags on their website. Having said that, I feel that care should be taken if you will be camping near the 15-degree mark. I have slept in this bag on several 20-degree nights and felt that it maxed my personal comfort range in the bag. It was hard to tell if adding a higher R-value pad would have made a difference or not.

Comparison to a Quilt and Quilt Pad Attachment System

The integration between the Mystic UL 15 and a sleeping pad is very similar to an ultralight quilt-based sleep system but with a few important differences.

  • The Mystic UL bag has a lightly insulated bottom that encircles your back like a regular sleeping bag. This eliminates any side drafts from entering the bag. Most quilts have open backs and you have to be vigilant about preventing drafts from entering.
  • The Mystic UL pad sleeve further blocks heat loss because it seals the gap between the bag and the sides of the sleeping pad keeping them tightly coupled. Quilts are not as tightly connected to a pad and side drafts have the potential to chill you.
  • The Mystic UL sleeping bag has a plush insulated hood and draft collar just like a regular mummy sleeping bag. When using a quilt, you need to use additional pieces of clothing to replicate these functions. It doable, but takes some trial and error to dial in an effective system.

I won’t argue that the Mystic UL 15 and its pad sleep are better than a quilt-based sleep system, but they are a lot more integrated and convenient to use. I have no doubt that a quilt-based sleep system can be lighter weight if you set your mind to it. You’ll have to decide whether the added complexity is more effective or worth the weight savings.

Comparable Sleeping Bags and Quilts

Make / ModelTypeTempWeight
Western Mountaineering UltraLiteBag2029 oz / 822g
Enlightened Equipment RevelationQuilt2019.18 oz / 544g
Therm-a-Rest Hyperion 20Bag2020 oz / 567g
Zpacks ClassicQuilt2017 oz / 482g
Feathered Friends Egret UL 20Bag2027.2 oz / 771g
Katabatic Gear Alsek 22Quilt2222 oz / 624g
Mountain Hardwear Bishop Pass 15Bag1537.3 oz / 1057g
Montbell Down Hugger WR 900 #3Bag3019.1 oz / 541g
REI Magma 15Bag1528.2 oz / 799g
Nunatak Arc ULQuilt2021 oz / 595g


The Big Agnes Mystic UL 15 is a pleasure to use and a good choice for trips where you don’t expect things to get too cold. It packs down small, it’s less confining than a regular mummy bag, and is a comfortable sleep system for a side sleeper like me. Going forward, I’m curious to see whether I can extend its range to accommodate colder temperatures by using a high R-value pad, wearing a heavier base layer to bed, or adding a thermal bag liner.

Disclosure: The author purchased this product.

About the author

Sven Peery is an all-season outdoorsman who enjoys backpacking, camping, hiking, skiing, and snowshoeing. He is also an experienced hunter and fisherman who is not afraid to wander off the beaten path. His wanderings have led him to hike and explore the vast trails of the High Uinta Wilderness, Wind River Range, and the Frank Church Wilderness in Utah, Wyoming, and Idaho respectively. Sven spent 8 years with a county Search and Rescue team in Northern Utah. His training includes man tracking, wilderness survival, backcountry, cave, and high angle rescue. Whether hiking in National Parks with family, rising up to 13,527 feet elevation of Kings Peak, or dipping nearly a mile below the rim to cross the Grand Canyon, he is always ready for the next adventure!


  1. I’ll give a big thumbs up for the REI Magma.

    And I want to request that reviews be dated. Things change.

    • I just use the date of the first comment as the approximate date that the story/review was posted. Thanks for date tagging this one. :)

      • Bug in the website template. We regularly move products that are no longer available or have been replaced by updates to the category called “Vintage Backpacking Gear.”

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