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.
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.
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.
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.
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 / Model||Weight||Fill Power||Draft Collar||Price|
|Big Agnes Blackburn UL 0||2 lbs 12 oz||850||Yes||$500|
|Big Agnes Orno 0||2 lbs 7 oz||850||Yes||$560|
|Feathered Friends Ibis EX||3 lbs 1 oz||900||Yes||$635|
|Feathered Friends Murre EX Women's||2 lbs 13 oz||900||Yes||$585|
|Feathered Friends Snowbunting EX 0||2 lbs 13 oz||900||Yes||$599|
|Marmot Lithium||2 lbs 9.5 oz||800||Yes||$499|
|Marmot Never Summer 0||3 lbs 3 oz||650||Yes||$279|
|Marmot Ouray 0 Womens||3 lbs 10 oz||650||Yes||$309|
|NEMO Sonic 0||2 lbs 11 oz||850||Yes||$479|
|REI Downtime 0||3 lbs 14 oz||650||Yes||$249|
|Sierra Designs UL Nitro 0||2 lbs 8 oz||800||Yes||$379|
|Sierra Designs UL Nitro 0 Women's||2 lbs 7 oz||800||Yes||$379|
|The North Face Inferno 0||2 lbs 14 oz||800||Yes||$519|
|Western Mountaineering Kodiak MF 0||2 lbs 12 oz||850||Yes||$675|
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.
- 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 (SectionHiker.com) with Sleeping Bag for this review.Editor's note: Help support this site by making your next gear purchase through one of the affiliate links above. Click a link, buy what you need, and the seller will contribute a portion of the purchase price to support SectionHiker's unsponsored and independent gear reviews, beginner FAQs, and free hiking guides.
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