Whether you're on a snowy summit or a frigid belay, a warm synthetic insulated jacket is a winter mountain essential. With a great active cut, climbing-friendly features, and cutting edge fill that keeps you toasty without tipping the scales too far, the Generator Alpine is ideal for UK winter use. While it's not equivalent to the very warmest synthetic belay jackets on the market, its performance is well judged for versatility, making this a layer built for movement, not just standing around in. It may not be cheap, but you're getting a lot for your money, and for the average weekend warrior it probably wouldn't make sense to spend more.

Weight and packability

On my kitchen scales the Generator Alpine in size Large weighs 585g, plus 13g for the stuff sack that comes with it. Rab say 540g size M. Considering this is really quite a warm, thick, and protective insulated jacket, I'd say that's surprisingly light. In fact, when it comes to the fabric, I wonder if it might even be slightly too light - see below. For ease of transport it can be readily squished down into its stuff sack, giving you a bundle it wouldn't be out of the question to hang off your harness for use on the belay if climbing sans pack.

Fit

The Generator Alpine comes in a good range of sizes, from XS to XXL, so although there's no specific women's version, smaller female climbers may find it worth experimenting with one of the smaller men's jackets. That said, it's quite roomy for its size, and in my standard Large I've certainly encountered closer-fitting insulated jackets. Nevertheless it's not like wearing a tent, and still has a trim enough fit in the body for active use.

As you'd expect from a belay jacket, it's sized to fit easily over several other layers without feeling restrictive. The hem sits well below the waist, which keeps the ming from your midriff, and drops lower at the rear to give total bum coverage; I find this generous length makes a big difference to warmth, and it's a solid mark in the Generator Alpine's favour.

Thanks to the overall length, and the articulated sleeves, I get minimal hem lift when the arms are raised. If you're climbing in a jacket, not just wearing it when stationary, then this may make or break your relationship with it. I've not yet had occasion to keep the Generator Alpine on when leading or seconding a pitch, but thanks to the active cut I know I can when called for. When the hem drawcord is over-tightened, especially if you're wearing it over a harness, I have noticed a tendency for it to puff up (see the top photo), but the answer is obviously to slacken off the drawcord a bit. A quick home test on a fingerboard suggests that if you're wearing it under your harness (you might do so on the coldest days) it should stay in place without pulling out.

The sleeves are very long, with plenty of space in the cuff to slide over a bulky glove, and a simple hook and loop tab to tighten the fit at the wrist to keep the elements out.

Insulation

The business part of the Generator Alpine is PrimaLoft Gold insulation with 'Cross Core Technology'. It's the second synthetic belay jacket I've reviewed recently to incorporate this new insulation, and while it's healthy to take a sceptical eye to marketing claims, my experience to date has been very favourable.

Said by some to be the warmest synthetic insulation currently available, its fibres incorporate aerogel, an ultralight synthetic material with extremely low density and low thermal conductivity; it's basically full of air. The 'cross core' bit, meanwhile, refers to an accordion-like structure. The end result is, we're told, an insulation that's warmer for a given weight than standard PrimaLoft Gold - which lest we forget was already a high performing fill itself.

By the standards of a synthetic fibre the loft does feel really good, and I certainly find the Generator Alpine toasty. Having hung around in it on various sub-zero summits and windy winter belays, I think the key thing for mountain use (where it particularly pays to reduce weight and bulk where possible) is that it offers loads of insulation for a comparatively low weight. For warmth when standing around, without being too hot and cumbersome when active, I'd say Rab have judged the fill of the Generator Alpine spot on.

You get a generous 133gsm of insulation in the body and arms, and 80gsm in the hood where a bit less thickness is required (and being thinner is better for both mobility and being able to hear what your partner is shouting at the far end of a 60m rope). It may be worth comparing with a similar product to use the same fill, Mountain Equipment's Fitzroy. This contains 120gsm in the body, with an additional layer of 60gsm across the shoulders, and 80gsm in the sleeves and hood.

Without carrying two jackets to use on the same day a head-to-head warmth-off is not on the cards, but I've worn both enough to have at least something of an opinion. In terms of insulating performance the end result is not dramatically different, but I feel the Generator Alpine just has the edge over the Fitzroy, thanks to its thicker sleeves and slightly greater weight in the body overall. If you wanted anything significantly warmer still (and it'd need to be a very long frigid belay before I did) then your obvious alternative would be Mountain Equipment's new Citadel; sticking with Rab, you'd have to switch to a down jacket, and accept the obvious damp weather drawback.

Fabric

The Generator Alpine comes in a choice of two colours, a bright green and the stealth black they sent me to review. Black seems ill-advised for a winter jacket, because it's both duller in photos and less visible should you need to be found. Unless you're an aspiring ninja, brighter is invariably better.

Two different 20D fabrics are used outside, a fact that's more immediately apparent with the green colourway. On areas of higher wear, over the shoulders, chest and much of the sleeves, Rab have gone for Pertex Quantum Pro, with something called 'Diamond Fuse Technology'. This, they explain, uses yarns with diamond shaped filaments, which "interlock seamlessly when woven, creating a flatter fabric surface than can be achieved with traditional circular yarns". The result is a "durable and snag resistant fabric that excels in abrasive environments with increased water beading properties, remaining strong and secure over extended wear and washing" they say. Despite being noticeably thin this does seem to be reasonably hard-wearing.

But a skimpier-feeling Pertex fabric is used elsewhere on the jacket, and while it might still be durable for its weight I do wonder if in pursuit of lightness Rab are sailing a bit close to the wind here. It might have been better to use the diamond stuff throughout.  A winter belay jacket inevitably gets abused, particularly if regularly worn whilst climbing. I've not yet damaged mine, but compared to similar products this does feel like more of a possibility. In addition, fabric this light has less resistance to billowing about in the wind, something that may compromise to some extent the sense of fortress-like weather protection that you'd want in a jacket designed to be donned on top of a shell. By comparison the thicker, stiffer 30D Gore-Tex Infinium used on the ME Fitzroy feels more confidence inspiring both in terms of abrasion and resisting being blown about. Pertex Quantum Pro shrugs off light moisture and snow well, but again Gore Infinium is more weather resistant still. But don't get me wrong. The Generator Alpine shouldn't be dismissed as flimsy, and in its favour it both weighs and costs less than its close rival. We're talking fairly marginal differences.

Hood

A belay jacket stands or falls on the fit of its hood, and in this case it's a real success. There's bags of space here to fit over a helmet, and even with the zip done right up I find the effect on head movement is only very slight. Remove your helmet and the three points of adjustment allow the hood to be cinched nice and snug. The rear cord lock is usable wearing gloves, while the side adjusters are neatly housed within the seam of the hood, with the cord tails directed inside to reduce external clutter. A small but effective brim with a stiffened peak keeps some of the weather out of your eyes, and helps the hood hold its shape in the wind. The collar comes high to cover your neck (and even your chin if you hunker down), and there's a nice brushed chinguard at the top of the zip. In terms of comfort, protection, and fit, this hood is hard to fault.

Features

With a double zipper for easy belaying, a massive glove-friendly zip pull, and a nice anti-snag backing strip, the chunky and robust YKK Vislon zip is exactly what you'd want in a winter jacket. It has a popper at the bottom too, so you can keep the hem of the jacket fastened when you pull up the zip to access your harness. The hem drawcords are also usable wearing gloves.

The two zipped hand pockets are big enough for bulky gloves, though given that you might be wearing the Generator Alpine to walk off a hill in a hoolie, it'd have been good if an OS-sized map was a bit easier to get in there (you can, just). These pockets are positioned high enough to remain usable when wearing a rucksack hip belt or a harness. I find the single chest pocket handy for little things like snacks, and because it's cut under the insulation it's also a sensible place to carry your phone (batteries don't like cold). Inside are two open drop pockets for keeping gloves warm on the belay. A stuff sack is provided, with a loop for attaching to your harness.

Ethics and environment

Rab has fairly recently joined the Fair Wear Foundation, which has awarded them 'Good' status. They say they'd like to do better. "We'll continue working with Fair Wear and other brands to challenge, influence, and develop the working conditions at our factories. Each year we develop a plan with Fair Wear and our manufacturers, targeting efforts to maximise improvements across our supply chain" they say.

Value for money

A price tag of £250 may be nothing to sneeze at, but quality design and materials do cost, and there's no question that you get a lot for your money with the new Generator Alpine. It's hard to put a price on staying warm (or staying alive). And think how much you might spend on a technical shell to go with it! Bear in mind too that its near competitor, Mountain Equipment's new Fitzroy, costs £330. The tough and highly weather resistant Gore-Tex Infinium fabric used in the Fitzroy may have the edge over the Generator Alpine's Pertex Quantum Pro in foul conditions, but it must also add to the cost. There will be users for which a marginal gain in performance is worth the extra outlay, but for the majority of walkers, climbers and mountaineers the Generator Alpine represents a more affordable buy, and one that's still well up to meeting the demands of winter in the mountains. This could be Rab's best ever synthetic insulated jacket.

Rab say:

Designed for climbing, winter mountaineering and all cold weather adventures, this insulated, weather resistant jacket thrives when the elements are at their worst.

Having spent decades on the sharp end in challenging and bone-chilling conditions, we know that synthetic insulation provides unrivalled warmth in wet weather. That's why we've strategically body mapped two weights of PrimaLoft® Gold Insulation with Cross Core™ Technology throughout this jacket for maximum warmth where you need it most. The PrimaLoft® fibres are infused with Aerogel, an ultra-low-density material composed of 95% air that was originally engineered to insulate space shuttles!

We've combined this highly effective insulator with an outer of durable, abrasion and weather resistant Pertex® Quantum Pro Diamond Fuse fabric, made from diamond shaped filaments that lock seamlessly together creating a snag resistant flat surface. The Generator Alpine jacket offers unrivalled protection from the wildest storms in the most rugged terrain.

• Sizes: XS-XXL (men)
• Weight: 585g size L (our weight)
• 1 YKK® Vislon zipped chest pocket with oversize puller
• 2-way YKK® VISLON® front zip with insulated internal zip baffle, chin guard and oversize zip pull
• Helmet compatible hood with stiffened peak and 2-way adjustment
• 2 large YKK® zipped hand warmer pockets
• 2 internal open stuff pockets

• Oversize cuffs with hook-and-loop adjustment for use with large gloves
• Articulated sleeves for mobility. Adjustable hem. Stuff sack included
• Combination of 2x 20D Pertex Quantum Pro fabrics 40gsm (Diamond Fuse front/ Recycled back)
• Lining - 15D Atmos (31 gsm)
• Primaloft Gold Insulation with Cross Core (133gsm Body / 80gsm hood)