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Big Agnes Mystic UL 15 Sleeping Bag

Big Agnes Mystic UL 15 Sleeping Bag

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Product Description

10627-BMUL1519

The Mystic UL 15 is one of our most technically designed Big Agnes System bags, using our Flex Pad Sleeve design, a very durable nylon ripstop shell fabric, and super cozy 850 fill power DownTek™ water repellent insulation. This bag combines ultralight versatility with a long line of packable creature comforts. Whether you're a 3/4 length-pad-carrying, fast fly tent-gram-counter or just gearing up for a trip where weight and packed size is important, this is the go-to bag for you.

Customer Reviews

3.7 / 5.0
15 Reviews
5 Stars
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Rated 5 out of 5
Great bag
I pair this bag with the AXL trail boss sleeping g pad and have no complaints.
August 23, 2021
Originally posted at
Rated 5 out of 5
Best Purchase of My Life
Like my heading says, best purchase of my life. Hands down. Had a TNF Cat's Meow before I got a mystic sleeping bag ~10 years ago (thing was amazing too btw, synthetic 20 bag).

Anyway, yep, bought 10 years ago, still going strong. Have spent over a year in this thing, if not two. Draft tube keeps zipper from getting snagged, just be smart about it. Still has loft, barely has lost any feathers, although occasionally see some here and there now. Can't speak of newer models but 10 years and still going strong. Love how it attaches to my sleeping pad, never had an issue. Perhaps I see myself getting a 2nd lighter/smaller bag but so far no need, although tempting.

This bag as paid for itself and then some on a few trips in the mountains, on the coast, backpacking, and in the dessert, and in the snow ....a few summer nights I had to sleep on top of it and then get cozy in it after it cooled off. In a bivy, a little condensation as expected or a, few other nights when moisture got it wet I still stayed warm.

Overall, not too hot, not too cold, this a Goldilocks bag, super cozy. I've slept out under the stars without a tent and this thing as kept me company. FYI- Male, 6' thin body type.

Some people have grand expectations for gear or a sleeping bag, I'm not sure if I did or I always enjoyed a good night's sleep. I do now though, go hard, play hard, and sleep soundly.
December 3, 2020
Originally posted at
Rated 2 out of 5
Temp rating should be 50, not 15.
I've been backpacking for 40 years, in PA and WV mostly, three seasons, also some trips to the West and Ontario. I have had a number of bags prior to this one. This Big Agnes bag is roomy, light, comfy, and looks well-made. I like the sleep system concept with the sleeve to keep bag attached to pad. I use this bag with a 25" x 72" Q-Core SLX Insulated pad. There is absolutely NO WAY this a 15-degree bag; not by by any stretch of the imagination. I actually happen be a warm sleeper, but on three nights with temps in the low to mid 40s (one in late spring, two in early September), I absolutely froze. I froze despite adding layers of clothing. First I had on smart wool long johns and a 1/4 zip long sleeve top, wool socks, and wool hat. Freezing. I added a thick polar fleece. Still freezing. Then I added a down vest and synthetic trousers over my longjohns. STILL freezing. I have never before had to wear so much clothing to sleep to try to stay warm--and totally fail. I was literally wearing all the clothing I packed and still froze. I could feel the cold coming right through the bag. Horrible. And low- to mid-40s isn't really even all that cold. Something is terribly wrong with the temperature rating. I have never been so miserably cold in any of my prior bags. I cannot imagine what it'd be like to try to use this bag at 15 degrees! Please, Big Agnes, get your temperature rating right. This was an expensive piece of gear, and it absolutely does not perform as advertised. Also, he adjustments for the hood are really poor, and impossible to operate.
September 18, 2020
Originally posted at
Rated 5 out of 5
Awesome Sleeping Bag for Backpacking!
This bag is incredibly warm and comfy, especially considering how light it is and how small you can compress it! I'm comfortable in it in just my skivvies right down to about 20 degrees F; if it gets lower than that add a couple layers or a hot water bottle and you're good.
September 7, 2020
Originally posted at
Rated 3 out of 5
Great SUMMER bag... NOT a shoulder season bag. 15º- NO
I spent years in the industry. In the shops I ran we sold Marmot, Western, Moonstone, TNF, etc etc.. We did not carry BA bags but I was always curious... So I picked one of these Mystic 15º UL up...

I got it because I had a TNF bag... I believe it was a "plover"... this is back in the early 90's (Not to be confused with FF's Current Plover... it had a sleeping pad pocket on the bottom of the bag... which kept you from rolling off the pad. I LOVED IT... took it to Nepal for a couple months and simply loved it... but it was on the heavy side so I left it there as a gift for a Sherpa Friend... Back state-side we don't have porters so I stuck to lighter weight bags (Western Mtneering)

Fast forward and I see the Mystic and I decide to try it out. Its awesome, fantastic concept... BUT NOT a 15º bag. I don't care what your R rating of your pad is, what your evening calorie intake was or how you measure convective heat-loss due to air.... OR what you're wearing... it aint no 15º bag.

So... great bag, great concept... but I only take this thing out when its dropping into the 40s... anything more and I grabbing one of 3 or 4 other bags who's ratings are more on the 'true' side. Great SUMMER bag... NOT a shoulder season bag.
September 3, 2020
Originally posted at
Rated 3 out of 5
Should be rated at 30°
My wife and I have been using these bags with BA pads for two years now and while they're comfy they are most certainly not 15° bags. We've been chilly in these bags at 40°, down to freezing we've had to put on a lot of clothing in order to not be cold, can't imagine how cold we would be at 15°. I've been backpacking avidly for 37 years, have multiple sleeping bags by various manufacturers (mostly Marmot bags though) from -40° to 40° and have never used a bag so far off a comfortable temperature rating. We got these bags with the BA pads because we wanted something fairly light weight, have really liked our other BA equipment and the integrated pad idea appealed to us, we really do like that aspect of the system. Compared to my older Marmot 15° (700 fill with DryLoft shell but long) this bag has about 1//3 of the loft. These are really comfy bags and work well for warmer temps but thought it was warranted to warn others not to count on that 15° temp rating.
August 11, 2020
Originally posted at
Rated 5 out of 5
Fantastic.
I took this bag down the Grand Canyon in February this year. I've owned a few 15º bags from Big Agnes over the years. This is the best yet. I'm 6' 190lbs. I'm happy with a Long size. Plenty of room. If needed I wear my down jacket and puffy pants to bed to increase insulation.

I'd be mindful of some of the reviews on this website. Many people misunderstand the temperature rating systems for sleeping bags. Some folks think that just because a bag is rated to 15º that everyone in the world will be fine sleeping in it at 15º. No manufacturer in the world would guarantee this. We all have different furnaces running at different rates. All gear is trial and error and it's important to test your gear before the big trip.....not on it. Just saying...it's too bad that some reviewers don't consider things more fully before commenting. I trust Big Agnes and their team of ninja testers.
August 10, 2020
Originally posted at
Rated 5 out of 5
More comfortable than a mummy bag
I just finished my first trip using the Mystic UL. I was out for two nights in the Colorado Rockies near Aspen, camping around 10,000 to 10,500 feet in July, with night time temperatures hovering around freezing. I read all the reviews complaining about the optimistic temperature rating, so I went into it knowing I was probably getting a 25 degree bag, not a 15 degree bag. I was just warm enough in shorts and a t-shirt the first night with my tent wide open, and plenty comfortable in a light fleece shirt the second, cooler night. I didn't feel any heat loss from the uninsulated bottom of the bag, confirming my suspicion that compressed down is useless and can be eliminated. I used it with a Big Agnes QCore SLX 72x20 pad, which fits perfectly into the sleeve. The bag wraps around the sides of the pad a little, so there's insulation all around. It's a really light and compact bag. Could you use this bag at 20 degrees or even 15? Yeah, I think so, if you're willing to sleep in a down jacket and long underwear. But this feels more like a summer/early fall kind of bag, not more than that.

I'm a little over 5'10", but I took a chance on the regular size (which is recommended up to 5'10), and I still had plenty of room at the end. I bet it would work for somebody 5'11 or even a little taller...but don't blame me if your toes touch the end. It feels a lot roomier than a mummy bag, and because it's attached to the pad, you can shift around, sleep on your side, and roll over, and it doesn't get all twisted or slide off the pad. I found it much more comfortable - a bit like a bed with a duvet. Overall I really like the bag, and I think it's a good choice for trips where you don't expect things to get too cold. I bet the warmer Blackburn would also be a good choice.
July 8, 2020
Originally posted at
Rated 4 out of 5
Best Night Sleep on the Trail
Absolutely love the Big Agnes sleep system! I've used the Mystic UL 15 with the Q-Core SLX sleeping pad for a season now and I'm still stoked every time I get in this bag. As a side sleeper I find traditional mummy bags uncomfortable with there limited mobility. The Mystic is a bit roomier and with the flex pad sleeve it allows you to shift in the bag without falling off your pad or getting turned around in your bag. The only issue that I've come across is about the temperature rating. I've found this bag to be a bit cold on some shoulder season hikes where the temps were in the mid to low 20's. I believe this is because the bag is a little more spacious, but I'm willing to accept this for the comfort it provides as a side sleeper. Just bundle up with another layer and you'll be fine. Overall, this is an excellent option for a 3 season hiker who wants to get their pack weight down without sacrificing comfort.
April 9, 2020
Originally posted at
Rated 2 out of 5
Very Disappointed! Not a 15^ bag
I was really excited about trying this sleep system. It seemed like a nice balance between going full quilt and traditional bag/pad. Received it last Wednesday and tried it out this weekend. Boy was I disappointed. This is not a 15^ bag. If you need a bag for below 30^, you should get the Blackburn.

First, lets go over the PROS.
I really like the sleep system. I'm a side sleeper who tosses and turns all night. The sleep system did exactly what I was hoping it'd do. It kept me on my pad and the tossing didn't result in letting in a cold draft. This is main reason I didn't get a quilt system. I'm afraid my constant tossing will let in cold drafts.

Now the 2 main cons.
1st, the pillow strap is useless. The hood just isn't big enough to fit your head and a pillow inside. Maybe a puffy jacked or fleece but not an inflatable pillow. I use a Sea to Summit pillow and once it's in there, your head sticks out of the hood. I had better luck with the pillow outside of the hood wedged in between the pad and bag. The sleep system keeps the pillow nicely in place.

THE MAIN CON.
This is not a 15^ bag. The first night got down to 27^ and the second night 29^. I was not comfortable and kept waking up. First night, I had a sleeping bag liner, thermal tights, and tech t-shirt. Ended up putting on a lightweight fleece and neck buff around 4am.

At 15^ above the rated temp, I should have been pretty comfortable. Second night, I wore 2 pairs of wool socks, thermal tights, hiking pants, tech t-shirt, quarter zip long sleeve, lightweight fleece, buff, and sleeping bag liner. My upper body was warm the second night but my legs and feet were still cold. I could feel the chill coming from above.

When I'd toss at night, I'd feel a thin line of cold where the pad meets the bag, but that wasn't a big issue. Not a lot of cold seeped in. It was mostly just feeling the chill from directly above. I would have to be in full down pants and jacket to even try this bag at 15^.
October 16, 2019
Originally posted at