Saturday, October 04, 2014

A beginner guide to clothing systems for the British Winter Mountains

This is an article I wrote a few winter ago for UKclimbing but it was never used for some reason. Anyway, it might be of use to someone (and I may even add photos at some point!) so here it is.

Buying clothes to wear for the winter mountains is an investment, winter climbing is not a cheap sport. So let’s start with the good news: modern outdoor clothing is, relatively speaking, cheap. Compared to any normal clothes you buy, the mark-up in the outdoor trade is rather small, and if you find something on a clearance rack at half price, the shop is almost certainly making next to no money on that sale. I bought my first Goretex jacket nearly 20 years ago with my savings from working the school holiday picking fruit on farms. It was about £130 and despite 19 years of inflation you can still buy a Goretex jacket for the same amount and it will work better than my 1990-vintage Phoenix Topaz. Secondly, modern outdoor clothing is really good. If you have the money to buy top of the range from any of the famous brands it is really, really, really good. But a sensibly-designed, own-brand fleece from Millets or Decathlon is going to be as good as the top of the range Berghaus or North Face fleece of twenty years ago whilst being a third of the price not even taking into account inflation. I might not go as far as to say you can’t go wrong, but like having SatNav – it’s getting ever harder to go really wrong.

Sometimes the British winter can get pretty wild - and this is just Wales! Scotland gets more knarly!

But good value is the result of competition and this comes from a huge choice. You can hardly moan about this, but of course this does making choosing exactly what you want difficult. This article aims to give some basic information for those who are new to the game, and perhaps a few alternatives thoughts to those who aren’t.

This article, being UK Climbing, is aimed primarily at winter climbers going to Scotland, Snowdonia or the Lakes. Climbers often need a little extra warmth than winter hill walkers due to pitched climbing necessitating standing still and belaying in foul weather, although otherwise the basic philosophy is the same.

Dry and still

Winter clothing keeps you warm by keeping you dry and from stopping the wind. You lose heat in two major ways – conduction and convection (forget radiation – there ain’t much that can be done about that). Conduction is heat energy moving (in this case away from your body) through solids and liquids. Convection is the same but through gas – the air, moving in the form of wind. Keeping dry is about reducing conductive heat loss. You can stand around naked in still air at -10 and if you are dry it is fine for a few minutes, but try getting into a lake where the water is 5 degrees and you’ll know all about it. We do both of these regularly in Finland, often together, so I say this from personal, and normally quite embarrassing, experience. Just to complicate matters, you can get wet in two ways – from the outside (snow, sleet, rain) or from the inside (sweat); your clothes have to stop moisture from either being near your skin. Keeping out the wind is about avoiding heat loss through convection. Anybody who has stood around belaying on a windy day without a windproof jacket will understand exactly how this works.

Dry and still conditions Nordic ice climbing, a micro fleece and vest was fine even if it was -10.
 Inside out

The layering principle is the standard way to dress for the winter mountains. There are clothing systems that claim they aren’t based on the layering principle, but due to basic physics they are really – it’s just a different take on it: normally combining two layers into one. It is best to think of the layering principle from the inside out starting with the clothes against your skin. This is the base layer – although often referred to by your granny as thermal undies. Base layers suck the sweat away from your skin as quickly as possible transporting it outwards to the next layer. This is called “wicking” - probably because “sucking up sweat” is such a horrible image. The quicker your base layer wicks, the dryer you stay – and as we discussed above, the warmer you will be. Next comes the mid layer – normally this means fleece these days. The mid layer is insulation that traps air which insulates you from the colder air outside your clothes. Your insulation mid layer also needs to be able to transport sweat outwards without holding the moisture. This is why it is rare to use a down jacket as a mid-layer, feathers hold moisture so it would get clammy from sweat and stop working well. Finally there is the shell layer. When I started climbing everybody just called these “waterproofs” and were done with it, but this is where things get a bit complicated because you have in effect two types of shell – those designed just to keep the wind out – windproofs – and those that keep both the wind and rain out – waterproofs. If you want to be down with the kids you can call the former softshells and the latter hardshells, but for the moment this unnecessarily complicates matters – so I won’t. Next we will go on to discuss the basic options available for these layers, before heading out to the extremities – hands, feet and head.

Base layers

Until some New Zealand sheep farmers hit on a really great business idea a few years back, base layers meant synthetics – mainly different types of polypro. I have synthetic base layers made by Helly Hansen, Karrimor, Jack Wolfskin, Berghaus, Lowe Alpine and others that I don’t recall. All work – even my 18 year old smelly Helly that I still regularly wear whilst cycle commuting in winter. There is not so much to distinguish them in terms of wicking – get any polypro base layer from a decent manufacturer and you won’t go wrong. Making sure they don’t have seams that rub or labels that itch is probably the most important consideration. One feature they do all share in common though is that if I wear them for more than ten minutes, they stink under the arms (and round your nether regions with the long johns). Different firms have claimed to have solved this issue over the years but none I have tried have succeeded. It seems that most blokes at least will make synthetic base layers stink. This is where we get back to those enterprising antipodeans. I reckon Merino wool is a real revolution in thermal undies. It still wicks to my mind as well as synthetics (others disagree on this but they seem to be a minority) but it is really quite spookily smell resistant. I can wear a cotton t-shirt for a day without it getting whiffy, but after two days it's not so great. I wore my favourite merino baselayer for four days ice climbing last Easter in Norway – and no hint of smell. To me this is amazing and in my experience the only downside to merino is that it tends to cost more and the material is a bit delicate in comparison to synthetics.

Mid Layers

Fleeces are pretty simple things – fluffy polyester knits that trap warm air and thus insulates you – but they come in bewildering range of styles and types. The fluffier or thicker it is, the more insulation that garment will offer. For climbing, simple and fitted is best. As increasingly with modern clothing systems we add insulation to the outer layer – the belay jacket idea (see below) - micro fleeces are amongst the best mid layer garments. They offer a fair amount of warmth but aren’t bulky and as shell layers become ever better cut and fitted, this is important. Hi-loft fleece is the fluffy type that makes you look like a brightly coloured sheep but is superb in cold conditions. They are far, far lighter than old heavy weight fleeces and compress well. They also make ace pillows once you are in the tent at the end of day – but many might find them too warm under a shell if climbing hard or moving fast. If you are sure you are going to be wearing your mid-layer all day, as most people will for winter climbing, consider a pullover rather than a jacket version: lighter, no annoying zip lower down near your harness and, best of all, normally cheaper.

Mid-layer for your legs is more complex because legs generally need less insulation so many find that if their leg wear has some wind resistance to it, it will actually be their outer layer for much of the time. Softshell trousers made out of a stretchy, breathable and wind-resistant material have become the legwear of choice for many winter climbers in recent years, but summer trekking trousers over long johns can also work well. But even the expensive Schoeller materials are not completely windproof (unless they are the expensive and less breathable membrane type) and in cold temps or when static for long periods I’ve found them to be not warm enough. This when you might have add some sort of shell over them, or pick a more specialist pair of trousers that are insulated in some way.


My great Marmot windproof on the top of a cold but (for once!) dry Scottish mountain.
A Rab windproof - super breathable for big ice pitch I'm about to try, Norway.
Shell layers are designed to mainly to make your insulation layers below work, not to offer insulation themselves. This is why many windproof smocks now weigh next to nothing. Nevertheless the various windproofs I have are amongst my favourite bits of outdoor gear; used all year round for climbing, ski mountaineering, cross country skiing, cycling and orienteering. By keeping the wind out they make your insulation underneath work, but unlike waterproofs they are (or at least should be) ultra-breathable and hence sweat produced from activity can escape easily. Pertex is the classic material for windproofs and perfect for the job, but any close woven and unproofed nylon will work.


Making a waterproof jacket is easy – the trick is to make it waterproof in only one direction. As you do any exercise you sweat. If this sweat can’t escape through your waterproof layer, it will wet your mid and base layers just like rain or melted snow from outside would do. Conducted heat loss then begins and you get cold. This is why your waterproof jacket also need to be breathable. Breathable simply means that the jacket material in some way to do with it physical structure or chemical make-up allows moisture vapour (sweat) through from the inside to the outside, but does not let liquid water (rain) in from the outside to inside.
Proper old school, get your Goretex on weather. Senja, Norway.

Materials are getting better – the Goretex of today breathes more readily than the Goretex of the jacket I bought in 1990 and there are now many competitor fabrics that seem to work adequately and particularly with eVent there is now a fabric that many believe is better than Goretex. But just as importantly is that designs have improved massively in the last decade. Designers are making jackets that are slimmer fitting, tailored to the needs of climbers or hikers, and use cleverer technologies like thin seam tape or bonding technologies that allow the material to breath better all over. In the early 90s I became a huge fan of Buffalo clothing because it meant I didn’t need my Goretex jacket for winter climbing and that meant getting less clammy and cold from sweat despite not being waterproof. I still won’t wear a Goretex for, say, skinning uphill whilst ski mountaineering, but my Arctryx paclite Goretex I can wear happily when ice climbing on drippy days, or hiking in sleety weather, without getting sweaty inside. It’s just a better designed coat made out of better material than the early 90s shells – and the fabrics of today are further improved than the six year old goretex of my Arctryx.

For the British mountains in winter, what you will want though is a shell jacket with a good hood - the best tend to have a wire in them to create a peak, and the hood needs to be big enough to go over a climbing helmet. Unsurprisingly, British companies (Berghaus, Mountain Equipment, Montane etc.) often have the best hoods for full on conditions – putting more emphasis on protection than peripheral vision. Some US firms have even made jackets designed specifically for the British market including bigger than normal hoods – showing the difference in design philosophy. More and more shells now use waterproof (water resistant some say) zips. These save weight, but some still prefer their winter jacket to have a storm flap that covers the zip for maximum protection.

Booster Layers

Booster layers – often called belay jackets – are insulated coats that you stick on over your shell (windproof or waterproof) when static or just really cold. Traditionally these were down filled, although down doesn’t mix with rain or wet snow well, so increasingly many climbers are going for modern synthetic fills such as primaloft. These keep their insulation value better if getting damp, but down is lighter, packs down smaller and last much longer if well looked after. See my earlier article on belay jackets for much more on this.

Alternative systems

For a long time the most famous ‘alternative systems’ in the UK to the layering principles outlined above were Buffalo and Paramo. You can read much more about both on their websites, but both avoided membrane waterproof fabrics like Goretex. What they lose in waterproofing they gain in breathability. The fans of both systems often have a slightly zealous air to them that come with having ‘seen the light’. I should know: in the mid-90s I was a hardcore Buffalo boy. I was living in Scotland and working in shop that stocked the system felt the urge to try and convert the Goretex clad infidels to the true and righteous (and slightly odd looking) path. I’ve never used Paramo, so their crusaders will have to speak up in its favour but back then Buffalo was without any doubt the best value for money mountain clothing system you could buy. Montane also make pertex and fibre pile products very similar to Buffalo. Pertex and fibre pile is not always perfect, but for serious winter climbing when on a budget it is still well worth looking at. Stephen Reed, owner of Needlesports has an excellent manifesto for the Buffalo system.

Feet, hands and heads

Keeping your extremities warm is one of the hardest parts of choosing your clothing system and my experience is that in particular finding the right glove system is an annoyingly expensive experience of trial and error. Hopefully some of


What boots you wear is dependent on what you are doing - winter hill walking, mountaineering and easy climbs, mid grade pitched climbing, or hard climbing. For hill walking and easier routes many will wear a B2 (link) rated boot - with a bit of flex to them and not too heavy. These can be super traditional leather walking boots, or more modern styles made with various synthetic materials. Boots for climbing in tend to be rigid - B3 rated for prolonged crampon use and built with warmth in mind. Boots for the hardest climbs are rigid but lighter, possibly sacrificing some warmth and support but anyone interested in those type of boots won't need this article. Opinions vary on what to wear inside. When I started climbing in Scotland in the early 1990s everyone wore plastic boots, and most people seemed to use inside a liner sock under a woolly sock. You didn't need to worry much about cold feet with that combo but it compromised climbing (and walking) performance. With better fitting leather boots wearing one pair of medium to thick socks inside seems to make more sense to make the most of the fit and climbing performance of your boots. Good mountain socks from manufacturers like Extremities, Thorlo, Smartwool, Bridgedale and the like are very nice but do seem horribly expensive for a pair of socks. I found that high wool content socks - normally sold as hiking socks - from even Tesco can do the job fine. My two pairs of Tesco hiking socks cost about seven quid but have kept my feet nice and warm inside my Nepal Extremes even when ice climbing in the bitter cold of the Norwegian arctic. The old Extremities mountain socks I have are a little warmer, but at something like eight times the price!

For UK mountains, I still think that gaiters are pretty vital. If you get water or mud over the top of your boots, you will get cold feet once above the snowline. The gaiters that come attached to many shell trousers might do a good job at keeping snow out of your boots, but not the boot sucking mud of many a British walk-in. Good gaiters are nice, but cheaper ones do the job well enough. Look for a pair with a front zip, these are much less hassle if you need to tighten your laces than the back zip models. Places like Decathlon do some very good value pairs with decent technical designs. Full foot gaiters like Yetis are great for keeping snow out of your boots on prolonged trips where you are camping in deep snow, but in my experience are a bit over-kill for day climbs. They do make boots slightly warmer by keeping snow off your boots and laces - but the majority of heat-loss from the feet is through the soles of your boots, so Yetis aren't the magic bullet to warmer feet that some people expect.


Glove and mitts are notoriously difficult to get right and, due to the complexity of the stitching and taping, expensive as well. Most winter climbers find a system that works for them after years of trial and error. Mitts are warm and often waterproof but most find them hard to do anything technical in. Softshell gloves are light and dexterous - picking the no. 3 wire of your racking krab is easy enough - but you quickly get cold fingers when belaying and water goes straight through them. Goretex or eVent mountain gloves are somewhere in between - a bit warmer and you can use your belay plate, but you might drop that wire. In my experience you need more dexterous gloves for Scottish climbing, particularly mixed routes where the majority of pro is rock gear. Softshell, or some other thinner types of gloves work well, with mitts for belays and the walk down. For pure icefalls, goretex (or similar) gloves work well - ice screws aren't too fiddly to use with them and they are warmer. Ice climbing in Scandinavia I have often just used my mountain gloves all day, for climbing, belaying and the descent, but for hiking up to Scottish climbs, takes something thin and stretchy for the approach; any old gloves will work fine including woolly ones, keep your main gloves dry and ready for the actual climbing. Finally, take some light, insulated mitts for belays, descents in horrible weather and for simply when your hands get really cold. Buffalo mitts remain a favourite, very light and pretty cheap, but if you think you might be wearing them to belay much get something with reinforcement on the palms. Dachstein mitts deserve a special mention as many and will go on at great (boring?) length about how they are the be all and end all of Scottish winter handwear. I'm unconvinced myself, finding them heavy, stiff and neither particularly grippy or warm - but a thousand happy punters can't be completely wrong so it may be worth trying them out.

Some specific recommendations: my current softshell gloves are by Ortovox - I got them mainly because I couldn't afford the Black Diamond Dry Tool gloves and they were the only other ones my local shop had, but they have turned out to be hard wearing, being three seasons old and surprisingly warm. If the price of softshell gloves puts you off, try Extremities Sticky Thickies over a pair of thinnies (or even cheaper no-brand 'magic' gloves) as a cheaper and surprisingly warm alternative. I used this system for a few seasons of regular Scottish routes and it worked great for me for more technical mixed routes where you are mainly placing nuts and cams. When it comes to a more general, waterproof, mountain glove; for about six years I used a pair of Goretex gauntlet gloves made by Mountain Hardware. These were absolutely superb: the palms and fingers were made with sticky and absolutely bomb-proof rubbery material that no number of abseils could wear out. They had minimal insulation, just a light brushed lining to protect the Goretex, but this meant they were very dextrerous and, for all but the most technical of routes, you could put them on and just keep them on all day. Of course they seem to have stopped making that model now, which all too often happens with a brilliant product! I replaced them last year with Rab Makalus - decent gloves but with some insulation making them less dexterous than the Mountain Hardware ones, and with a less good cuff arrangement. The eVent does seem very good though. If buying waterproof climbing gloves one really important thing is get them to fit your finger length; any floppy bits at the ends of the fingers seems to be magically attractive to the gates of any karabiner you are trying to handle - not what you want whilst desperately trying to get a quickdraw onto your ice screw. For mitts, bargain bins in climbing shops in the summer or somewhere like Decathlon have proven good bets for me in the past - any loose fitting nylon-covered and pile-lined mitts should be pretty warm. My current favourite belay mitts are Extremities and were bought in TKMaxx for about a tenner.


For ultimate warmth and protection you want a balaclava - I like light and stretchy ones because I tend to carry it much more than I wear it, plus with a black powerstretch balaclava you are also always ready to attend fancy dress parties as a ninja. Back out on the hill, wearing a hat and some sort of fleecy neck tube is far less likely to get you arrested as a bank robber and is more flexible an arrangement. And remember: bobble hats both look ridiculous and don't fit well under climbing helmets, so buy a good looking beanie and you can also use it for bouldering, as long as you remember to take your top of first.

Saturday, May 03, 2014

Edelrid Beast Lite Crampon review update have just published a review I have written of the Edelrid Beast Lite crampons. This post is basically an addendum to that noting a problem that has since arisen with the crampons since I filed the review.

I got the crampons unfortunately near to the close of this year's rather truncated Finnish ice climbing season but I still managed to get out and use them on several hundred metres of steep ice divided over lots of pitches and a number of days. It wasn't a whole season of weekend usage, but I still felt pretty confident in my conclusions on the strong and weak points of this rather novel model of crampon. The most noticeable thing about the Beast Lites is (unsurprisingly, considering their name) their low weight, so when I was packing for a ski-mountaineering trip to Norway just before Easter I threw the Beasts into my bag because, despite being a technical climbing crampon by design, they weigh less than even a pair of 10-point walking crampons I own. It was actually on taking them out of my pack to use to scramble up on to the blocky summit of the Senja peak Kvænen where I noticed that plastic heal clip of one of the crampons had cracked through (see the pictures below).

I know that heel bail clips like these don't need to break and that plastic is a very suitable material to make these components from; my Grivel G12s are now well over a decade old and the heel bails are fine. and on my first pair of DMM Terminators it was the metal frame of the crampon that broke after 8 seasons of hard use not the bails. It seems very unlikely that this could be simply bad luck after, relatively speaking, so little use. So I suspect there is a problem either with the type of plastic that Edelrid have used here or the shape and design of the heel clip that led to the breakage. It maybe that in trying to make the crampons so light they simply used too thin plastic here.

Wednesday, February 19, 2014

Marmot Isotherm Hoody/Polartec Alpha Insulation - some thoughts.

The Marmot Isotherm Hoody being used as a mid-layer on a -20 day.
The Isotherm hoody is Marmot's first go at using Polartec's new Alpha insulation, an insulation originally developed by Polartec for US Special Forces. The military wanted a synthetic insulation that while warm was also breathable and could be kept on during periods of activity. With body armour, plus all the equipment carried by modern soldiers on their webbing, donning or taking off layers of clothing below all that is obviously difficult in anything beyond relaxed and safe situations hence the requirement for insulation that breathes well when you are active in it. The Alpha insulation is knitted onto a mesh, allowing 'sheets' of the insulation to made and sewn into garments. The knitted construction is very air-permeable meaning breathability, but also means the insulation is stable and drapes well. This allows for simple garment designs that don't require extensive channels through the construction, as would be necessary to hold a loose insulation like down. The Alpha insulation is though encased in an inner and outer shell. Again because of the stable, knitted structure of the insulation, manufacturers can use as an inner layer a very breathable and light mesh material (again a loose insulation like down would escape through such a material). With the Isotherm Hoody Marmot have used a mesh version of their own dri-clime material for most of the inner liner. 

Graphic from Polartec, click here for more info.
With Polartec Alpha's unique selling point being breathability I feel the choice of outer material to the insulation is vital. Insulation works by holding air still, creating a barrier of stable, warm air between the person inside the clothing and the colder, moving air in the environment beyond – so there needs to be a windproof layer over the insulation to allow for this to happen. This would seem to be where the central dilemma with Polartec Alpha lies – for the material to insulate to its maximum amount you need to use a windproof outer fabric but this may not be particularly breathable, but to get the most out of the insulation's structure which is what Polartec says makes Alpha more breathable than competitor synthetic insulations, you want an outer fabric that is highly breathable, which generally means less windproof. The new buzz-phrase in the industry for this is “air permeability”; for instance the new Goretex Pro is air-permeable as is Polartec NeoShell making both of these fabrics more breathable than other waterproof fabrics, but the other way of putting this is “not as windproof” and that might not sell as well in the 'performance outdoor clothing' market. For the outside of the Isotherm Hoody, Marmot have gone with Pertex Quantum, a light ripstop nylon. There are different types of Pertex and I'm not sure if certain forms are more windproof and less breathable than others – but Quantum is an ultra-fine and smooth weave so I would imagine that it isn't particularly air-permeable, although of course being windproof allows the Alpha insulation below to insulate all the better.
Some slightly freaky google auto-produced multilayer pic of me mountain-biking in the Isotherm.
So that's the physics theory section; but what about in use? Well, in truth it is a bit of a mixed bag. Let's start with the good stuff; the Thermo Hoody is very light (387 grams in medium, about 20 grams less than Marmot claim) and very compressible – it is very easy to stuff into a bag to take along 'just in case'. The hood (a simple under-helmet design) adds instant warmth when used although it isn't designed to zip up and protect your face. Considering how light the materials are too, it seems relatively tough – it survived a day of gritstone cragging when it was far too cold for me to worry much about not scuffing and scraping it. Marmot have used a stretchy light softshell material over the shoulders to make it a bit tougher for use with a rucksack. It's easy to care for too, particularly in comparison to lightweight down tops. If it gets grubby you just chuck it in the machine for a wash. For me the downsides are the fit and the design. I've been a big fan of Marmot for years as their quality seems top-notch and their size medium has always fitted me well, but with the Thermo Hoody the medium is too tight across the shoulders although it's not particularly trim around the waist so its seems a bit oddly proportioned. Perhaps related to this I've also found it doesn't work brilliantly with a harness with a tendency to pull out after some high reaches. A climbing harness covers the hand warmer pockets too, which considering Alpha is all about “active insulation” - so insulation to wear whilst doing stuff, like climbing! - seems a shame. Another minor design flaw is that the lining in the sleeves is too loose so when ever you pull the jacket on the inner fabric pulls out, protruding beyond the cuff. It goes back in easily enough if you hold the cuff and stretch the sleeves a bit, but it's enough of an annoyance to notice.

And finally to the big issue – is it breathable and how much warmth does it offer? Used as an outer layer for things like autumnal mountain biking I found the Isotherm snug, wind resistant and warm but perhaps a bit too warm. Despite not riding particularly hard or doing long climbs, in positive single digit temperatures the inside of the Isotherm was getting rather damp with sweat being worn over just a thin base layer. This would dissipate with time, but then I've found the same true of traditional synthetically insulated pieces like the Marmot Variant. Through the damp cool autumn Alpha didn't strike me as that different and I was left wondering – was it the Pertex Quantum material holding in the sweat or the Alpha insulation itself? For cool weather rock climbing, the jacket was more successful, not getting sweaty inside and keeping me reasonably comfortable while climbing on what was a ridiculously windy day at a rather exposed English gritstone edge. The day was so windy though, that although it wasn't particularly cold (about +5) I still needed to use a duvet over the Isotherm when not climbing – in those gales you notice the air permeability by getting cold! So good for less aerobic activities like rock climbing but I don't think the Isotherm is necessarily the best cragging top for the reasons mentioned earlier; it would need to be longer, have differently positioned pockets and perhaps a slightly heavier, tougher face fabric for that. I have used it as a mid-layer under a Marmot NeoShell jacket in cold and very cold conditions, and here it worked impeccably. This has included Nordic skating at -20 and alpine skiing at just below freezing. But in both these cases I suspect a hi-loft fleece would have also worked as well as a mid-layer.

Cragging at the Roaches in the Isotherm Hoody
I'm left wondering rather what “active insulation” pieces like the Isotherm are really best suited for? I think for me, at least, a microfleece (the grid-pattern ones wick and breath very well) and a windshirt will work as well; being more versatile, costing less and not weighing much more. Nevertheless we all experience the outdoor environments differently and for some people I'm sure the Isotherm will be the best midlayer they've ever tried. There is a lot of like about it, a certain silky luxurious snugness in particular! But still – for me – I'm not sure if it does a job better than pre-existing solutions. It will be interesting to see how Polartec Alpha is used in conjunction with different shell materials in the future, because I still don't quite see how to square the circle of having an insulation that is both air permeable itself and is encased in air-permeable fabrics that will still work well in anything other than windless conditions.

Wednesday, January 08, 2014

Physics you can sleep on; a design weakness in the Alpkit Numo sleeping mat.

Bikepack bivvying - my Alpkit Numo under my tarp
Warning: very geeky camping gear post follows; surf away now if you don't care and most well adjusted people probably don't.

A camping mat is pretty fundamental to getting a decent night's sleep when camping – whatever you pick is a compromise; the light ones might not be tough, the tough ones not light, the light, tough ones not cheap etc. In summer you can get away with more (well, actually less); find some softish, non-rocky ground and even the lightest closed cell foam mat can be great, but on hard uneven ground and as winter approaches the mat becomes more important.

Winter bivvy, in a laavu (Finnish lean-to log shelter)
I got an Alpkit Numo a few years back and have like it. It's one of the new generation of air mattresses - you blow into it and it fills up like a balloon - very light and compact. I had always thought that just getting you off the ground, so that heat can not move by conduction away from your body into the ground as you sleep, was central to how sleeping mats of all types worked but using the Numo demonstrates it's more complicated than that. Once inflated the Numo probably is about 10 cms thick – a lot more than most Thermarest style mats (a couple of cms) or closed cell foam mats (>1 cm). This makes it super comfy but also allows for some interesting physics – because the Numo is just air inside (thermarests hold air in a complex lattice of open cell foam that it inside the mat) you get convection currents in it. Because the air can move inside the mat as it cools it will move around - not working well as insulation. Alpkit obviously knew this as in the body section of the mat (about shoulder to bum) they put insulation, this was some sort of synthetic strands stuck to the two inside-sides of the mat. When you blow the mat up this stretches forming a lattice structure and stoping convection in that section of the mat. 

Late summer bivvy.
The difference this insulation made is very noticeable – I first sussed this on a wild autumn night in upper Glen Nevis, near Steall waterfall. It wasn't terribly cold, maybe around 5 degrees and I had a bag plenty warm enough. I slept fine but it was quite noticeable that whilst my body was warm my legs (where there is no insulation in the Numo) were getting cold from below – just like the feeling of trying to sleep on ice with a too thin mat. Hence despite being both really comfy and also light and the most packable of my mats, I decided it was best to use it for 3-season camping only. 

In the pictures above, on the left you can see the insulation still adhering to one side of the mat but on the right you can see where most of the insulation has come detached and collapsed back on itself. 

This is issue is compounded by the design problem with the mats – the insulation comes unstuck from one side of the mat and collapses back against the other side hence doing nothing. When I first noticed this with my first Numo, Alpkit in their normal very customer-first way said “no worries, we'll send you a new one”, but then the same thing happened with our second Numo (my wife had discovered how much comfier my Numo was than her old thermarest), and then more recently with the replacement to the original one. I've come to the conclusion that you can get about two weeks use out a Numo before the insulation peels away. I used one of them in that state on my recent bikepacking trip where it was just below freezing at night and even sleeping on the wooden floor of the laavu, I got cold enough from below to wake me up (the first night in my tent in the car park I had slept perfectly on my much thinner foam Z-rest). So the failure of the insulation really limits the Numos to summer use only.

Slightly grumpy bikepacker in the morning twilight after a long chilly December night on a not warm enough mat.
Alpkit admitted that the problem is that when you breath into the mat blowing it up – they aren't self inflating like Thermarests – the moisture in your breath gets trapped and the sogginess inside makes the glue holding the insulation in place fail. Alpkit have stopped making the Numos and aren't going to do any more – they told me they're redesigning their whole mat range for next summer – so of course that makes this whole post sort of pointless: if you don't have a Numo you can't buy one and if you do have one and it fails in the way mine did, Alpkit can't really do anything about it now. But at least I've proven to my own satisfaction that air alone isn't sufficient insulator for sleeping. I guess it has to be stable air that can't circulate, and the problem also shows how small a sealed space -inside an air mattress- is enough for convection currents to have a significant impact on the insulation quality of that mat.