Waterproof know-how
How to give your waterproof the love it craves – and repays.
THE MEMBRANE SYSTEM
The vast majority of fully waterproof jackets use the same concept: a breathable membrane sandwiched between a tough, water-resistant external shell and a softer internal lining. The membrane includes millions of microscopic holes which are small enough to stop rainwater droplets coming in, but big enough to let moisture vapour (from sweat) escape to the outer surface. Hence the clunky but descriptive phrase waterproof/breathable membrane. Gore-Tex remains the ‘gold standard’ of the membrane system, but many brands use their own versions which are often just as effective for our needs.
THE DIRECTIONAL SYSTEM
The main alternative to the membrane system is the Nikwax Analogy fabric used by Páramo (Nikwax and Páramo are sister brands, both created by British entrepreneur Nick Brown). Modelled on the ultra-efficient moisture management of animal fur, it uses a pump liner to transfer liquid water (i.e. sweat) away from your body, rather than waiting for it to become vapour and then moving it out, as membranes do. By dealing with moisture more swiftly, you reduce the risk of your clothing becoming soggy. Externally, the fabric ‘directs’ rain and condensation off the outer surface.
WHAT DEFINES ‘WATERPROOF’?
The ‘waterproofness’ of a waterproof is measured by its hydrostatic head, which is the water pressure at which its fabric begins to leak, measured in millimetres. Here’s how it’s calculated in the lab…
1. A strip of the waterproof fabric is placed at the bottom of a pressurised cylinder.
2. The cylinder is slowly filled with water. (Side A)
3. The height of the water column at the point where it starts to seep through the fabric is noted. (Side B)
4. The amount of water it took to reach that height becomes the fabric’s hydrostatic head rating.
To be classed as waterproof, a fabric must have a minimum hydrostatic head of 1500mm (or 1.5 metres if that’s easier to visualise). But most waterproof fabrics in the outdoor market far exceed that baseline, with top-end Gore-Tex having a hydrostatic head in excess of 30000mm (30 metres). But it’s always a trade-off, because a higher rating can reduce the fabric’s breathability. Some examples from our test:
● Sherpa Kunde: 10000mm
● Quechua MH500: 15000mm
● Rohan Ridge: 20000mm
● Berghaus Glissade InterActive
(Gore-Tex): 28000mm
HOW DO I KNOW IF IT’S FAILING?
A functioning waterproof should cause water to ‘bead up’ as pictured above, because the water molecules cannot penetrate the outer fabric layer. If the waterproof fabric has failed, it will ‘wet out’, i.e. water spreads out over the surface in a dark blotch – and you’ll quickly start to feel cold and wet. But it’s not the end of your jacket – see Care and Repair.