Breathable, Waterproof Hard Shells

Everybody remembers their first waterproof jacket don't they? Especially if you were growing up in the 1970s, it was likely to have been a cagoule - a smock like thing, or if you were posh a jacket with a full length zip and a flap across the back to let some of the sweat out. What we remember was how we sweated at the slightest exertion. A few decades on, we now have jackets made with magical fabrics that let sweat through one way, but not rain the other. Back in the '70s there were waterproof and breathable fabrics but they were rare and hugely expensive. Ventile was such a thing, a war-time invention of a tight weave of very fine cotton whose fibres expanded when damp to effectively seal out water. It was made into aircrew immersion suits, or taken on Arctic expeditions. It's a lovely fabric but heavy.

So how do today's waterproof and breathable "hard shells" work. Whilst there are many manufacturers the fabric of all hard shells exploit the same properties. Let's start with a look at the difference between sweat and rain, they are quite different. Sweat, before it condenses out onto a cold surface, is in the vapour state - think steam. You don't really notice it as it's just a gaseous fug of molecules floating about without any real droplets. Rain is easy to see as each droplet contains many, many molecules - think of a mug of water chucked in the air. So if you can have something that has holes in it that are large enough to pass a vapour molecule through, but not a rain drop, you have the holy grail of waterproof and breathable fabrics.

It turns out one of the properties of a thin film of polytetra-fluoroethylene (PTFE) is that when stretched the structure splits forming zillions of microscopic holes. Fortunately these are plenty big enough to allow our water vapour through, but are way too small for a raindrop to get through. So now we have our perfect barrier, or membrane, to make into a jacket, trousers, survival suit or whatever.

On it's own the membrane isn't much to look at, and being essentially a bin-bag style thin film it's somewhat vulnerable to damage. So to make it look nice and withstand normal wear and tear, it's laminated to other fabrics to protect it. These are called face fabrics and to a large extent determine how the jacket, or other item of clothing, performs day to day and wears over time.

You'll see when you shop around for a hard-shell jacket or over trousers, that they are described as 2, 3, or even strangely 2.5 layer fabrics. This refers to how the membrane is being protected, with the inner surface being the differential.

A 2 layer fabric is where the inner surface has no protection for the membrane, only the outer face is laminated to a face fabric. 2.5 layers means the inner has another coating, like carbon, to give it a bit of durability. Obviously a 3 layer fabric is one where the inner is another layer of fabric which will look after the membrane. (The diagram above shows a 3 layer Gore-Tex fabric with more complex additional protection).

It is important to wash these sort of jackets in the proper way because they are also coated with a DWR, which stands for Durable Water Repellency. This is another essential part of the system, as if the outer fabric wets out, or gets saturated with rain, the microscopic holes of the membrane are in effect blocked resulting in you marinading in sweat again. The inside of the jacket is also under attack from the oils your skin produces, which again can block the membrane. So it is absolutely critical to keep it all clean. Usually this means your hi-tech clothing needs to be washed regularly in a detergent-free soap based cleaning agent, and occasionally the DWR should be restored with a re-proofer. Failure to do so will eventually mean it becomes a very expensive 1970's cagoule again!

I've deliberately not mentioned any names of manufacturers in this article as I wanted to keep to the basic technology involved. Suffice to say that once you do start looking in further detail it can get very complicated indeed.

Schematic of a composite Gore-Tex fabric for outdoor clothing from Wikipedia and used under a Creative Commons CC BY-SA 3.0 licence.

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