The Therm-a-Rest Parsec 0 is a VERY lightweight cold-weather sleeping bag suitable for year-round use. It’s a very warm bag with premium box baffling and 60% of the insulation positioned over the chest and 40% on the bottom in order 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 so you can 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 with 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 very 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 actually 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.
You can’t tension the draft collar to tighten it on this bag, which is a plus because it makes the bag simpler and a lot more comfortable to use compared to sleeping bags that have more elaborate velcro tensioning systems, which tend to get crufty with age and use. But it can also be a 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.
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 has also 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.
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.
The pad straps complement the 60/40 top and bottom distribution of the goose down in the Parsec 0 by ensuring the proper utilization of an appropriately rated sleeping pad. When you sleep in a sleeping bag, the sleeping bag 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.
Therm-a-Rest has been supplying sleeping pad straps or sleeves like these Synergy Links with their cold-weather sleeping bags for many years. They use the same approach with their quilts, including the awesome Vesper 20, Vesper 32, and Vesper 45, which I also highly recommend.
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 with a fairly conventional shape that favors back sleepers. There is an external cordlock that 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 out 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 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 / Model
|Down Fill Weight
|Marmot Lithium 0
|3 lbs 0.9 oz
|The North Face Inferno 0
|2 lbs 14 oz
|NEMO Sonic 0
|3 lbs 4 oz
|Therm-a-Rest Parsec 0
|2 lbs 6 oz
|Western Mountaineering Kodiak 0
|2 lbs 12 oz
|Feathered Friends Snowbunting EX 0
|2 lbs 13 oz
|Mountain Hardwear Phantom 0
|2 lbs 10.6
|Thermarest Polar Ranger -20
|3 lbs 4 oz
|Western Mountaineering Puma MF -25
|3 lbs 7 oz
|Feathered Friends Snow Goose EX -40
|4 lbs 2.9 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.
Disclosure: Therm-a-Rest donated a sleeping bag for review.SectionHiker is reader-supported. We only make money if you purchase a product through our affiliate links. Help us continue to test and write unsponsored and independent gear reviews, beginner FAQs, and free hiking guides.