Guide to Layering - Part 4

Waterproofs

The most technically advanced outer layers will be waterproof AND breathable, meaning that whilst the fabric prevents water droplets (rain) entering from the outside, it also allows the release of some of the water vapour (perspiration) from the inside. 

So, how does the system work?

All waterproof fabrics have an outer layer (face fabric), its main function is to provide a flexible and durable outer shell. The inside of the face fabric is coated or laminated with a protective moisture barrier which keeps you dry and comfortable in wet conditions. The choice of face fabric and barriers determines the level of brathability and waterproofness. 

To understand why breathability can differ, we must first understand how breathable fabrics work. Fabrics don't breathe, they work by temperature differential. When the inside of the fabric is warmer than the outside, a driving force is created. The higher the differential, the higher the driving force and the more the fabric will 'breathe'. 

Because of the structure of the inner protective moisture barrier, it allows the smaller water vapour molecules (perspiration) to pass through, but prevents the liquid molecules (rain) from coming in. 

Breathability is measured as a rating of how much moisture vapour can pass through a fabric in a given time, usually measured in grams per sq m of fabric in a 24h period. For general recreational use, 10,000gm/m2/24h and above is desirable. 

The efficiency of a waterproof garment will depend on the quality and number of coatings applied and the fabric construction (eg a 3-layer laminate is more durable and efficient than a 2-layer laminate). 

Waterproofness is measured by testing a garment's resistance to water. This is referred to as "hydrostatic head" and means that if a a fabric can stop a 5 meter column of water penetrating, then it has a hydrostatic head of 5 meters or 5000mm (or it exceeds a water resistance rating of 5 meters hydrostatic pressure). 

To be truly waterproof, a garment should also have seams which are sealed against water entry (taped seams). 

Your choice of waterproof garment will depend on your level of activity, as you will need a more lightweight packable garment for ultra light mountaineering or other active sport, whereas extreme protection and waterproofness will be required for high altitude mountaineering.