You’re looking for a sleeping bag that keeps you warm on the coldest of winter camp nights? Could that be TETON Sport’s Altos Down 0°F?

Fi takes one through its paces on a December bivvy camp.

My current down sleeping bag ‘Big Orange’ has been with me for nearly 15 years now. It’s still in pretty good shape but could do with a professional clean and ‘service’ and doesn’t keep me quite as warm as it used to.

Which is why I suggested to the team at Teton, that I test and review their warmest down sleeping bag, the Altos Down 0°F.

I didn’t pay for the Altos bag but am under no pressure to write a positive review.

That said, it’s going to be hard to fault this competitively-price winter sleeping bag. Despite a sloping sleep platform, near-freezing temperatures and an unhelpful bladder, I was toasty warm in the Altos all night.

And I’m usually a cold camper.

On unpacking this sleeping bag, my first thought was, ‘That’s going to be too bulky to carry’ but it comes in a sturdy compression sack, which squeezes it down roughly to football size This is bigger than some winter-weight down sleeping bags but would still fit in my rucksack.

I was hoping the bag would make up for its size when I got it out of the bag, and I wasn’t disappointed. It’s made from lovely soft fabric and the down started to loft almost immediately. I laid it next to my existing sleeping bag to compare the sizes and would definitely agree the Altos is ‘oversized’.

I wasn’t too keen on the black outer fabric colour but thought the bright red inside was gorgeous. Everything felt super soft.

4/5 – no lofting bag

PROS

Oversized for plus-size warmth

Fantastic lofting

Super warm all night

A proper compression sack

RDS down certification

CONS

No lofting bag

Hood too small for my pillow

Temperatures didn’t quite get down to freezing on my winter camp but they came pretty close, and I was super warm all night. So warm that at one point, I had to remove layers of clothing, including my socks. Before heading to camp, I tried the bag at home and the hood was so cosy it felt like I had a radiator on my head.

This Altos sleeping bag is rated to 0°F or minus 18 °C. Whilst most sleeping bag manufacturers give separate comfort and extreme warmth ratings, Teton only give the extreme level, which is fine as long as you remember that at -18°C the bag should keep you warm enough to be safe but not necessarily comfortable. From Teton’s suggested calculations, I would suggest this bag has a comfort rating of minus 7 °C.

It’s not just the fabric and warmth that make a sleeping bag comfortable, it’s also how it interacts with you and your other sleeping equipment. And for me, the extra size in the body and footbox areas made changing position much easier.

One thing to note is that the Altos range only comes with left-hand zips. This isn’t necessarily a problem but if you tend to sleep on your right side, you might have issues with doing the zip up.

My next comment would be on the size of the hood. It’s been designed to offer a warm, snug fit and this really does work but if, like me, you’re used to fixing a camping pillow in place in your sleeping bag hood, you might require a rethink (or a smaller pillow).

Otherwise I can’t fault the comfort levels of this silky soft sleeping bag. The filled zip and shoulder and neck baffles are effective and its extra space (ten whole centimetres more than my other down sleeping bag around the middle) really does help with both comfort and warmth. At 5’8″ and a size 16, I was able to move around inside the bag without squashing the down and causing cold patches at the sides, something that has been a problem for me in the past.

On cold camp mornings you want a sleeping bag that’s going to pack away quickly and fit into your rucksack without a struggle. There’s always a balance however between pack size, weight and warmth. I would always prioritise warmth and, if necessary leave something else at home. Lost sleep is lost energy and on winter expeditions, energy is a vital commodity.

The Altos sleeping bag was easy to stuff back into its compression sack. There are always going to be smaller and lighter bags available for the same warmth but they come with a high price premium and I felt this sleeping bag struck a good balance.

Some experienced thought has obviously gone into the design of this sleeping bag. The two-way zip allows for footbox ventilation, and the inside pocket is sensibly positioned and big enough for a mobile phone (keeping a phone warm can preserve battery life). The bag has hanging loops to help you air it after use, but it doesn’t come with a lofting bag. I’m going to have to improvise because I don’t want to lose any of this lovely lofting by keeping it compressed.

The zip isn’t a YKK one but slides really easily and doesn’t snag. I did find this easy slide meant that the bag sometimes undid on its own when I was wriggling around, but found this far preferable to a snaggy zip, which can be a real nuisance in the dark.

In the UK this sleeping bag is currently (December 2025) retailing at around £250 (Amazon). Compared to other, similarly rated bags, this would appear to be an average price. Alpkit’s Skyhigh 700 would set you back £229 and the Mountain Equipment Helium 600 £370.

Sustainability and longevity go hand in hand and I haven’t yet given this sleeping bag a long term test. That said, TETON Sports do offer a lifetime warranty.

This sleeping bag is manufactured in China using RDS animal welfare certified down.