So your heading to your local outdoor store to do some shopping before an up coming trip. You look at waterproof jackets and notice they all have ratings to determine how resistant they are to the weather.
Lets have a look at what they mean.
A waterhead rating is found on fabric items, it is used to determine the point at where water passes through the weave of the fabric or membrane.
To obtain a waterhead rating a sample of the fabric is placed on a 1 inch diameter tube. The tube is then filled with water until it passes through the fabric. A measurement is taken at the height of the column of water, for example a 1,500mm result would simply mean that the one inch diameter column was 1500mm high. A 20k waterhead rating, you guessed it 20,000mm high column of water.
A waterhead rating is not the end of the line though, other factors will come into play to determine how waterproof a jacket may be.
A jacket may have a high waterhead rating and still leak like a sieve if the seams of the fabric are not sealed. The seam is where the fabric has been stitched together, most seams are generally double stitched then a waterproof tape is applied over the top to plug the holes created by stitching the item together. Cheaper jackets may not be taped or sealed by any method and provide a point of weakness to water to enter. Some manufacturers apply a seam tape or seam sealer to critical points only, leaving the other areas to leak.
Zips are another point of entry for water to get into your jacket, many manufacturers use waterproof zips or waterproof zips with a storm flap over the zip to reduce to volume of water the zip is exposed to.
Breathability is the ability of the fabric to move water from the inside of the jacket to the outside of the jacket. Why is there water on the inside to begin with? Well, as you hike you will begin to perspire, if your jacket doesn’t breathe then that perspiration will build up on inside the jacket and you will end up just as wet as if you didn’t have it on.
Breathability is measured in grams of water vapor that can pass through one square meter of the fabric in a 24 hour period.
The result is reflected in a figure similar to a waterhead rating, you may get a result of 15000 which means 15,000g of water vapor, per square meter, over 24 hours can in theory escape the jacket.
There are some factors that will reduce the breathability of the jacket, if the jacket is dirty then those pores that allow the sweat to escape will clog and prevent the jacket breathing. The humidity of the area its being used will also affect the breathability. Part of the process relies on the humidity inside the jacket being higher than outside. So if you are using a jacket in the tropics it will breathe less than if your are using it in a drier humidity environment.
DWR or Durable Water Repellent, is a coating that is applied to the outer layer of a jacket. The purpose of this is to prevent that outer ‘face’ fabric from becoming wet and the jacket wetting out. DWR wears off over time and water fails to bead off the fabric, instead it soaks in. DWR can be reapplied through wash in methods or spray on. Heat in the form of a warm iron or tumble drier then sets the DWR into the fabric.