Of course, without giving it much thought, the obvious answer is to keep us dry. When we give it some thought and consider all the different scenarios in which we would use rain gear, we see that it isn’t necessarily being dry that is important, and often, that being dry is not only impossible, but counterproductive. Particularly as the level of exertion rises, the more unreasonable it becomes to stay dry. If you have moisture falling outside and your exertion is high enough that you’re producing heat, your body will sweat to cool itself down. So, you have moisture outside and moisture inside. Not only is it going to be impossible to stay dry, but it also wouldn’t even make any sense. Why perspire a bunch of your body’s moisture out under a rain shell? You’d be a fool to not take advantage of the external moisture to cool your body and conserve your body’s water.
So when we boil it down, most often, the real point of rainwear is to keep us warm. It can be to keep us warm in the moment, and also to keep our insulating layers warm for later. If we approach rainwear from this angle, it opens up a lot of different potential and shifts our actions and practices around it. So, the main questions become “How do we use rainwear to keep us warm and preserve our insulating layers?” and “What type of rainwear is best in what type of scenario?”. Let’s list out some scenarios and speculate on what might be the best approach.
Sitting around in cold rain or snow with static insulation (like a down puffy) as mid layer insulation under a rain shell. This scenario likely favors a waterproof breathable membrane fabric. An impermeable rain layer would tend to accumulate internal condensation because the mid layer is trapping the body heat near the body and the surface of the rain layer is cold. Any internal moisture is going to want to move away from the heat and condense onto a cold surface. This condensation would then soak back into the top of the down puffy. Down is susceptible to loft degradation from moisture. A WPB membrane might be able to move enough moisture from inside to outside to mitigate this effect a bit, but I suspect that there would still be some condensation issues. If the mid layer were less insulative and more hydrophobic, the scenario is much better, for either type of rainwear. This allows more heat to reach the surface of the rain layer so that heavy condensation is less likely, and what little there is will not soak into, or degrade a hydrophobic layer as much.
Running in cold rain, with significant exertion. This is really jumping straight to the other end of the spectrum and really tends to favor an impermeable rain layer. There is going to be a significant moisture load coming from both the inside and outside. We’ve sort of established above that if it is warm out, we really should remove all rain layers and allow the moisture to cool us. It is only when the moisture is putting us a risk of being cold, that we need to consider using a protective layer. During the times when it is warm and we are going to “embrace the wet” we would want to ensure that we are not getting essential insulating layers wet, but just getting fast drying, hydrophobic layers wet. Something like base layers or an Alpha Direct hoodie that can basically be shaken dry. Or better yet, only use the impermeable rain layer over your skin. This can provide some warmth and protection, without getting anything soaked. When exerting at higher levels, there is inevitably going to be considerable buildup of moisture inside. So why not a WPB membrane here? Well, for a few reasons. None of these WPB membranes have anywhere near enough moisture vapor transmission to keep up with exertion. Even the very, very best are only capable of keeping up with pretty low-level activity, at best. So, at a running output, these would see the same internal moisture build up. So why not have the WPB there to do what it can and then mitigate the build up from there, just the same as you would with an impermeable? Because many WPB fabrics come with pretty heavy drawbacks that aren’t worth it when you consider the small effect they would have in a scenario like this. The obvious ones are that they can be heavy and fragile. Susceptible to abrasion degradation. However, I think the biggest drawback to many of them is their tendency to absorb water. In situations like the one described above, there will be a heavy moisture load on both sides. The layers present in many WPB fabrics will soak this up and hold it. This will make a heavy shell even heavier, and it will keep you cold for long periods of time. Not all WPB does this, but many do. An impermeable layer can’t absorb any water, so it holds strong as a barrier between the moist and cold outside and the moist and warm inside. Mechanical ventilation becomes key to mitigating the internal temperature and moisture level, but there is little long-term consequence since the layer can’t absorb moisture. Just the same as we would do when “embracing the wet” without a rain layer, we would also want to choose a mid / base layer that can get wet without major consequences. You wouldn’t want to use a critical, sensitive static insulation layer in this scenario. You’d want a base layer or Alpha Direct hoodie or something.
So we’ve outlined what I think are clear winners on the two opposite ends of the spectrum. There are a lot of scenarios in between to discuss so this page is a work in progress. More on this later……