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.