Monday, 9 November 2020

My waterproof jacket still leaks!

Just on 6 years ago I posted an article on my old blog "Running Late" entitled "My waterproof jacket leaks!" I've noticed a bit of a swell in Facebook discussion on this topic again recently, so I thought it was maybe time to revisit this, update it as we're always learning of course, and hopefully make it a more concise and easier read. So here goes.

You'll have seen the discussions of course. Runner A recommends a jacket as the best he's ever used, then five minutes later Runner B says he had one of those but it leaked like a sieve. They can't both be right............can they? And nowadays we get instructions that we must have hoods, taped seams, this, that, whatever; Joe says how can he find the lightest job that will still satisfy the kit inspectors while Bill says take the heaviest you can carry or you'll die. What's it all about?

Well, for what it's worth, here's my take on the subject. The usual health warnings, my views come from a combination of experience in the hills and training as an engineer; I hope they make sense, I'm sure not everyone will agree, but if they prompt a bit of thought that's enough and OK. Also this is an area where it's easy to get bogged down in the science; I'm hoping to get by with only the most  important bits and I might take some shortcuts which will offend the purists, but I'm sure I'm like most runners in just wanting to understand why things that affect us happen and what we can do to try and control them. On the other hand, forgive me if I'm explaining things you already know, but not everyone may and it is really important to understand a bit of the physics behind what is happening. And if you've read my blog before you'll know that these odysseys of mine can go on for a while, so be prepared if you're going to stay the course.

Before we start talking about jackets we need to start by covering a few of the terms that everyone uses but maybe not everyone really understands.

1. "Sweating". This is your body's way of dumping unwanted internal heat generated by physical exercise. A fluid that is mainly water is pushed out through small holes in your skin (pores), carrying some heat with it. What happens next depends on external conditions.  If you're wearing no clothes and the air humidity is low, the water (sweat) will evaporate, gaining latent heat from your skin as it goes and so cooling you down more (when water turns from liquid into gas - it "evaporates" or "boils"  -  it needs a heat input to bring about this change of state without changing the temperature; this is called the "latent heat". In a boiling kettle this heat input comes from the power you put in via the element or burner, in evaporating sweat the heat is drawn from your body, which then feels this heat loss). That's the way it's designed to work. But when your body gets hot and generates sweat, it can't control what's happening on the outside. If you wear a technical "wicking" shirt, the sweat gets drawn through to evaporate on the outside, almost as well as without a shirt. A cotton shirt will absorb the sweat, but it will eventually start evaporating from the outer surface, although you won't feel so comfortable. A rucksack tight against your back will prevent any evaporation at all in that area so you will have a wet back. But if the air outside is already saturated  -  that is, it has already absorbed all the water vapour it can hold, a condition known as 100% Relative Humidity (RH)  - then sweat won't evaporate whatever you're wearing.  But we should say here that the air won't normally have reached 100% RH until you see visible signs of it; the water vapour starts to condense in the air, and you're in mist, fog, a cloud (or the bathroom). The fact that it's raining alone does not mean that the air is saturated (but for sure if there is rain about the air will be already holding more moisture than it would have under a cloudless blue sky). But the closer the air gets to 100% saturation, the less of your sweat evaporates; what doesn't evaporate stays on, in, or inside your clothes, making them wet.

2. "Breathable".  This is generally accepted to be a material which has "holes" small enough to prevent water droplets passing through while big enough to allow the passage of water vapour. This isn't the precise truth, but it's good enough for us here. It doesn't matter what the material is, or how the "holes" are created, these details might affect the overall robustness of the material but not the explanation of how it might work. The theory is that you can wear such a material when it's raining; sweat then evaporates on the inside of the jacket and so can pass out through the holes, which at the same time are not letting the rain in.

3. "Waterproof". A string vest is not waterproof. Wear one in the rain and you'll get wet. A plastic material can be made completely waterproof - think of the bladder in a Camelback - and if you make a jacket out of it, it won't let the rain in, but it won't let any vapour out either, under any conditions. What we think of as modern "waterproof-breathable" materials are a compromise between these two. The smaller the "holes" the more they lean towards waterproof, bigger holes mean more breathable. These materials should more correctly be referred to as "water resistant".  Furthermore, a material that is resistant to a steady rain may not be so resistant if the pressure of the water goes up  -  a tropical downpour maybe, or if you sit in a puddle. That's why modern jackets sometimes have some sort of hydrostatic rating on the label  -  a measure of their actual degree of waterproofing. As an aside, it's also why you see the frequent claim that waterproof socks "never work"; the pressure created by having them continuously immersed in wet bog for a few hours is far greater than if the material was just subject to rainfall.

So now we have these mechanisms nailed down a bit let's look at what actually happens to you when you're out in the weather.

Let's say you are out running on a coolish but not cold day, wearing a long-sleeved technical teeshirt, and it feels pretty comfortable, not too hot, not too cold. The heat you're generating is being dissipated by your sweat into the air. Your systems are in balance. Then it starts to rain steadily. It's not unpleasant to start with, but after a while you start to feel cold. What's happening here is that the raindrops hitting you and then eventually dripping away are more efficient at conducting the heat away than the air was  (we'll get to this in a bit more detail later on), so now you are losing more heat than you're producing and you start to cool down.

There are three options from here. (i) You run faster to create more heat to warm up, (ii) you increase the insulation by putting on a thicker layer, say a fleece, or (iii) you pull on a waterproof. Intuitively, the best option is (iii) so on goes your lightweight waterproof but breathable jacket. This sheds the raindrops much more quickly and efficiently than your teeshirt so it cuts down the heat transfer, and this enables you to warm up. But as you warm up, you start to sweat again, and now the sweat has a much harder route to escape and evaporate. First of all, it now has to evaporate not into unrestricted air, but into the small space between your jacket and the layer inside it. And when it reaches the shell it (a) has to find its way through the restricted holes in the surface, and (b) even if it gets out it will be faced with air more humid than before you put your jacket on. Sorry, but a lot of your sweat is either not going to evaporate, or evaporate then re-condense as it hits the inside of the shell. You probably still feel quite comfortable, because the heat being taken away now matches again that which you are producing by the running effort, but inside your jacket you are unlikely to be dry.

Most outings in the rain start and end like this. When you stop running you get into a warmer place, change your clothes, and think nothing of it. Your jacket did a good job. But on a longish run, there may be the opportunity for a few other things to happen to affect the balance again, such as:

- you get tired, so you slow down to a jog or maybe even a walk, so you're putting less energy (and therefore less heat) into the system, but the outside conditions are still taking out what they did before.
- the weather gets significantly worse, heavier rain, lower temperature, higher wind, so that although you're still putting in the same energy (heat), the outside conditions are taking it away faster.
- it's a long time since you last ate anything, so more of the work you're doing gets used internally and less goes to warm the surface.

The first thing you realise in any of these scenarios is that you're getting cold. Then comes the realisation that your clothes are wet, because instead of being warm and wet which was comfortable, they are now cold and wet which isn't  -  "my waterproof jacket leaks!" Yes, I'm prepared to stick my neck out and say that so long as you're wearing a jacket that claims to be in some way waterproof, then 99% of the moisture on the inside will have come from you, not the sky. OK, nice to know, but once you get into the "I'm cold and wet" scenario experience shows that you're on a downward spiral to ending your participation in the race (and in anything else if you're really unlucky), so what can we do about it? Before we go on to that, let's just home in on a couple of points that are key to the situation.

1. The first and most important thing to realise, which I hope I've drawn out by the little scenario above, is that when you are running in the rain, the inside of your jacket will ALWAYS be wet. So, you may say, what's the point of wearing it? Well, this is where we come to the "lightbulb" moment. This is that the overriding demand of your clothing system when running in bad conditions is not that it must keep you dry, but that it must keep you WARM.  Being warm and wet is not unpleasant (think of a warm shower) and it keeps your core temperature where it needs to be. But being COLD and wet is not only unpleasant but eventually dangerous. You need to get away from the idea that you are dealing with a waterproofing problem and understand that it is all about minimising heat transfer.

2. Breathable clothing was originally developed for activities less intense than running, such as walking, climbing and the like, and was found to work well. But these pastimes are less energetic than running and less sweat is necessary. Walkers and climbers would also not normally use a waterproof shell unless it was part of a three (at least) layer system - base layer wicking, to keep sweat away from skin, middle layer insulation (fleece, etc), then outer shell to keep rain or wind out. The whole system is under far less stress than in running, and the wicking/insulation layers hide any deficiencies better - they keep you feeling warm. Further, these activities are often intermittent, where bursts of energy are interspersed with easier or inactive periods allowing any accumulated moisture to evaporate while none is being produced. Finally, walkers and climbers (sensible ones at least) layer up and down frequently to keep in balance with changing conditions, whereas a runner will begrudge the time spent in doing this.

So why do we start feeling cold when we get wet? Well, your body temperature needs to be kept at around 37 degC or it doesn't work very well. Normally in our sort of latitudes the outside air is colder than this, so potentially, you are always losing heat. But also normally, this heat loss is mitigated by (a) wearing some clothes (insulation) and/or (b) generating some internal heat by food and/or exercise; this pushes the body temperature up so dumping what is not required is not only OK but essential. If the imbalance is small the excess heat is removed from your skin surface by conduction/convection in the air; if you overheat the further excess is removed by sweating and evaporation as explained earlier. The sweating/evaporation is needed because air is a poor heat transfer medium. Water on the other hand is very good at transferring heat, something like ten to fifty times better than air depending on the exact situation (for example that's why the great majority of car engines use water rather than air as the primary coolant). So as soon as you get a continuous layer of water between your skin and the outside air you can really start losing heat in a big way pretty fast. On top of that, if it is actually raining you have a continuous source of new water which is colder than the outside air temperature (because it condensed into rain at a much higher altitude than you are at now) hitting the outside of your clothes.

The ONLY way you can stay comfortable and safe is to manage the balance between the rate you  are producing heat and the rate at which the outside conditions are trying to take it away.

So where has all this got us so far? You know now that I'm sceptical that you'll stay dry inside any waterproof jacket, whatever it claims, when you run in it. You know that being wet isn't in itself bad, but being wet (especially if it's raining) dramatically increases heat loss, and if you can't manage that heat loss you can get into big trouble. So is there a strategy that we can adopt that might keep us out of jail? Well, for what it's worth, here's mine.

1. Choosing and maintaining a waterproof jacket

I personally don't think there's an overall winning brand; after all they're all using the same science and mainly similar materials. I've tried a lot of the reputable guys, North Face, Salomon, Montane, OMM and can't really detect any difference in performance. Their designs are better thought out than cheaper jackets but their quality depends on how well they manage their manufacturers and they're not all great at that. Better to go with what fits you best and has the features you personally think worthwhile (pockets, hood design, cuff details, etc). If you're not fussed about design details then cheaper brands can often work pretty well. I have run in some nasty conditions in less costly jackets and they have not let me down (to be clear, I'm not saying here "go out and buy a cheap jacket", but rather make sure that what you are getting for your money makes sense to you after a critical technical appraisal). I'm not sure that I buy into the taped seams thing but a lot of Race Directors require it and all of the reasonable makers do it anyway. But whatever brand you choose, think about the following.

(a) The outer surface needs to be kept in good condition so that it "sheds" rain as efficiently as possible. Manufacturers apply an outer surface which effectively makes it more "shiny"; if you hold your jacket under a tap the water should immediately form "beads" and run off. But this surface is subject to deterioration over time, think of it as "rubbing off". This will be happening all the time due to abrasion, even when the jacket is rolled up and in your pack. You can delay it by careful handling and regular washing, I store mine on a hanger at home and only keep them rolled or screwed up as little as possible. When the surface has deteriorated badly then water will appear to be absorbed into the surface rather than running straight off, an effect sometimes referred to as "wetting out" and this condition increases the heat transfer significantly. This is the time to restore the surface by application or washing in of an appropriate treatment (I use Nikwax Wash-in but there are others). When people refer to "re-proofing" of modern breathable clothing, this is what they really mean, restoring the shine to the outer surface.

(b) The "breathable" performance of the jacket will also deteriorate over time because elements of your sweat that are not water will gradually clog up the pores in the garment. I have stressed throughout that a lot of sweat doesn't get out anyway, but you really want whatever has a chance to get through, otherwise you might as well be running in a plastic bag. Again, regular washing is the key.

(c) Everything in this game is about compromise. Lightweight, flexible materials can be made just as waterproof as as heavier ones. Heavier materials have a rigidity that may not feel so great to wear but in bad weather will keep the jacket away from what is underneath in a lot of places, creating air gaps. These air gaps are your best possible way to slow down heat transfer. Conversely, the lighter weight materials will tend to "cling" to the garment underneath (often aided by the wind) to give a much more continuous contact surface with no air gaps, so faster heat transfer. If you can afford it, it's worth having a lighter jacket for warmer summer events where bad weather is just a possibility, and a heavier one for winter when the temperature difference between you and the air outside (and therefore the heat transfer rate) is much greater and bad weather is more or less inevitable. 

2. Don't put on a waterproof unless you really need to

It doesn't matter how brilliantly technical your jacket is, it will be nowhere near as breathable as if you don't wear it. The sweat inside starts building from the moment you put it on. I'm amazed at the number of runners I see in events putting on waterproofs at the very first sign of rain, even if it's clear it's only going to be a short shower. I'm quite happy to carry on in a baselayer in gentle rain, or even a light fleece if it's chilly. A light windproof which weighs nothing in your pack can often be a much better bet in gentle rain than a full waterproof. I always leave the decision to break out the waterproof until it's clear that the rain has really set in and I eventually have to choose between "cold and wet" and "warm and wet". As soon as the rain stops, unless you are cold, I think it's worth getting your jacket off as soon as possible so that what's underneath has a chance to dry off and get you back to square one.

3. Once you've got a waterproof on, manage the layers

This won't be popular with the faster runners, but less necessary for them either as they continue to generate plenty of heat energy throughout the race. But if your pace drops to a jog/walk or the conditions worsen, be prepared to manage several layers underneath your waterproof. You have to play with the level of INSULATION you are wearing to get back into balance between heat generation and heat loss. Insulation means getting air-trapping fabric layers between your skin and the inside of your shell, to slow down the heat transfer rate. They will all get wet but the non-absorbent fibres and the air trapped within them will do the job. Thicker layers will insulate more than thin ones. This might mean having long and short-sleeved base layers (either worn together or separately - the  increase in well-being you feel from not having your lower arms pressing directly against the clammy inner surface of a jacket can be quite dramatic), a light fleece, a light primaloft or similar pullover or gilet, etc. The exact combinations you decide to take and use is very individual depending on how "hot" you run and other variables, including the conditions you predict for any given event. Personally I have found that a good base layer (something like a Helly-Hansen Lifa rather than a conventional running tee) and a light fleece will see me through all but the wintriest of conditions so long as I can keep moving reasonably well, but I always keep another layer in reserve if the event is particularly long or committing, just in case.

4. Don't stop

I don't mean don't stop for the odd cup of tea or jam sandwich, but you have to realise that unless you can stop in a warm place, you really change the heat balance dramatically by stopping. Even walking you are still generating a lot of useful heat. If you stop you'll lose a lot of heat and start shivering fairly quickly, so get going again and get the balance back before you lose it completely. This means that if you have to stop, say to get out a torch or some food, or another layer, you need to manage it so you choose a relatively sheltered spot if possible and are stopped for the absolute minimum time necessary. This means having everything organised before you start. This is not the time for a "now where did I put that pork pie?" moment.

None of us have all the answers so we should be prepared to learn all the time.

For completeness I better add just before closing that I have limited this piece to the performance of  recognised "hard shell" jackets, because they are generally on the kit requirement of all events these days so you have to carry one. Whether you then choose to combat the elements using a system not employing a hard shell, such as Paramo or Buffalo. would be the subject of a whole different discussion.

Wet outings where you get cold can turn into varying degrees of misery. Wet events where you manage your temperature and are comfortable can be strangely satisfying, maybe even enjoyable. As Brits, we ought to be able to manage a bit of rain  -  we get plenty of practice.

No, your waterproof jacket still doesn't leak.

Finish of UTMB 2010 (re-routed due to bad weather). Base layer, light fleece and Regatta jacket.


  1. Well written post as ever Mr Cole.
    Keep up the good work

    Dan Milton

  2. Great article, I carry a hard shell just to meet the tick box. I wear Paramo if cold with base later if not vented and next to my skin.

  3. Excellent article. Nice to see some science based discussion for a change! Thanks.

  4. Thanks. Very helpful, although it makes me question the wisdom of adding weight to ensure a higher level of waterproofing if sweat build up is inevitable anyway. Also, I guess wind is also a major factor to bear in mind when it comes to cooling?

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John Kynaston

The last contact I had with John was early this year when he left a comment on a blog post I had just published  -  "Great write up as ...