Monday, 30 September 2013


Me, pressure testing a Nookie Assassin suit, back in the day.
I loved that shirt, wonder where it is?

Waterproof Breathable Fabrics (WBF's) are these days regarded as essential in outdoor clothing. In a previous post I gibbered on incessantly about the breathability aspect - now we're going to talk about waterproofness and how the two figures are expressed.

If a manufacturer quotes the fabric performance it will usually be as follows:


 - which is waterproofness (or WP or Hydrostatic Head) / breathability (MVP or WVT or similar)

Typical figures for a waterproof garment are 10,000 (mm) / 5,000 (g/m2/24hr)

The waterproofness figure in this case, 10,000mm, means that you could put a 10m column of water on the fabric without it leaking, or that you could take it down 10m underwater.  Sounds a lot, eh?

The truth is you aren't going to be even 1m underwater very often, but that pressure could be simulated by the hydraulic pressure of being hit by a wave, for instance. And nobody really knows how much of that we are exposed to. However, experience has shown that anything with a WP rating of over 4000 doesn't really leak at all in use. And that's lucky, because what the manufacturers never tell you is that the seams are not as waterproof as the fabric. 

Your garment is full of seams, and they are tape sealed to make them waterproof. The quality manufacturers use a water pressure testing device to check their fabrics and this can also be used on the seams - generally speaking seams pass the test if they withstand 4000mm pressure. Seams tested in the factory are very unlikely to withstand more than 8000mm, and in any case it's not desirable to test this high on a production garment because of the damage that kind of pressure inflicts on the fabric and seam. 
So, irrespective of fabric rating, straight out of the factory the real waterproofness of your garment is closer to 4000mm, but that's OK - it's more than enough. However, over time repeated flexion of the seams will weaken the adhesive and lead to a constant deterioration in waterproofness. This can be repaired at home with glue, tape or patches until it becomes too laborious to do so, at which point it's time to replace the garment. How long this takes is entirely dependant on usage and care - a pro boater like one of our test team who paddle almost every day might arrive at this point in three months. A recreational user might take three years. It's impossible to say.

Thanks to Ardmel Automation Ltd, The University of Life, The A Level of Intuition, and a GCSE in Making Kayaking Equipment for 20 Years.