The Therm-a-Rest Altair HD 10 sleeping bag is built to handle cold nights in the winter without weighing a lot. They engineered the Altair with several weight saving features. But, they don’t take away from keeping the bag comfortable. This is not a super slim sleeping bag, nor do they cut any corners to make it lightweight. The Altair is a full functioning winter sleeping bag meant for backcountry ski and splitboard touring. How did they do it?

Start with the down – 750 hydrophobic (water resistant) down treated with Nikwax to keep the moisture out. The down is box baffled and zoned in the areas where you need it most. Meaning, most of the down is on top and sides. Only 15% of the down is on the bottom of the bag. Thermarest figures that the down under you will be compressed and you will be relying on the warmth of your sleeping pad anyway. So they keep the majority of the down where it will not get compressed and left do its job of keeping the occupant warm.

Next, Therm-a-Rest adds a reflective “ThermaCapture” liner to the bag to keep heat in. Think of it as a softer version of a space blanket. This liner is built in and does a good job of keeping the heat in and radiating in the sleeping bag. I liken it to the way they build their Neo Air line up of sleeping pads. I have used those pads in the past and found that the reflective technology does work. So, now they have a similar idea within the sleeping bag. Genius!

The “SynergyLink” connectors on the bottom of the sleeping bag are designed to accommodate a sleeping pad and keep the entire system working together. The idea is that the user would slide their sleeping pad into the two sections and this will keep the sleeping bag and sleeping pad together. I was able to get the two elasticized straps around 25” wide sleeping pads.

Form and function

The Altair HD 10 sleeping bag is not slim and restricting like many other lightweight sleeping bags. I was able to do a tree pose (figure 4) in the bag. I do not like tight mummy bags, so this was a welcome relief. One downside of having more space is more air to keep warm. I usually sleep in my clothes and wear a puffy to bed, so this was ideal to have the extra space. At first, I was confused why the inner fabric of the sleeping bag felt like it was thicker fabric (it is actually 20D polyester) and realized that this was actually the ThermaCapture liner at work. The outer fabric is 20D polyester as well.

Therm-a-Rest Altair sleeping bags only come in a left zip, sorry all you right handed zipper lovers! Actually, I am a right-hand zipper lover and had no prob switching over to left… Adapt people! The bag I tested was the long version as I am 6’1”. I am glad they sent the long as it was a perfect fit for me. If you are on the fence, size up. The weight of the bag was a respectable 2lbs 10oz. Not too bad for a winter sleeping bag.

Temperature ratings are very nebulous and hard to rely on. This winter sleeping bag would be in the 5-20 degree range in my book, though I am sure others would disagree. I tend to use a bag rated at least 10 degrees lower than the temps we will have at camp. The coldest it got was in the low 20s during our testing, so take that with a grain of salt. Your mileage may vary. Therm-a-Rest is very honest with their ratings and calls this a 10-degree bag. I would concur.

The comfort of the bag is excellent. The zippers were smooth. The sipper draft collar was sufficient. The neck draft collar was a bit smaller than some other winter bags we have tested, but it worked fine once you adjusted the internal pull string. There is even an external zippered pocket on the right side.

The Altair HD 10 degree sleeping bag packs up well and would make an excellent winter or shoulder season sleeping bag for camping, backcountry ski, snowshoe, or splitboard trips. With water-resistant down, integrated reflective liner and straps to hold onto your sleeping pad – this is a great sleeping bag. MSRP $639 for long (as tested).  $589 for regular.


Regular Long
Color Midnight Midnight
Weight (Standard) 2 lbs 7 oz 2 lbs 10 oz
Weight (Metric) 1.12 kg 1.2 kg
Length (Standard) 72 in 78 in
Length (Metric) 183 cm 198 cm
Fits 6 ft / 183 cm 6 ft 6 in / 198 cm
Girth, Shoulder (Standard) 62 in 64 in
Girth, Shoulder (Metric) 158 cm 163 cm
Girth, Hip (Standard) 58 in 60 in
Girth, Hip (Metric) 147 cm 152 cm
Girth, Foot box (Standard) 40 in 40 in
Girth, Foot box (Metric) 102 cm 102 cm
Packed dimension (Standard) 8 x 18 8 x 18
Packed dimension (Metric) 20 x 46 20 x 46
EN Comfort (F) 23 F 23 F
EN Comfort (C) -5 C -5 C
EN Limit (F) 10 F 10 F
EN Limit (C) -12 C -12 C
EN Extreme (F) -25 F -25 F
EN Extreme (C) -32 C -32 C
Fill 750Fill Nikwax Hydrophobic Down 750Fill Nikwax Hydrophobic Down
Fill weight (Metric) 0.58 kg 0.64 kg
Shell fabric 20d Polyester 20d Polyester
Liner fabric 20d Polyester 20d Polyester
Zipper side Left Left
Country of Origin Made in China Made in China