Rain shells are a complex and not entirely truthful article of clothing. Their main purpose is obvious – to protect the wearer from precipitation – and for the most part all rainshells perform this task admirably, but they won’t keep you dry. And here the dry rainshell misnomer resides. This article will provide the information you need to understand how it all ties together and why the $20 shell may in fact be better than the $400 alternative.

Invention of the breathable fabric

The original rain shell was a heavy plastic sheet that completely protected against rain but did little to prevent body sweat from naturally escaping. The user frequently found themselves, during high activity or prolonged exposure, soaked completed from the inside. It was annoying to attempt protection from wetness while becoming wet in the process, especially for athletes, and so was born the waterproof breathable (WPB) fabric. WPB fabrics contain micropores that are small enough to prevent water from passing but allow water vapor to disperse. This was an amazing feat. In theory, a use would be protected from precipitation while perspiration was allowed to vent, effectively keeping the user dry and warm. With the ongoing development of WPB fabrics tests with concrete metrics were put in place to measure the effectiveness of their claims. This is done in two ways: Water resistance is measured by the amount of water, in mm, which can be suspected above the fabric before water seeps through. Lab tests involve suspending a section of fabric and filling it with water until it begins to seep. Moisture vapor transmission rate is measured by the rate at which water vapor passes through, in grams of water vapor per square meter of fabric per 24 hour period (g/m2/d). This is tested by stretching and sealing a square metre of fabric over a beaker filled with water, which is then heated to simulate perspiration, and the amount of water vapor escaping is measured. Simply: if the fabric keeps out a ton of liquid water and allows a ton of water vapor to exit, it gets high marks and is considered a good WPB.



Here is a list of the current (2012) WPB fabrics and their test results:

WPB fabrics and DWR coatings

People soon discovered that having a WPB fabric alone was flawed. Water from perspiration or precipitation would collect on the fabric and eliminate its breathability, thus causing the user to ‘wet out’. Wetting out is when a fabric becomes saturated or the breathability is blocked by clinging water particles. You’ll know when this occurs if the fabric stops beading water or if there is excessive clamminess on the inside of the shell. So scientists developed a durable water-repellant finish, or DWR. DWR coatings are a hydrophobic (water-repelling) chemical applicant on the external surface of a WPB fabric. It causes water to bead and prevents water from soaking in or blocking fabric breathability. DWR coatings degrade over time and must be re-applied frequently or risk wetting out. DWR and WPB fabrics go hand in hand and it is important not to neglect the DWR coating.

Humidity and the importance of base layers

A user wearing a rain shell has a microclimate inside their jacket and their perspiration adds to the relative humidity. Humidity is measured as the percent water saturation in the air. This humid air will escape to less humid regions away from the body. But if the air outside the shell is more humid the perspiration will remain locked inside the shell and wet out. Pit zips, loose-hanging ponchos, or opening the front zipper can aid in ventilating perspiration but expect to get wet. To stay warm it’s important to use proper base layers that wick moisture from the skin and provide warm air pockets. A common practice prior to the advent of plastic ponchos was to layer in wool. Wool fibres do not collapse when wet and will wick water for a time, so even when soaked a wearer will have tiny pockets of warm air to stave off hypothermia. Modern materials such as polypro, Capilene, and advances in wools have made the heavy wool sweater a thing of the past but they operate on the same principle. People often make the mistake of adding more layers if they get cold but in a situation where your underlayers are going to get wet it’s best to strip down to the wicking layers + rain shell and increase body heat through activity. When you set up camp, strip out of wet layers and put on dry ones to warm up.

The $20 alternative

In extended rain we want to remain warm and dry and will spend top dollar to do so but we find that in this case a WPB is no better than your $20 plastic poncho. In high humidity it won’t breathe, the DWR coating will degrade on thru-hikes, and oftentimes a manufacturer will forgo pit zips and ventilation strategies to cut back on weight and rely solely on fabric breathability. For cloudbursts a WPB shell provides storm-proof protection with proper breathability while a plastic poncho could leave much to be desired. But for rain that only lasts a few hours is it really worth spending $250+ on a shell? If you are in the mountains, on exposed ridges, or on remote expeditions then I’d say yes, it’s important to be as much protected as possible in these high-risk areas, but for the average hiker spending anywhere from 1-14 days on a managed trail it’s harder to say. If expecting short bursts of rain I can pack a cheap shell or an emergency plastic poncho if I feel bold, but if I’m expecting lots of humidity or constant rain I know I’m going to get wet regardless and will focus more on base layers with a well-ventilated shell.

Get rid of the rain shell?

Some people forgo the rain shell altogether and instead use their wind shirt over wicking base layers. This philosophy accepts getting wet but mitigating the negative effects by blocking heat-robbing wind and staying active so your body temperature aids in the wicking process. For light or moderate rain over only a few hours this is not uncomfortable, but for sustained storms base camp begins to look very good at the end of the day. This option saves weight, bulk, and money but requires experience because hypothermia can set in quicker when hiking wet.