Saturday, 31 August 2013

So how does fabric breathability really work?

Is it really possible for a fabric to keep the water out while letting perspiration escape at the same time?

Good question... 

I originally wrote this article for the Nookie website, and it may have appeared in Canoe And Kayak UK magazine as well. It's reared its ugly head again because I was doing some research on sailing gear recently, and some of the comments on the forums amused me - it was like stepping back in time. 

Back when I first became interested in technical outdoor gear, it was still a widely held belief that waterproofness and breathability were inversely proportional. A lot of ocean sailors and fishermen seem still to believe that, while yet bleating about how wet they get inside their oilskins from perspiration!

There is a mechanism by which a waterproof material can still allow water vapour to pass through it. The physical chemistry of this is fairly horrendous so I'll gloss over it and say that it's called diffusion and was defined by a clever chap called Fick, and you can read what he worked out here if you want your head to hurt a lot. The way a waterproof barrier can be made to allow diffusion varies and is even more complex than the inner workings of Fick's mind, but they fall broadly into two categories - microporous and hydrophilic. Both types of barrier allow diffusion but microporous ones being a bit more natural do so fairly simply, following Fick's hypothesis to the letter. Hydrophilic barriers have an anomalous diffusion mechanism, using transient hydrogen bonding of water molecules to complementary functional patterns on the polymer chains. It is sometimes described as a ‘‘molecular stepping-stone mechanism’’ but I think a better way to visualise it is that the water molecules are passed from one place to another like a bucket passed along a line of people. You don't want to know how though... you really don't! 
Luckily for us both of these methods work pretty well. But, how breathable do the fabrics need to be? Well, truth is nobody knows. We can work out how much vapour we need to shift at a given workload. But estimates of how much of that is lost through the head, or on the breath, vary enormously. And the laboratory tests that indicate how much vapour will pass through the fabric per unit time are, frankly, a bit rubbish.
WVT (water vapour transmission) is technically (SI units) expressed in kilograms (of water) per square metre per second. However, the so-called "trivial unit" of grams/m2/24hrs is the more common measure of breathability in industry. The figure that the manufacturer gives, however, is obtained in a laboratory test rather than under true biological conditions. There are at least three International Standards that can be applied to WBFs, each using different test conditions and expressing results in completely different ways. The ‘‘sweating hot-plate’’ method, called  the skin model (ISO 11092:1993) was developed from research into the effects of clothing on human thermoregulation under varying workloads, and yet the Standard test parameters (35˚C; RH 100% to 40%) are pretty meaningless for outer garments worn in real-life situations.
The problem is that the barrier doesn't just breathe on its own. It requires a vapour pressure gradient between the inside and the outside. And that depends on temperature, relative humidity (RH, above) and a bunch of other stuff. You could force yourself to understand Fick's hypothesis at this point, or you could just believe me. Either or...
Just because a fabric has a high breathability score in a lab test, doesn't mean it's an oustanding performer in the field. Because in the field the figures are all substantially different. And we wear one or more layers of thermal clothing, the performance of which is as or more important than that of the waterproof shell! In many wet conditions, the thermal and vapour pressure gradients across the actual waterproof fabric are negligible, so any vapour transmission will be approximately nil too, irrespective of any other performance characteristics. But the manufacturers have to say something, so they quote what they can. There is, however, no substitute for human testing in real operating conditions, and this can reveal surprisingly excellent performance even in counter-intuitively difficult circumstances. In some conditions, a good waterprooof breathable shell CAN expel all the moisture we need to for thermoregulation, and be dry inside at the end of the day. I recommend strongly that you test garments before buying, or buy something a trusted fellow has used and liked, or at least buy from a manufacturer you can trust, rather than one that quotes spurious and irrelevant lab results in an attempt to garner your interest.
Before I go, a few myths to dispel. Microporous fabrics DO NOT leak, or clog up with salt. The diffusion process CAN reverse and breathe backwards, but if the circumstances for this persist for more than a few seconds you are probably already dead or something very weird. The fabric being wet or frozen DOES NOT in itself stop the diffusion process, and can in some circumstances accelerate it. And the fact that other gear like a spray deck/skirt, buoyancy aid/pfd, or backpack are covering vast swathes of the breathable area may not help, for sure, but it DOES NOT completely stop the fabric from breathing there.
So, there you go. That's breathability. Check back soon for a rant about waterproofness. See ya!

Thanks btw to the Royal Society of Chemistry, The University of Life, The A Level of Intuition, and a GCSE in Making Kayaking Equipment for 20 Years.

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