Understanding Waterproofs

A guide to the performance of modern waterproof fabrics

You know, once waterproof clothes were just that. Waterproof! You put them on to keep out the rain. Unfortunately these mobile saunas soaked you in sweat as soon as you attempted any simple task. Then came W.L.Gore...

Certainly the first breathable waterproof fabrics were bug-ridden yet they did start the process that was to radically change the outdoor industry, improve the quality of our life on the hill and bring about some of the biggest misconceptions of what a product can do - the biggest of which being that breathable fabrics stop you sweating.

So, what is it all about? First, let us clear up the sweat bit. Without sweat you're dead! A person performing tasks will sweat to cool the body. The idea of a layering system is to remove this sweat and keep the body dry while maintaining the body's "microclimate" (ie the body's normal operating temperature and humidity). All this and keep out the dangerous elements too.

Where do breathable waterproof fabrics fit in? These fabrics do not breathe. What they do is transfer body moisture vapour between its surfaces using the difference between the temperature and humidity next to the body and that of the cool outdoors as a driving force. Wow! Loosely speaking the body sweats. This sweat is picked up by your layering system which transports it to the surface away from your body. Here it evaporates to form a vapour. Because it is hot and sweaty inside your jacket ( ie warmer and more humid) than outside the jacket there is a difference in pressure across the fabric. In an attempt to equalise this difference vapour is driven across the fabric to the outside.

This is a sensitive equation failure of which is often incorrectly put down to base layers not wicking. You see, as it gets hotter and more humid outside the jacket the differential gets less which reduces the force to drive vapour to the outside. It becomes more humid inside your jacket and the sweat is then unable to evaporate away. It collects in your thermals and you get wet. Sometimes the system can overload as you sweat too much and it cannot be removed fast enough. In this case, to stabilise the equation (known as the moisture vapour transfer rate - MVTR for short), stop and ventilate to remove excess sweat. If the former case is a problem then find a good pub to sit in or learn to live with the terrible weather!


How does this remarkable "breathable" fabric keep out the rain?

Breathable fabrics come in three forms. The first is not truly waterproof as it relies upon the close weave of the fabric to keep out water. This means that fluid can still be forced through the fabric in certain conditions. The waterproof properties of close weave fabrics can be enhanced with the use of chemical water repellents. It would certainly appear that the Nikwax Paramo system has overcome the above problems in an off-beat way that has heralded a new look to waterproofs and can be recommended. The other two forms rely upon either the hydrophillic (water loving) or microporous qualities of materials which come as either a coating or a laminated film.

The molecular chains of hydrophillic materials are stepping stones by water molecules. The molecules are passed from chain to chain by the force of the temperature/heat differential, until they are released to the outside. Water droplets cannot pass back across the fabric for it is non-porous.

Microporous materials are created to have tiny holes within their structure. These holes are large enough for water vapour molecules to pass through yet many times too small to allow the passage of water droplets.

Bi-component materials are a combination of both the above materials.


How many ways can a material be made waterproof yet stay "breathable"?

There are six basic ways of creating a waterproof/breathable fabric. These mainly involve spraying a face fabric with layers of coating to form a waterproof coat. The more layers, the more waterproof (and often less "breathable"). Likewise, a plate can be sprayed and the dried coating removed to create a film that can be laminated to a fabric.

The following are brand names associated with specific waterproof methods:

  • Microporous coatings - TriplePoint
  • Hydrophillic coatings - Milair Scantex
  • Microporous laminates - Aquatex
  • Hydrophillic laminates - Sympatex
  • Bi-component coating - Entrant G2
  • Bi-component laminate - Gore-Tex

What is a laminate? A laminate is where a waterproof/breathable film is bonded by a special gluing process to a fabric. This could be to the outer fabric (2 layer) and, in some cases, have a protective scrim bonded to the back of the film to create 3 layer lamination. The fashion industry also laminates the film onto either a lining or scrim to form a drop liner. This allows them the facility to create clothing with a better drape and handle.


How about clothing construction?

If clothing is to be truly waterproof then the seams have to be sealed against water entry. This is normally done by glued tape applied under heat and pressure. Further, many manufacturers are bound to create clothing with certain features such as guttered storm flaps over the zip. These features are designed to take water away from the high-risk areas of leakage. The outside of garments are normally treated with a durable water repellent to prevent the fabric from becoming soaked which would interfere with the M.V.T.R. With use the repellent will wear off and have to be replaced to maintain the garment's performance.


Is there a standard to compare waterproof/breathable fabrics?

Unfortunately not. Most figures quoted by manufacturers are arrived at by different tests, however do not despair. To be classified as a waterproof, garments have to be equal or above those standards laid out by British or European Institutes.

This does not quantify breathability. There are plans to introduce the RET scale which is the evaporative resistance of a textile. Hopefully, in a few years time, we will be able to relate a complete layering system's performance and comfort factor in the same way that we can judge the warmth of something by its TOG rating. Until then...


What is Gore-Tex?

It is a bi-component membrane constructed from pure expanded PTFE which has an oil-hating substance integrated into it. The latter component is has hydrophillic properties. The Gore-Tex membrane contains nine billion pores per square inch! These pores are 20,000 times smaller than a droplet of water yet 700 times larger than a molecule of water vapour which finds it easy to escape through the membrane. The pores will not clog for they are too small for dirt to enter. In fact, the pores are 4 times smaller than bacteria! The PTFE is also protected from contamination by the oil-hating polyalkylene oxide.

For almost 20 years W.L.Gore have refined their product to a point where they offer very attractive guarantees to its performance. Being inert and very stable only misuse and poor construction should impair its performance. W.L.Gore restricts the fabrics use to manufacturers who construct approved garments under license; this maintains high standards of construction. Only you can look after your clothing once it leaves the shop!

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