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Therm-a-Rest Parsec 0 Sleeping Bag Review

Therm-a-Rest Parsec 0 Sleeping Bag Review

The Therm-a-Rest Parsec 0 is a VERY lightweight cold-weather sleeping bag suitable for year-round use. It’s a warm bag with premium box baffling and 60% of the insulation positioned over the chest and 40% on the bottom to provide enhanced warmth when used with an appropriate sleeping pad. It has a draft collar and zipper draft tube to hold in your body heat with a two-way zipper to vent the bag if you feel too warm. The Parsec 0 has generous girth measurement to accommodate extra clothing layers and to let you store gear inside, like boot liners or water bottles, which you don’t want to freeze overnight. Special sleeping pad straps help prevent you from rolling off your pad at night, while exterior loops let you layer compatible quilts or blankets on top to boost the Parsec’s temperature rating, so you don’t need to buy a separate sleeping bag for colder conditions.

Specs at a Glance

  • Gender: Unisex
  • Temperature Rating: 0F (-18C)
  • EN Tested Lower Limit: 0F
  • EN Tested Comfort Limit: 14F
  • Insulation: 800 fill power goose down
  • Water-Resistant Down: Yes, Nikwax Hydrophobic (PFC-free)
  • RDS-Certified Down: Yes
  • Construction: Box Baffles
  • Shell and liner: 20 Recycled Nylon
  • Zipper: YKK Snag-Free, 3/4 length
  • Weight: 2 lbs 6 oz / 1077g
  • Down Fill Weight: 26 oz /  738g
  • Length: 72 inches tested (66″, 78″ also available)
  • Girth: Shoulder (63″), Hip (58″)
  • Compression Sack Weight: (included)
  • Storage Bag: Included

Sleeping Bag Features

The Parsec 0 has all of the standard features you’d expect on a premium cold-weather sleeping bag, including a draft collar, two-way YKK snag-resistant side zipper, zipper draft tube, taped zipper stiffeners, spacious dimensions, and a very warm hood. But some of the things that set this bag apart from others include box baffling construction that lets the down loft fully while preventing downshift, a 60/40 distribution of goose down insulation with more on the top and less underneath, and gear loops integrated into the shell that let you attach Therm-a-rest quilts or blankets. While I haven’t used this layering functionality yet, it’s a compelling feature for winter use when camping in minus 10F to minus 20F weather.

Continuous Draft Collar and Zipper Draft Tube

The draft collar is a dow-filled tube that wraps under your upper back or shoulders if you lie on your side, and continues without a break to cover your upper chest. It prevents warm air heated by your lower body from escaping when you move around at night. The collar is a continuous extension of the zipper draft tube running along the side zipper, so there aren’t any gaps between the two where leaks or drafts could occur.

The Parsec 0 hood has a draft collar that runs behind your back and over your chest.
The Parsec 0 hood has a draft collar that runs behind your back and over your chest.

You can’t tension the draft collar to tighten it on this bag, which is a plus because it makes the bag simpler and much more comfortable to use than sleeping bags with more elaborate velcro tensioning systems, which tend to get crufty with age and use. However, it can also be negative if the bag volume is too large to create a draft-free seal. It is easy to compensate for, however by looping a sweater or lightweight insulated jacket around your neck to create a tighter seal.

Two-Way Zipper

The Parsec 0 has a 3/4 length zipper that ends around your knees and has two sliders. Both sides of the zipper are taped with grosgrain to stiffen the zipper and prevent snags. The top zipper also has a special YKK zipper cover designed to prevent snags, and while it’s not foolproof, it’s pretty effective. While snags are annoying, their biggest danger comes from ripping open the shell fabric alongside the zipper and leaking the insulation. I’ve done it before on other bags and it really sucks.

The Parsec 0 has a two-way 3:4 lngth zipper that helps reduce weight without compromising access.

There is a snap at the top of the zipper at chin height that you should close before pulling the zipper up, when you’re lying in the sleeping bag. This helps take the pressure off the zipper while helping it to track more easily. When the snap is closed, you can also vent the chest area by pulling down the top zipper while keeping the hood snugged around your head. As stated above, there is a thick draft tube running alongside the zipper, which slides over the zipper when it is closed and blocks cold air from leaking through the zipper and chilling you.

The bottom zipper slider does not have the same YKK zipper cover since there is much less pressure on the zipper, and the sides stay well aligned. That bottom zipper is good for venting the foot area of the bag and helps extend the bag’s range in warmer weather.

Pad Straps

The pad straps complement the 60/40 top and bottom distribution of the goose down in the Parsec 0 by properly utilizing an appropriately rated sleeping pad. When you sleep in a sleeping bag, the insulation underneath you can’t trap warm air because you’re lying on top of it and preventing it from lofting. Called Synergy Links, these pad straps are removable if you don’t want to use them, but they do help keep you on the pad, especially if you roll around a lot at night.

The Parsec 0 includes pad straps, called Synergy Links, to prevent you from rolling off the pad at night.
The Parsec 0 includes pad straps, called Synergy Links, to prevent you from rolling off the pad at night. Hint: this is the upside-down view.

For many years, Therm-a-Rest has supplied sleeping pad straps or sleeves like these Synergy Links with its cold-weather sleeping bags. They use the same approach with their quilts, including the awesome Vesper 20, Vesper 32, and Vesper 45, which I also highly recommend.

Wide Girth

Dimensionally, the Parsec 0 is cut wide so you can wear extra clothing layers, and store extra gear, like your boots, boot liners, or water inside with you when you sleep to prevent them from freezing. That extra width also lets you sleep on your side or move around at night with ease. If you have to spend 12 hours in a sleeping bag in winter, you’ll appreciate the extra room!

The Parsec 0 has a heavily insulated mummy hood.
The Parsec 0 has a heavily insulated mummy hood.

Mummy Hood

The Parsec 0 has a heavily insulated mummy hood with a fairly conventional shape that favors back sleepers. An external cordlock controls the size of the hood opening and can be cinched down quite small for maximum warmth retention. However, if you’re a side sleeper, the hood will not rotate with your head like the jacket-style hood on the NEMO Sonic 0. This means that the inside of the hood will get wet as you exhale at night, which can get nasty and uncomfortable if you’re out for several nights in a row and don’t have a chance to dry the bag in the sun. It’s perfectly usable for single-night excursions.

External Chest Pocket

There is also an external pocket on the side of the upper chest, which provides a good place to stash a headlamp so you can find it for a nighttime bathroom break. It’s quite large, actually, but it’d be a whole lot more convenient if you could access it from inside the bag without having to unzip the upper half of the sleeping bag to reach it. I prefer putting my headlamp in the pocket of the hoody I wear to bed in cold weather instead. If you’re a side sleeper or roll around at night, I’d caution against putting a smartphone into this pocket unless it’s in a sturdy protective case.

Comparison table: 0-degree sleeping bags

Make / ModelFill PowerWeightDown Fill Weight
Marmot Lithium 08003 lbs 0.9 oz33.9 oz
The North Face Inferno 08002 lbs 14 oz29.3 oz
NEMO Sonic 08003 lbs 4 oz28 oz
Therm-a-Rest Parsec 08002 lbs 6 oz26 oz
Western Mountaineering Kodiak 08502 lbs 12 oz30 oz
Feathered Friends Snowbunting EX 09002 lbs 13 oz25.3 oz
Mountain Hardwear Phantom 08502 lbs 10.630 oz
Thermarest Polar Ranger -208003 lbs 4 oz34 oz
Western Mountaineering Puma MF -258503 lbs 7 oz36 oz
Feathered Friends Snow Goose EX -409004 lbs 2.9 oz42 oz


The Therm-a-Rest Parsec 0 is a cold-weather down-insulated sleeping bag with premium features including a box-baffled construction and body-mapped 60/40 down insulation. In addition to being warm. I like it because at 2 lbs 6 oz, it is one of the lightest-weight 0 degrees sleeping bags available today, which is a real win for cold weather or winter use when your skin-out pack weight is edging close to 40 pounds. With excellent features and good ventilation options, the Parsec 0 is flexible enough for use across a wider temperature range than just winter, ranging into autumn and spring when nights drop close to freezing.

Note: The temperature rating for the Parsec 0 is based on the ISO sleeping bag temperature rating standard, so that 0 degree Fahrenheit temperature rating is good for men and warm sleepers. If you are a cold sleeper or a woman, the temperature range be closer to 10 degrees Fahrenheit. The difference is due to the fact that women aren’t generally as large as men and don’t generate as much body heat.

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Disclosure: Therm-a-Rest donated a sleeping bag for review.

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  1. As a side sleeper who moves a lot during sleep, I don’t get it why the other manufacturers don’t use the pad straps.

  2. Important detail. Stuff size 8×9, pretty darn good!

  3. I reviewed the Parsec 0C 32F for Backpacker magazine. Other than the temperature rating all the loops, straps and doodads seem the same as this one and I found them not worth their weight ( I am a weight-weenie). If I rolled over, my face was on the floor and the mattress on my back like your photo ha ha. If I roll over inside my bag, my face is in the ‘hood’. I do keep my Unightie pillow in there however. I didn’t try the quilt anchors. At present I am using this bag over top of my Zpacks bag. So eventually I cut them all off! There was an downfilled extra footbox in mine but I cut that out too and stuffed the down into the main bag (very hard to do as the down floats everywhere yikes). Although I also sneered at the little pocket with its overkill zipper, I was pretty happy to tuck my gloves liners in there as I’m always ‘losing’ things in my sleeping bag. Great idea to access the pocket from inside and maybe ditch the zipper and just have a bias-taped overlapped opening.
    You and me should design gear!

  4. As a side sleeper, I would not get a bag with the features you have pointed out. Here is why: If I turn on my side within the bag, I now have to breath into the hood. The pad straps work for a back sleeper along with the 60/40 top/bottom loft thickness. But as a side sleeper, I don’t want to breath into the hood, so I need to take the bag with me when I roll on to my side. Now my back is insulated with a layer that is 1/3 thinner (or less warm) than the top layer.

    For me a bag that does not have this 60/40 loft is desirable. I like the rare bag that has a center zip and no side block baffles or has all vertical baffles that are consistently lofted for top and bottom.

    These pad straps and 60/40 loft would work great with a “hoodless” or hood separate sleeping bag. Then I could rotisserie inside the bag.

    My two cents,

    • What Scott says is food for thought. My FF swift20 is not really keeping me warm enough in shoulder seasons when nighttime drops into 30’s especially 20’s. I was thinking the Parsec 0 was the one till now. I was attracted to it’s warmer rating, doable weight, but especially generous dimensions (due to big shoulders). With FF bag I just roll it with me so as keep breathing (and moisture) outside, so now I guess will just keep the Swift and supplement it with my puffy+.

  5. I appreciate this article a lot. I sleep cold and I am looking for a new sleeping bag. Really like the Parsec 0 but ca not seem sot get Thermarest to give me the size of this bag compressed in a compression sack. An what sack I need to purchase.

    Talked to CS but they were no help, their website says 8 x 10″ but I know that is wrong.

    Do you have any idea?


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