Rab’s lightweight Neutrino 400 sleeping bag has been our faithful sleeping companion for around six months now – it’s done it all from lightpacking through to slumming it on mates’ floors and crashing out at race events and it’s never put a foot wrong.

Neutrino is Rab’s weight no object, lightweight class and designed to do anything from full-on lightweight mountaineering to bike touring, anything where weight, pack size and performance matter.

We chose the 400 to test because with a comfort rating of 2°C, a limit of comfort of -4°C and an extreme rating of -20°C – that’s the lowest temperature you’re likely to survive at rather than enjoy – we figured it should cope with all but the very worst UK conditions.

So did it work out that way?

Technical Lowdown

Rab’s sleeping bags got a thorough revamp this season. Rab worked with Leeds University to calculate the ‘golden ratio’ of down to baffle volume for maximum performance. The idea is that each baffle chamber gets exactly the optimum amount of down it needs for maximum warmth, so you get an ideal balance between warmth and weight.

Then there’s a differential cut between inner and outer shell to optimise lofting – the inner is smaller – the baffles themselves are cut with slanted walls, ‘trapezoidal’ Rab calls them, which help to hold down in place and reduce migration and the walls themselves are made from a very fine mesh  for the same reason.

The down itself is high spec, European goose down with a fill power of US 800, meaning it gives a lot of volume and insulation for its weight and the fabrics are lightweight Pertex Quantum.

It’s a proper science and you can read more about it at rab.uk.com/technology/sleeping-bags_1/down.html but all you really need to know is that an awful lof of research and expertise has gone into making Rab’s latest bags work better.

Performance

What summed up the Neutrino 400 for us was that several friends who borrowed it came back with the same comment: ‘It’s warmer than you think it’s going to be’. And it is. It packs down small, feels light, but once you’re inside it and it’s lofting up, it’s deceptively cosy in that reassuring, cossetting way that makes a good sleeping bag something close to a friend…

We happily used it around freezing level and we reckon, depending on how warm you sleep and with judicious use of additional clothing, it should cope with all but real freak UK winter conditions. It’s certainly well up for three-season stuff.

As you’d expect from Rab, it’s a mostly well thought out piece of kit. The slanted, box-section toe gives plenty of foot room, there two-way zip with anti-snag strip is a YKK and works well, allowing foot venting if it’s warm, though you can snag the fabric if you’re clumsy.

Effective baffles at neck and zip and a well-shaped, down-filled hood – the down stays in place too – make the most of the insulation and there’s a zipped pocket for valuables on the inside of the top section. A couple of people commented on the chunky cord-grips, which can be uncomfortable if you roll onto them at night – neoprene ring-type cord-adjusters would be one solution, but it’s not a major issue.

One thing to be aware of is that the fine Pertex Quantum fabric isn’t very water resistant, so you need to be careful both in use and when carrying the bag. It also means you need to take particular care bivvying in UK conditions.

Fortunately Rab supplies an excellent taped waterproof stuff-sac which incorporates both a drawcord initial closure and a dry-bag fold-over style final seal. It’s a nice touch, though it’s not the lightest stuff sac out there. If you really care, you could go for one of Pod’s lightweight compression stuff sacs to save grammes.

Verdict

You can blather on about feature and techical calculations till the geese migate back to Siberia, but the real question is does it keep you consistently warm and comfortable in the conditions it’s intended to and the answer to that is a resounding yes.

It lofts well, the design makes the most of that loft and it packs down neatly and is light for the performance. You could go lighter still with the 850+ down used for Rab’s Expedition bags and even lighter Pertex Quantum GL, but as a lightweight, packable, ‘most of the year’, all-round sleeping bag, the Neutrino 400 is pretty much bang on.

Pros

Warm, light, compact, top notch materials and down, redesigned for optimum lofting and it shows. Waterproof stuff sac is a real bonus.

Cons

No DWR finish, chunky cord adjusters can be uncomfortable during the night.