Review: Big Agnes Mystic UL 15 Sleeping Bag

The Big Agnes Mystic UL 15 is a full-featured ultra-light sleeping bag for your camping adventures.

On a recent backpacking trip to Fish Lake in the Uinta Mountains of Utah (the Uintas are the only East-West mountain range in the USA). I was able to test Big Agnes’ Mystic UL 15° sleeping bag. You may ask why use a 15° bag in the summer? Well, at 10,500 feet and with a late summer cold snap, a warmer bag was needed to sleep comfortably. I don’t like to shiver while sleeping! Who does?


The Big Agnes Mystic UL 15° bag has 850 fill-power RDS DownTek down. What does all this mean, and why should you care? Fill-power is the loft of the down. So, 700 fill-power will require more down to achieve the same insulation value, and 850 fill-power down requires less to make a 15° rating. With less down it compresses into a smaller stuff sack and takes up less room in your backpack compared to a 700 fill-power down or synthetic sleeping. That way, you can downsize your pack and carry a lighter load on your next adventure. I’m able to compress the Mystic sleeping bag into an 8L stuff sack easily, so it doesn’t take up a lot of room.


What is DownTek? It’s a coating that is applied to the down to make it water-resistant. The benefit of this is if you are sleeping in a single-wall tent and get condensation on your sleeping bag, it will help maintain its thermal properties and will dry quicker while retaining its loft. That’s the downside of down compared to synthetic insulation. When down gets wet, it loses its warmth, but the weight-to-warmth ratio is unbeatable, which is why so many backpackers choose down.

What is RDS down and why should you care. RDS means that the down used in the Big Agnes Mystic 15° sleeping bag is Responsible Down Standards certified. That means that the down is sourced from ethical practices and is not live-plucked from geese. 


One of the excellent features of the sleeping bag is what Big Agnes calls their Flex Pad Sleeve design. The Flex Pad Sleeve design enables the sleeping bag to fit around your sleeping pad so that while you roll around while sleeping, the sleeping bag stays centered on your pad. The other benefit of this design is that they can eliminate the down beneath you while sleeping. When down is compressed underneath you it loses its thermal properties so if you pair the Mystic with an insulated sleeping pad; you are all set!

Another nice feature is the cell phone pocket inside the sleeping bag. I didn’t think this was a necessary feature, but my phone battery stayed nice and charged during the cold temps at night thanks to it being warm inside my bag

The last thoughtful addition to the Big Agnes Mystic 15° sleeping was the pillow pocket which keeps your camp pillow in its place as you roll around while sleeping. 

If you are a weight-weenie gram counter, this may not be the sleeping bag for you due to the additional features like the Flex Pad Sleeve and full-length zipper. But, if you want a fully-featured ultra-light sleeping bag, the Big Agnes Mystic 15° sleeping is just the ticket.

We want to acknowledge and thank the past, present, and future generations of all Native Nations and Indigenous Peoples whose ancestral lands we travel, explore, and play on. Always practice Leave No Trace ethics on your adventures and follow local regulations. Please explore responsibly!

Chris EngelsmanAdmin

Clear eyes, full heart, can't lose!