Lightweight insulation used to be straightforward, you bought fleece, it came in three weights - 100, 200 or 300 and the higher the number, the warmer - and bulkier - it was. If you were going somewhere very cold and dry, you might choose down instead.
But things have changed, now fleece is under attack not just from more complicated, designer fleece - yes, really - but from lightweight, shelled, synthetic garments, shelled microfleece and even lightweight down tops.
They all have different pros and cons, so here's a run through to help you choose from the wide range of lightweight insulation things...
Insulation all works in the same basic way - it traps air which has been warmed by your body close to your skin and minimises heat loss. If there's no wind, the warmed air will sit there regardless. If there's any sort of breeze, the wind will strip the warm air away, your body will have to warm a new layer of air and you'll lose heat. The more air you can trap, the warmer the garment will be relative to its weight - but windproofing matters too.
As far as lightweight insulation goes then, it's all about how much air you can trap with as little weight as possible. But of course there's more to it than that - wind resistance will make the garment warmer in real world use, while more or less breathability will affect its suitability for active use, as will technical features like hoods, baffles and cut.
Fleece is a decent insulator, it dries quickly and wicks well enough, which means it works pretty well worn under a windproof or waterproof.
Standard fleece, however, has limited wind resistance, which means that worn on its own in windy conditions, trapped air is rapidly stripped away, making some sort of additional shell advisable in anything other than completely still weather. If you run remotely warm, anything warmer than a microfleece may well be too warm in temperate climates.
It's also relatively bulky for the amount of air trapped, something that's tackled by new more 'furry' looking fleeces, which use pile-type fibres or grid patterns to improve their warmth-to-weight and bulk ratios. Again, few of these offer any wind resistance.
Fleece is robust though and, at the end of the day, has a high 'huggability' and 'feel good' factor, which most of its competitors lack.
Shelled microfleece uses a very light micro-velour liner rather than actual 100-weight fleece. It works in a very similar way adding a little extra insulation along with excellent wicking properties and a nice comfortable next-to-skin feel.
These make fantastic all-round hill garments and add perhaps the additional thermal warmth of a thick baselayer. Ultimately, breathability depends on the outer windproof shell fabric. Really closely woven windproofs don't breathe that well, however, and you might want to look at wind resistant rather than windproof, with better breathability and wicking performance. The choice is yours though - whether you run hot, in which case you might want to go for wind resistant, or a rather cooler person, in which case windproof might be better for you.
Garments of this type are almost soft shell - great for active use on the hill, but not really warm enough for prolonged stops when you'll need additional warmth. The totally windproof versions can feel clammy when teamed with a waterproof shell.
Synthetically Filled Lightweights
We're not talking full-on belay jackets here - those generally use too much insulation to be practical for anything other than sitting about - more like lightweight sweaters and jackets filled with synthetic fibre insulation like Primaloft.
The Pertex and other shell fabrics used tend to be completely windproof, having some sort of water-resistant treatment to shrug off showers. The insulation copes reasonably well with dampness, meaning they have a knock-about robustness that down lacks.
The downsides are that in active use, the combination of the shell fabric and synthetic insulation, while it's completely windproof, doesn't breathe or wick particularly well, stick a waterproof shell on top, and it's even worse - meaning that this type of garment is best kept for lower energy use or for stops and overnight use.
Finally, synthetics like this lack the feel-good factor of down - you simply don't want to snuggle into a windproof outer shell...
Down offers the ultimate warmth to weight ratio complete with a windproof outer shell, so it should be the ultimate answer, right? Unfortunately it's not quite that simple. For active use, most down clothing shares the breathability / moisture transfer issues of the synthetically-filled equivalents - vapour is trying to pass through two windproof layers and a layer of warmed air...
Down scores for its warmth-to-weight ratio all right, but it doesn't deal brilliantly with wetness. If down does get wet, the surface tension of the water overcomes the ability of the down to loft, the tiny tendrils are swamped, and the result is a flat, non-insulating porridgy mess. In damp conditions that's quite important, though in cold, dry, high mountain areas, it's less of a factor. It also means you need to carry any down items in a waterproof pack or stuff bag.
Most down strays into belay-jacket warmth areas, but there are a couple of lighter exceptions which are much closer to fleece in warmth terms.
So how many of you had thought about using wool as an insulating material in a jacket? Probably not the first thing that springs to mind, and when you think about it, you might immediately dismiss it as being too bulky or heavy when wet ....... But as insulation within a waterproof jacket - maybe time to think again?
Wool offers the most insulation with the least amount of weight, and naturally retains its shape. It is light and naturally hygroscopic meaning it absorbs and evaporates moisture more effectively than most other textiles, making it a good temperature regulator. On a cold day, it will keep you warm, but on a hot day it will cool you down. Wool fibres preferentially pull moisture out of the air until they reach saturation. This can take considerable time, because wool can absorb 33% of its own weight of water before it feels damp and allows condensation to take place in the air trapped within garment insulation. The precise rates of transfer of moisture are complex and vary with the design of the garment, the other layers being worn, wind speed, external temperature and humidity.
In addition, wool is flame and odour resistant, as well as an entirely renewable and biodegradable natural product, that can be safely recycled at the end of its life, reducing environmental pressure. So, a naturally adaptive wool-based insulation, such as HDWool® Active Insulation, is an excellent alternative to the traditional down and synthetic insulation.
Some of the most minimal insulation pieces out there will save weight and complexity by keeping things simple - maybe a short zip, basic Lycra cuffs and so on, but if you want more than just that, here are a few extras with their pros and cons...
Pockets are down to personal choice, but hand-warmer pockets are a nice extra, though if you plan to use them on the move, make sure they don't get in the way of a pack belt. It's also handy to have a small zipped pocket somewhere, say to hold loose change or a mobile phone. Talking of zips, windproof versions will benefit from a strip or even baffle behind any main zips.
For active use, a drop tail will protect your backside from cold and thrown up damp and dirt up to a point. Another extra which you'll either love or hate is a hood. On the one hand hoods allow quick and easy head insulation without fiddling with a hat, on the other, they get in the way if you throw a shell over the top and, of course, if you don't have a hat as well, you'll have to insulate both body and head, which you may not want to do.
As you've probably deduced, what you choose all depends on what your priorities are.
Fleece is a good all-rounder, but needs additional wind protection. It feels nice, dries fast and can be very versatile when layered with either wind or waterproof shells.
Shelled microfleece makes a great active-use garment combining wind and water resistance with just about enough additional insulation. It wicks well and dries fast. It's not warm enough for prolonged stops however when most people will want an extra warm layer.
Synthetically-filled lightweights by contrast, while offering wind and water protection and good warmth, tend to be a tad clammy and sweaty for active use, making them best kept to throw on at stops and for evening campsite wear. They don't feel as nice as fleece either.
Wool is definitely something to think about. All the insulative properties of the other options, lightweight, renewable and natural, moving moisture way from the body and stays warmer longer than other materials. Make sure it is within a fully waterproof jacket, though!
Ultra-lightweight down is nice, light and compact but again, best for stops. Its achilles heel is an aversion to wetness, which makes it best saved for cold, dry environments and requires careful transportation to maintain its lofting ability.
There are a huge number of garments dubbed with the 'soft shell' label, with a massive and different array of qualities and properties. They range from highly air permeable and fairly wind resistant garments right through to the slightly ridiculous idea of fully waterproof, taped soft shell...
What is Soft Shell?: There's no 'right answer' here. If you were going to be logical, you could say it's something you can wear instead of a fully waterproof shell for most of the time. Rather than being fully waterproof, it should be weather resistant, with the trade-off of having much better breathability.
Why do you need it?: Because most of the time fully waterproof shells are simply not breathable enough for many users. The idea is that you're trading off some of that weatherproofing, for greater breathability.
Soft shell can be seen as a continuum - at one end you have fabrics which are extremely breathable, but only moderately wind and water resistant, though they'll stand up to light showers and most snowfall.
At the other end of the scale are totally windproof and very water resistant, even waterproof, materials. They're almost as protective as a full waterproof jacket, but the pay-off is much reduced breathability.
Which Fabric Is For You?: People vary massively. If you're trying to decide which fabric will suit you, think hard about how hot you run and how active you really are. If you're a 'sweat bucket' sort of person who races around pouring out heat then you'll probably be better off with one of the more breathable, less protective soft shell materials.
If you run cold, then a more protective, less breathable, membrane-based soft shell fabric may make more sense. The fabric will be less breathable but significantly more protective.
Finally, bear in mind that most soft shell fabrics are simply that, shell, and offer very little insulation though there are some with a fleecy inner surface designed to be worn in cold conditions. There's no reason why you can't add an insulating layer inside for cold conditions and wear them just over a baselayer when it's milder.
Cross breeds...: Because soft shell is really just a marketing term, there are all sorts of garments out there that may or may not be soft shell depending on, well, opinion really. They include Paramo, Buffalo-style pile / Pertex garments, and all sorts of shelled microfleeces.
Are they soft shell? No one really knows .... If they do what you want them to, that's fine. It's a bit like arguing over whether mass-produced lager is real beer; if you like the taste, then drink it.
Because we're talking about such a wide range of different clothing under the one 'soft shell' banner, it's again a question of deciding what you need for your activity and preferences.
For full-on technical climbing and mountaineering softshells which will be used in mountain conditions, you will probably want to be looking for similar features to those you'd want on a conventional mountain shell. That means a full hood with some sort of stiffened peak, cuff, hem and neck adjusters, possibly some venting, particularly if you've chosen a membrane-based fabric and so on. There aren't many garments out there meeting this spec. Most soft shells seem to be based on a simple fleece jacket cut.
For more general mountain walking use, hoods are still really useful. They are ideal for stops and make the jacket more versatile generally. Many softshell tops seem to be viewed as fleece alternatives rather than as full shells, so it's hard to find much that matches up. If you don't want a hood, you'll have a much wider choice.
Other general features are the same as with other technical jackets. Think about whether pockets will clear a harness or waist belt. Do you have one that fits a map, if you carry a map in your pocket? Can you tighten hems and cuffs? Are adjusters one-handed? Can you use them with gloves on if you need to? Fast movers might also prefer a drop-tail to protect the backside, but without hindering movement.
Softshells and Rain: There are waterproof softshells out there, but they're no more breathable than normal waterproofs but the softer face fabrics used often absorb water more easily reducing their breathability.
Most 'soft shell' fabrics will work well in light rain or snow, but if it really buckets down, you'll need a lightweight waterproof jacket that you can sling over the top for proper rain protection. Even though the membrane in, for example, Windstopper, is effectively waterproof, water will still penetrate the seams making you wet.
You'll be able to wear soft shell for longer before needing a waterproof, but generally, you will still need one.
Confused?: A couple of years back, US outdoor shops voted soft shell as the most confusing concept to come out of the outdoors industry. If you think of soft shells as being weather resistant to varying degrees, but not generally waterproof, you won't go far wrong. What you really need to decide is just how much weather resistance you need - and remember - the more weather resistant the soft shell is, the less breathable it's likely to be.
Conversely, really protective soft shells may be little different from conventional waterproofs when it comes to handling sweat and heat. It's your choice.