Buyers' Guide - Lightweight Insulation

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 it was and the bullkier to carry about. 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 serried ranks of lightweight insulation things...

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The Basics

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 the new layer of air and you'll lose heat.

As far as lightweight insulation goes then, it's all about how much air you can trap with a little weight as possible. Simple eh, 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.

The more air you can trap, the warmer the garment will be relative to its weight. There's more to warmth than just trapping air - windproofing matters too.

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Fleece

We actually have an entire Buyers' Guide to fleece, but in nut-shell form, it's pretty much this. 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 will be too warm in most UK weather.

It's also relatively bulky for the amount of air trapped, something that's tackled by new furry designer fleeces like Polartec's Thermal Pro range, 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.

Excellent feel good factor makes it ideal for pub wear and kicking back. Lack of wind resistance requires some sort of shell layer in anything other than still conditions.

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Shelled Microfleece

Most shelled microfleece use 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.

There are plenty of examples out there - Marmot's DriClime for one - and they 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. What we've found over the years is that really closely woven windproof, Pertex-type fabrics don't breathe that well, which is why Rab's Vapor-rise version is our favourite of the type. The Pertex Equilibrium fabric used is wind resistant rather than windproof, but the pay-off is better breathability and wicking performance.

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.

High wind resistance and low insulation levels are great for active use.  Ultimate insulation values low,  not always that breathable.

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Synthetically Filled Lightweights

We're not talking full-on belay jacket here - those generally use too much insulation to be practical for anything other than sitting about, more lightweight sweaters and jackets filled with synthetic fibre insulation like Primaloft.

Rab's Generator Smock is a good example with a combination of Primaloft 1 fill and Pertex Quantum inner and outer fabrics giving a very lightweight 340-gramme top that's as warm as quite heavy fleece equivalents.  Mountain Equipment's Compressor - right - is very similar.

The Pertex and other shell fabrics used tend to be completely windproof and have some sort of water-resistant treatment to shrug off showers and 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 Pertex and synthetic insulation, while it's completely windproof, doesn't breathe or wick particuarly well, stick a waterproof shell on top, and it's even worse meaning that this type of garment is best kept for low-tempo 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...

Good warmth to weight ratio, generally completely windproof, knockabout robustness in damp. Tend to lose out to fleece-type garments for breathabilty, wicking and feel good factor.

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Ultra-Lightweight Down

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 synthtically-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 the UK 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 exceptions from Mont Bell and Rab, with the new Microlight range which is much closer to fleece in warmth terms.

Ultimate warmth to weigh ratio, windproof, very packable. Great in cold, dry conditions.  Not really suitable for active use,  allergic to wetness making it of limited use in the UK.

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Technical Features

Some of the most minimal insulation pieces out there will save weigh 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 clear a pack belt. It's also handy to have a small zipped pocket somewhere, say to hold loose change or a mobile. 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 butt from cold and thrown up damp and dirt up to a point, though it may blunt your sartorial edge somewhat. 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.

Add versatility and functionality keeping heads and hands warm and valuable safely stowed.  Also add weight, bulk and complexity so not for ultra-lightweighters.

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Summing Up

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, but 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 for active use. 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.

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.

Happy choosing :-)

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More Help And Advice

This article should have given you some basic pointers. The good news is that there's a load more advice on OUTDOORSmagic:

Ask on the gear forum about general issues or specific items.

Check the member reviews section for user experiences of kit.

See OM editorial reviews on the front of the site for our impressions.

Ask Richard Gear if you have a specific question you need answering.