Welcome! Ultralight backpacking is my passion, and keeping up on new technologies, gear, and techniques relevant to UL backpacking is what floats my boat. I'm always looking for the lightest, most functional gear to improve a lightweight or ultralight backpacking kit, and report my impressions and field testing results here. For hikers wanting to keep up on the latest and greatest ultralight backpacking gear, this is a good place to hang out. Also, there is a lot of information here (and on our informational website Southwest Ultralight Backpacking) on useful techniques and backcountry etiquette -- food for thought for hikers wanting to lighten their load and their impacts.

My goal for Ultralight Insights is to understand, test, and report on new technologies and gear of interest to lightweight and ultralight backpackers. It's a passion after all, so we just plain enjoy talking about it. I hope readers will add their own wisdom and comments, respond to my questions, ask their own questions, and correct me if I get something wrong. Happy hiking! Will

Tuesday, June 20, 2017

GEAR REVIEW: Marmot Phase 20 Sleeping Bag

By Will Rietveld

There was a time when I recommended: wearing your camp clothing inside a 30F sleeping bag is all the warmth you need for summertime backpacking in the mountains. I was a bit younger then, and I stayed warm using that technique. Now I give the same advice, based on a 20F sleeping bag. I get cold easier as I get older, and I suspect others need extra insulation too, and it probably doesn’t have anything to do with age J.

Whether you need a 30F or a 20F bag, the same question arises: which bag do I recommend that is lightweight and has an honest temperature rating? Marmot has been making ultralight sleeping bags for a long time, and their Phase 20 (and 30) is state-of-the-art. Do I recommend them unconditionally? Not exactly; read on to learn the caveats.

Marmot Phase 20 Sleeping Bag in size Regular. (Manufacturer photo.)

Specifications and Features

Marmot (www.marmot.com)
Model Tested
Phase 20 Regular
Models Available
Men’s Phase 20 Regular and Long, Women’s Phase 20 Regular
850 fill-power down treated with Down Defender for water-resistance, shell and lining are 10-denier Pertex ripstop Nylon (0.88 oz/sq yd)
Measured weight 23.85 oz; manufacturer specification 23.3 oz
Anatomic Wrap-Around Toebox provides more insulation and room, Smooth-Curved Baffles reduce down shifting, Nautilus Multi-Baffle hood, full-length locking 2-way #3 YKK zipper with anti-snag slider, EN tested, internal stash pocket, snagless insulated draft tube, hood drawcord, stuff and stash bags included
Regular $459, Long $479, Women’s $479


The specifications table lists all of the relevant features of the bag; the following photos provide a good look.

Size Regular fits a person up to 6 feet tall; size Long fits to 6 feet 6 inches. I am 5 feet 11 inches tall and 160 pounds, and normally wear a men’s size Large shirt. Size Regular is sufficiently long for me, so that’s not an issue. However, shoulder girth for size Regular is 60 inches and Size Long has 62 inches, and that's where the issue lies. Remember those numbers.

Marmot Phase 20 zipped. The bag is very anatomically shaped. (Manufacturer photo.)

Note that the Phase 20’s left side zipper has a very distinct curve near the head end that extends to the base of the hood.

The zipper has a zipper garage at the end, a good feature to avoid snagging on the sleeper’s hair, facial or otherwise.

There is a zippered stash pocket inside the topside of the bag.

The hood has a single drawcord adjuster on the outside. There is a baffle around the front of the hood to seal in heat.

Field Testing

I tested the Marmot Phase 20 on four backpacking trips in Utah and New Mexico in fall 2016 and spring 2017. This photo was taken in April 2017, sleeping under the stars in a sandy wash in the backcountry of Canyonlands National ParkUtah. The nighttime low was about 30F.

What I like:
  • All materials are state-of-the-art for an ultralight sleeping bag, as good as it gets.
  • The Phase 20 is EN tested, has excellent loft, and is realistically rated. Having said that, note that most bags are not toasty warm when the air temperature is at the bag temperature rating.
  • Most sleepers will like the full-length zipper (it actually ends about 17 inches from the foot end of the bag), which enables the bag to be used as a blanket on warmer nights.
  • The footbox is roomy and warm.

What I don’t like:
  • The bag’s shoulder girth is simply too tight.
  • Because of the bag’s tightness, the zipper is very hard to close, which is exacerbated by a curve to the base of the hood. It is even more difficult to unzip.
  • The bag is not amenable to wearing camp clothing inside the bag to extend its warmth. It’s simply too tight and I can’t get the zipper closed past my shoulder.

The best way to use the Marmot Phase 20 is to wear a single layer of clothing inside the bag. That keeps the bag clean and roomier.

A snug bag is warmer because there is less air space to warm up.

The locking feature of the zipper is also its nemesis – if there is any sideways tension on the zipper (from the bag’s tight fit), the zipper won’t slide. The best way to operate the zipper is to hold the track straight with one hand and zip with the other.

I struggled with the hood to obtain a breathing hole aligned with my mouth.

On warmer nights, my preferred method to use the bag is as a blanket by unzipping it and wrapping it around me.


For a person my size, the Phase 20 in Regular Length is not a good choice. If you are a smaller person, chances are you will get along just fine with the Phase 20. Although I am an “average” sized male, it would appear that I am at the upper limit (or beyond) for the Phase 20 in size Regular. The Long version of this bag would accommodate me much better. In fact, I initially requested the Long, but it was not available at the time. The Long has two more inches of shoulder girth, which should be sufficient.

Speaking from experience, I need 62 inches or more of shoulder girth in a sleeping bag in order to wear my camp clothing inside my bag. That is a weight-efficient technique to extend the warm of a sleeping bag to stay warmer while reducing overall pack weight.

In addition to the bag’s tight shoulder girth, the curved zipper at the head end is another factor to take into account. The zipper can be very hard to close because of the locking feature. Also, because of the lightweight #3 zipper used on the bag, its a recipe for a broken zipper. My advice is to test the bag at an outdoor store to be sure the fit and zipper closure works for you.

Note that the women’s Phase 20 weighs 29 ounces, about 6 ounces more than the men’s Regular, so it obviously contains more down and is warmer. That is the norm nowadays – the women’s version is a bit heavier but warmer. The shoulder girth is 58 inches.

These issues may not affect everyone, and if you are a smaller person than me, the Marmot Phase 20 is a very good choice for an ultralight warm sleeping bag. It’s made of premium lightweight materials, and 23.3 ounces is an excellent weight for an ultralight 20F bag. If you are a bigger person, try out size Long to see how it works for you; I was not able to and wish I did.


  1. I have been searching for one-person and two-person sleeping bags to replace my outdated models. I am certain the marmot phase 20 sleeping bag will be a good addition; thank you for the review. I also found the big Agnes dream island 15 –person sleeping bag here: http://wildernessmastery.com/camping-and-hiking/best-2-person-sleeping-bag.html