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.