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Therm-a-Rest Altair Winter Down Sleeping Bag Review

Therm-a-Rest Altair 0F Sleeping Bag

The Therm-a-Rest Altair 0 degree down sleeping bag is a very comfortable cold-weather sleeping bag that is excellent for side-sleepers or winter backpackers looking to shave more weight off their winter gear list without breaking the bank.

Elastic Synergy Link straps keep the bag on your sleeping pad
Elastic Synergy Link straps keep the bag on your sleeping pad

Often called a top bag in Europe, the Altair 0 places most of its down insulation on the top of your body and to the sides instead of underneath you, relying on an insulated sleeping pad to keep you warm from below. This works thermally because compressed down does not have the ability to loft (puff) and retain warmth if you lie on it. It also saves considerable weight and bulk when compared to other top-rated 0 degree sleeping bags, and at 2 pounds 7 ounces, the Altair 0 weighs 8 ounces less than other comparable, top-rated down sleeping bags from Montbell or Western Mountaineering.

Unlike regular sleeping bags, the Altair comes with two elastic straps that are attached to the underside of the bag called Synergy Links. These hold the bag onto the pad and prevent cold air from flowing between the bag and the pad where the Altair’s down insulation is the thinnest. At the same time, the bag itself provides you with plenty of interior room, particularly around the shoulders and hips, so you call roll onto your side without compressing the shell against the bag’s baffles and losing loft and insulating power.

The coldest temperature bag in the Therm-a-Rest sleeping bag product line, the Altair works best with other Therm-a-Rest cold weather sleeping pads like the NeoAir XTherm or the NeoAir All Season which are narrow enough to easily slide through the Synergy Link straps attached to the back of the Altair 0 and have a high enough R-value to insulate you from cold ground in zero degree weather. It’s best to do this after you’ve inflated your sleeping pad and to align the hood and the bag’s foot box so that it covers the top and bottom of the sleeping pad. I only own NeoAir insulated pads, so I have been unable to test the Synergy Link straps with sleeping pads from other vendors, but I suspect that any 21″ wide pad will work just fine.

Puffy Draft Collar
Puffy 3″ Draft Collar

Must-Have Features

There are a few must-have features that I look for when evaluating sleeping bags for my personal use and the Therm-a-Rest Altair 0 has all of them:

  • Wide shoulder girth so I can easily move around inside the bag
  • Adjustable draft collar to prevent hot air from escaping when I move at night
  • Reliable, non-snagging zipper
  • Down-filled draft tube behind zipper to prevent cold drafts
  • Flatter-style hood that does not cover my mouth if I lie on my side
  • Water-resistant shell fabric

In addition to a roomy fit, the Altair has a big 3″ deep draft collar that wraps over the top of your back and shoulders, preventing warm air from escaping like a bellows when you move around at night. An adjustable cord pull on the right-hand side of the bag, below the hood, lets you easily cinch the collar to retain warmth or to loosen it to vent excess heat.

Extra Beefy Zipper Draft Tube
Extra Beefy Zipper Draft Tube

The Altair’s side zipper runs along 3/4 of the bag and is a two-way YKK zip so you can pull it up to vent heat around your legs in addition to pulling it down from shoulder height to vent heat from the chest and torso areas. The fabric running along the inside of the zipper is triple stitched to prevent the liner from catching in the zipper teeth and works marvelously without requiring the need for a heavier zipper guard. There’s also a 4″ wide down draft-tube running the length of the zipper to prevent drafts from entering the bag and to prevent warm air heated by your body from escaping.

Flat Hood
Flat Hood

The Altair has a very flat hood, shaped like a shallow crescent instead of a highly articulated hood that sticks out perpendicular to the bag. This makes it easy to fold it under the underneath the bag when it’s not needed and to use the Altair more like a quilt. The inside of the hood has plenty of down baffles inside it and still provides lots of warmth if you are a back sleeper.

However, the flatter hood also make it possible for a side sleeper to exhale at night without drenching the side of the hood with moisture from their breath (very irritating below freezing) because there’s enough space above the draft collar and below the top of the hood when pulled down over the forehead, for a side sleeper to exhale moist breaths. I can’t completely understand why this design hasn’t been used before on other sleeping bags, although satisfying the needs of side sleepers is a relatively new market direction for many companies. Thank you Therm-a-Rest for leading the charge.


  • Large draft collar prevents hot air from escaping when you move around at night
  • Highly compressible and 1/2 pound lighter than comparable down mummy bags
  • Reliable non-snagging zipper with a large down-filled backing tube to prevent drafts
  • Flat hood is easy to fold out-of-the-way when you don’t want it, but also contains plenty of insulation when needed.


  • While 750 fill power goose down is less expensive, the Altair 0 would be even lighter weight if it used 800 or 850 fill power down
  • Only available with left side zipper at this time
  • The comfortable temperature range is probably closer to 0 to 10 degrees rather than the US rating of 0 degrees Fahrenheit.

Comparable 0 Degree Down Sleeping Bags

Make / ModelWeightFill PowerDraft Collar
Big Agnes Orno 02 lbs 7 oz850Yes
Feathered Friends Ibis EX3 lbs 1 oz900Yes
Feathered Friends Murre EX Women's2 lbs 13 oz900Yes
Feathered Friends Snowbunting EX 02 lbs 13 oz900Yes
Marmot Lithium2 lbs 9.5 oz800Yes
Marmot Never Summer 03 lbs 3 oz650Yes
Marmot Ouray 0 Womens3 lbs 10 oz650Yes
NEMO Sonic 02 lbs 11 oz850Yes
REI Downtime 03 lbs 14 oz650Yes
Sierra Designs UL Nitro 02 lbs 8 oz800Yes
Sierra Designs UL Nitro 0 Women's2 lbs 7 oz800Yes
The North Face Inferno 02 lbs 14 oz800Yes
Western Mountaineering Kodiak MF 02 lbs 12 oz850Yes


My favorite part of cold weather camping is sleeping comfortably on long cold nights. It’s the only time of year when I can stay in bed for 10-12 hours at a stretch, so I’m very picky about the sleep system components I choose to bring on backpacking trips. But I am jazzed about the Altair 0 sleeping bag and really look forward to using it on more trips in the coming months before the deep freeze hits New England. I think Therm-a-Rest has a winner with the Altair’s and hope they continue innovating around this design with an even colder weather version of this sleeping bag in the coming year.

Manufacturer Specifications

  • Weight: 2 lbs, 7 ounces / 1120 g
  • Fits: 6 ft / 183 cm
  • Girth Shoulder: 62 in / 158 cm
  • Girth Hip: 58 in / 147 cm
  • Girth Foot Box: 40 in / 102 cm
  • Width: 30 in / 76 cm
  • Length: 75 in / 190 cm
  • Packed Dimensions: 8 x 18 in / 15 Liters
  • EN Comfort: -5 C / 24 F
  • EN Limit: -11 C / 12 F
  • EN Extreme: -31 / -23 F
  • Fill: 750+ Fill Power Goose Down
  • Fill Weight: 1 lb 4 oz / 580 g
  • Liner Fabric: 30D Nylon
  • Shell Fabric: 3D Nylon Ripstop with DWR
  • Zipper: Left

Disclaimer: Therm-a-Rest provided Philip Werner ( with Sleeping Bag for this 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.


  1. The top bag looks good. But, you really do not save that much over a full sleeping bag. The bottom is likely a single sheet of some heavier fabric, not .8oz Pertex or Microfiber. And the heavier elastic straps make up most of the weight difference for two full shells. The down actually takes up about 40-70% of the weight. For example, the WM Mitylite has 12oz of fill with a total weight of 26oz, or around 46% in fill. The WM Kodiak has 68%. The Altair is around 51%. Most of the difference is in the entire bottom fill. So the weight is good.

    I would suggest that the bag needs some more development. Sort-of like a quilt, or a bag, the top bag concept seems to reduce drafts associated with quilts (like a full bag) but doesn’t offer any lofting for the bottom. Yes, yes…I understand you lay on it, but the parts you do NOT lay on will be much warmer, even with the NeoAir under you.

    There are several concepts that are ignored:
    ** Loft Pockets That is, those areas that are allowed to loft under you while being supported by other areas. The Klymit Inertia does this easily, they work. But the NeoAir does this to some minor degree along the horizontal baffeling.
    ** Lofting is not the only way the insulation keeps you warm. Down conducts heat less than any other material used for insulating bags when compressed. It is not completely lost as an insulator.under you.
    ** Body area, contacting/compressing on the pad, is much less than the bags bottom area. Lofting around your boddy would keep you warmer, though heat rises without baffeling.

    A full bag, (using smaller, more distributed, loft pockets in the pad than Klymit uses) would do better in a larger size to allow you to put the pad inside.

    I would suggest that, perhaps, less fill is needed on the bottom, ie, maybe 1.5″ of down as opposed to 3″, but still be able to do the job, adequately when coupled with a pad. Relying on one or the other does not seem to work as well as using both, together. Most quilt users, quickly abandon the quilts, or tuck them under, rather than risk 0F weather with only the pad.

    • Compare the weight of the Altair zero with zero degree down bags from Montbell and Western Mountaineering:You’ll see that the Altair is still 8 ounces lighter despite the use of a thicker shell and lower fill power down – which makes the comparison even more impressive in my mind.

      While there is still room to further improve the Altair, I’m not certain that Therm-a-Rest wants to. Why add expense when the bag is already suprerior in several ways, and gives them access to a much larger pool of would-be customers. I think they’ve nailed this one,
      and targeting side sleepers is also simply brilliant!

      Re: Klymit. I think the entire loft pocket discussion lacks any basis in fact. Sounds like marketing drivel to me. In addition, If you have a sleeping bag with tight shoulder and torso grith, you will have shell-to-loft compression. No, I like the idea of keeping ones sleeping pad outside the sleeping pad especially in a colder weather bag where I am likely to stick extra clothes, water bottles, etc. at night.

      Just my 2 cents.

      • Well, loft pockets DO work. I tried the Inertia and found that they work quite well adding some warmth. As does the pad IN a bag. My big complaint was there were too many bloody holes. I poked my foot/leg through it twice.The pad in the bag works, too. It adds a bit of warmth (though I think this is more due to bleeding heat out the side being minimized.)

        But I do agree that larger winter bags are wanted. Clothing, hot water bottles and an interior pad all make sleeping in a WM bag confining. I went with the old Marmot Super Stretch Long that has additional girth and space in the bottom for clothing, water, etc. Seemed to work well. Like quilts, these top bags don’t seem to cover all the bases in colder weather for me.

  2. I must admit i was really interested here with your review. I liked everything. I am pleased that they have come up with a simple solution to keeping you on your mat. (As well as keeping the bottom of the bag at the bottom of course). I had a figure in my mind of 750grams, so it was a bit of a shock when i got to the end to read its over a kilo. Not a ridiculous weight but would i be persuaded to buy it over others such as PHD or WM. I doubt it. Shame.

    • I’m replacing a western mountaineering Ultralight 20 with the Altair 0 because I can’t stand the WM hood system which forces me to lie on my back. I hate it, I hate it, I hate it. I use an Therm-a-Rest Alpine blanket, for the warmer months which is essentially a quilt system, coupled with a NeoAir Xlite sleeping pad.

      I am likely to be selling 1 or possible 2 western mountaineering bags in the future. Their insistence on not EN testing the warmth of their sleeping bags is just another nail in the coffin for me.

  3. Not sure how that Racoon sneaked into our tent!

  4. So, as one with zero experience in sub-zero weather, what do you do when you know that the temps. will be far below the bags rating? How low can you reasonably go and how do you do it?

  5. Stay home and go bowling or hike out. People have all kinds of schemes to push the temp limits of their bags, but if you’re off by more than 10-15 degrees you might as well go home or build an igloo/snow cave if you have snow.

    To elaborate:

    Put nalgene bottles with boiling water in them in your sleeping bag
    Open chemical heat packs in your bag
    Wear all of your clothing
    Stuff all of your extra clothing into yor bag with you
    Stuff your sleeping bag with dry leaves and pine needles
    Sleep in front of a roaring fire, deflected by a back tarp or rock pile
    Sleep in a vapor barrier bag (probably the most effective)

  6. I think the reason you don’t see this “flat” style hood in other bags is that when a side sleeper rotates (back to side, one side to the other) in a conventional mummy, you turn the bag with you. Of course that wouldn’t work on a “top loft” bag since you would then be exposing an area of the bag with minimal loft.
    It has just become second nature for me to extend my elbows when I turn so that the bag turns with me. I’ve been sleeping in mummy sleeping bags since my first down bag purchased at REI in 1964 (cotton “balloon cloth” shell with 1/2 top zipper!) and I personally love the modern articulated, or pieced hoods. I never have an issue of breathing moist exhaled air into my bag because the hood closes so neatly around my face. I guess that is why I will never be comfortable with the Big Agnes or Thermarest bags that have little or no insulation on the bottom.

  7. Exactly how long is the zipper?

  8. Philip, I take back my comment about the weight. Apologies, I was thinking 0C. Not 0F.

  9. Hi Philip,

    Will you be keeping your WM Puma?

    • I plan to sell it as soon as I can figure out what to replace it with but I’m also not in a rush to do so. It’s warm warm bag, but I’m not entirely happy with it because it’s harder to side sleep in.

  10. This reminded me of my friend Rodney; he use to sleep exclusively in sleeping bags LOL!…its uncanny how much that sleeping bag looks like the one he use to have! :)

  11. Hi Philip!
    I’m a side sleeper looking for a winter mummy sleeping bag and a three season mummy bag, with insulation all-around (rather than one withou insulation on the bottom that attaches to a pad). My biggest problem with hooded mummy bags isn’t so much not being able to turn over in my sleep, but rather the hood. When I’m lying on my side, I’m breathing right into the side of the hood, making me feel like I’m suffocating or causing condensation problems. I’ve tried turning the bag with me when I lie on my sides, but I find it much too difficult. I much prefer to turn in the bag rather than with the bag. Do you have recommendations for your current favorite winter and 3 season mummy bags that have flat hoods that allow you to breathe while on your side? The other possibility I’m considering is a hoodless mummy bag with an adjustable down collar and pairing that with a down balaclava. If you’ve experimented with such a set-up, do you find it just as warm as having a hooded mummy bag? I was thinking of the Feathered Friends Flicker 30 or 20 with such a set-up as a 3 season solution, but I haven’t been able to find a similar winter bag (hoodless mummy bag with adjustable down collar rated to -20 or 0 degree F). Do you know of any such bag?
    Thanks so much as always for your help!

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