Daniel Neilson braves the cold to test some of the warmest sleeping bags on the market. On test Therm-A-Rest sleeping bag

Therm-A-Rest should be applauded for coming up with a truly interesting solution to cut weight, and for listening to their customers. This is the second iteration of the Altair bag. Like others in the collection, it is notable for having straps on the back that fasten around a sleeping mat (any sleeping mat!). This means you won’t fall off as easily, but it also means the bag doesn’t need the same insulation all the way around. The part on the mat has a less down in it – the argument being that there’s insulation anyway, plus, when you’re sleeping on it, the down loses its insulating properties. I’ve not slept in it in -18°C, but at five below it was great. The original bags had no down at all at the back, now, after comments from users, it has a lighter amount there. The down can then be concentrated around the 3/4 around the top, sides and just under the back. Also new in the 2015 bag (available January) is hydrophobic down which I’ve found to be a great innovation in down products. It’s the only bag submitted that uses it. It’s on the snugger side of bags tested here, and also comes in a long version. It’s also worth noting that the comfort range is to -5°C so it’s on the border between three and four season.