So came the complaint. “I have paid a lot of money for this and expect that it keeps me dry while running on our trails”.

I work for the Adventure Inc, the distributor of Mountain Equipment and Ronhill among several other leading brands. Every winter I get at least one complaint like this. Here is how I explain what are possible contributing factors to this situation.

The Summary:

• Decide very carefully if you indeed need to wear a Waterproof Breathable Jacket or if a shower proof jacket will do.
• When you exercise you produce more sweat than any Waterproof Breathable jacket can handle. So if you are exercising in a Waterproof Breathable Jacket you are going to get wet from the INSIDE.
• You have two choices: you can either slow down your rate of exercise (slow down running or start walking or stop walking altogether) or you can deal with the moisture build up INSIDE your jacket.
• Option 1 is not always feasible when running a race. Option 2 is ok as long as cold does not become a problem.

  

The Theory:

Waterproof Breathable Jackets should all be made out of some type of membrane or laminate. This prevents water coming INTO the jacket form the outside (waterproof) and allows water vapour OUT (breathable). (It has got nothing to do with being able to breathe through a jacket.) This magic layer actually works in conjunction with the rest of the jacket’s layers in order to produce the best performance. The membrane will only work if the outer fabric is dry. Check out my post one why this is important here. Also the membrane works best if not in direct contact with your skin.

If all these conditions are met then we can measure the theoretical breathability of the fabric. (Note we are talking about the fabric breathability and not the jacket’s breathability. We will get to that later.) When you start exercising, irrespective of which brand of membrane you choose your body will produce more moisture (in the form of sweat) than the membrane is able to deal with. So now you have a problem. What happens is that you build up heat and moisture on the inside of your jacket. This in itself is not an issue as long as you are able to continue moving at the same pace. ie if you slow down you will produce less heat, your wet layers will cool you down and you will feel the COLD! This is much more pronounced when you are wet rather than dry.

Waterproof Breathable membranes don’t work by themselves: They need a differential of temperature, pressure and moisture to work best. In other words the hotter and sweatier you are on the inside of your jacket and (more importantly) the drier and colder the outside is the better the membrane will work. But those are hardly the conditions that require a WB Jacket.

Jacket breathability: Manufacturers now take the membrane and produce a jacket out of it realising that the membrane’s breathability alone is not enough to keep you dry. Hence the need for pit zips and other gadgets to be able to vent. This venting is good in part to dump moisture but also heat. Add onto this that you will more than likely be carrying a pack. This will cover half of the jacket’s breathing surface area (your back). Also if you put a big plastic covered map in your chest pocket then this will reduce the breathable surface area further.

So if all these jackets do not actually work well while running in the rain then why carry them? For if you are unable to continue with your run in bad weather. For example when you are walking out slowly or worse have to sit out a rain storm.

Getting wet is not actually a problem. It is getting wet added to cold or wind then you get very rapid cooling and trouble.

So what features do I look for when choosing a Waterproof Breathable Jacket for my next run?

First of all I don’t intend to run in the garment. It will spend 90% of its life in my pack so it needs to be light enough so that I don’t get tempted to leave it behind. It needs to work well enough to get me out of the poo when it hits. This is an emergency layer only. (unless you were at Skyrun last year. And then you were not doing much running in the rain) But I would always still take a windproof layer with as a running layer.

If I was ever that cold and desperate to need a waterproof jacket then I would also want the waterproof pants to go with it.

Breathability figures as indicated in advertising on comparative tests is an issue but not one of the core issues for me living in the Cape. If I was going to need it more often, living in the UK then I would take this into more consideration.

Hope this gets you thinking about how this emergency layer will work best.

Please let me know your thoughts below.