How is waterproofness measured?

When looking to purchase a new rain coat, tent, ski wear or general outdoor gear, it is helpful to know and understand about waterproofing and breathability.

There are two types of waterproof fabrics: waterproof non-breathable and waterproof breathable. Waterproof non-breathable products may be suitable for those wanting complete water protection however may become uncomfortable during wear due to a build up of water vapour in the inner lining. Waterproof breathable fabrics aim to give breathability from the lining whilst also keeping rain and moisture from seeping through the outer shell of the item. This option is more expensive but may be necessary for high intensity activities or tents to allow the material to breath and prevent a build up of sweat and moisture through condensation etc.

Waterproofing and breathability are achieved through fabrics which contain a membrane containing millions of microscopic pores much smaller than water droplets but much larger than water vapour molecules allowing the fabric to remain waterproof on the outside yet still allowing moisture to escape from the inside.

How is waterproofness measured?

The waterproofness of a fabric is usually decided from the results of two tests:

The first is the Rain Room Endurance test where a fabric will be placed in a room where different climatic conditions are reproduced. Leakage is then recorded in relation to the amount of time and pressure of simulated rainfall.

The second is the Static Column Water Resistance Test. In this test a column of water is placed on top of a fabric and the level at which the water begins to seep through is recorded. The results are measured in mm; as such a 5,000mm rating means that the water has permeated through the fabric at the 5,000mm level so it is not as waterproof as a fabric with a 10,000mm resistance.
 
A standard waterproof fabric rating is about 5,000mm but in general this will only be rainproof and not waterproof.

A good waterproof rating is between 10,000-15,000 which unless under serious pressure should remain totally waterproof.

High quality waterproof fabrics are in the 15,000mm - 30,000mm range. These are totally waterproof even under serious pressure. These items will be able to withstand shallow-depth submersion without leakage.

How is breathability measured?

Breathability is measured over a 24 hour period by the rate at which water vapour passes through a fabric. This result is recorded in grams of water vapour per square meter (g/m2) or just "g". As is the case with waterproofness, a higher level of breathability will mean a higher "g" rate. A standard fabric might rate at 5,000g where as a higher end product could reach up to 20,000g.

Tips for buying new outdoor gear:

Taped seams are very important for waterproofness, as the seams and fastenings of a product are the most vulnerable to leakage. A high quality waterproof garment or tent therefore will have fully taped and sealed seams.

Often outdoor gear will be coated with Durable Water Repellent (DWR). This works by encouraging moisture to bead up so it can roll off the surface of an outer lining readily leaving space for the vapour from the inside lining to be released.

Gore-tex is an alternative innovation to be aware of when comparing waterproofing and breathability. This high-tech material is a leading name in waterproofing. It is both waterproof and breathable yet is not measured in the standard mm. It is however thought to be comparable to the waterproof fabric rated at between 20,000-60,000mm.

Other factors to consider:

Most companies test the waterproofness of their fabrics themselves, with different variations of the tests. For example- some will only test the fabrics once, whilst others will continue to test fabrics after several uses or washes.

Additionally, many will test just the fabric and not the completed product or garment. The fabric is often only used as the membrane or laminate in the garment so does not mean that the overall product will have the same mm rating as the waterproof fabric has.

Also, the tests for waterproofness only measure the pressure of water when remaining still and do not take into account the movement of a fabric.

It is generally considered that no test yields results that are completely applicable to real life situations but the key points listed in this article are may be significant when considering purchasing outdoor gear.