Why is my waterproof jacket not waterproof?
This is a lament we hear often on our courses and guided hikes, sometimes spoken in frustration, sometimes in anger, and most often at the end of a wet day.
So why is our precious waterproof clothing failing to perform? In many cases it is because we are simply asking too much of it.
The techi bit….
So called ‘breathable’ fabrics have a micro pore membrane bonded to a hardwearing outer layer. This membrane allows water vapour to pass through, but not liquid water. This in turn means that the moisture you produce when working hard is allowed to escape, whilst preventing rain from penetrating your garment from outside.
The ‘breathability’, or Moisture Vapour Transmission Rate (MVTR), is measured in laboratory conditions over a 24 hour period. Manufacturers can then make claims about their materials based on these results.
The problem…
The issue is we are not in a laboratory, we are slogging our way up a damp, humid hillside, and our own moisture transmission rates vary considerably, depending on our activity levels and metabolism, amongst other things.
Exercise vigorously on a wet and humid day and your body will pump out a lot of moisture (sweat), much more than the ‘breathable’ fabric of your waterproof jacket can cope with. Water vapour will condense on the inside of your jacket and your clothing will begin to feel damp. You might think your waterproof is leaking, but it is not, it’s coming from you!
For the water vapour to escape effectively it is important that the outer surface of the material is not saturated with water. To prevent this your jacket is treated with a DWR coating (Durable Water Repellancy), but this coating wears off over time and needs to be replaced. If your jacket is no longer ‘beading’, i.e. the water is forming into small droplets and running off the surface, then it is likely to become saturated, and will ‘wet out’.
This will slow the movement of water vapour through the jacket pores and lead to more condensation on the inside of your clothing.
In addition, humans are not well designed for waterproof layers, any clothing has to have large holes designed into it to accommodate our head, hands and legs, and water can enter through these holes resulting in us getting even wetter!
So what can we do to try and stay dry…?
First and foremost, don’t wear your waterproof jacket if it’s not raining. I see so many waterproof jackets being used solely for insulation and wind proofing, and this leads to a build-up of condensation when exercising vigorously.
Far better to wear a ‘softshell’ jacket if possible. This is essentially a jacket that offers a degree of insulation, can be fully windproof, and will allow moisture to pass through, (in both directions so they are not fully waterproof). None of your moisture will condense on the inside and you will feel much more comfortable. Some softshell jackets are designed to keep out a light shower, but might be less ‘breathable’ as a result.
However, when it starts to rain you must exchange your soft shell for your waterproof shell. Remember to put on your waterproof pants too, or water will run off your jacket and soak your trousers/pants.
Don’t overheat…
If you have to put on your waterproofs in warm weather, then be sure to remove a layer first. By adding an extra layer you are adding more insulation and will be getting warmer.
This will lead to more water vapour being produced by your body and will put extra pressure on the waterproof membrane to perform, resulting in more condensation inside your clothing.
Wash and reproof your jacket regularly…
When the DWR layer wears off and the rain water no longer ‘beads’ on the outside of your jacket it will become saturated or ‘wet out’.
This layer of water on the outside of your jacket will impede the movement of water vapour through the membrane, and more condensation inside your clothing will result, making you feel wetter.
Reproof your waterproof clothing with NikWax Wash-In, or a similar product, on a regular basis. The more you use your jacket the more often you should treat it.*
Check on the build quality of your jacket and pants…
The material may be waterproof but rain can still penetrate through seams, pockets, zips and those large openings for your head and hands!
Check that the seams are sealed, or ‘taped’ on the inside, some top end jackets now have welded seams.
Consider waterproof zips when you purchase your waterproofs, both for your main zip and ancillary zips on your pockets and elsewhere. These are much better than conventional zips at keeping water out, though more expensive.
Check that the inside of your pockets are made of the same waterproof material as your jacket. If your pockets do not have waterproof zips and the pockets lining is not waterproof then you are going to get very wet around your midriff.
Make sure the hood fits well around your head and can be pulled in tight without impeding vision. Check that the cuffs are adjustable, and can be closed down with Velcro to reduce the amount of water running up your arms!
Don’t buy waterproof pants with simple slots for pockets, this will just let in water and soak your underwear.
Avoid cotton T shirts and underclothes…
Cotton will absorb up to 20 times its’ own weight in water, and the hollow fibres of cotton won’t release it easily, so it stays with you and makes you feel cold and clammy. In colder conditions this can also increase the risk of hypothermia.
By wearing synthetic wicking layers the moisture your body produces will be moved to your outer layers and away from your skin, making you feel much more comfortable.
It is not possible to stay completely dry when battling through horizontal rain on a windswept mountain, but by caring for your waterproof jacket and using it appropriately you can stay comfortable, if a little damp, even in the worst of weather.
*I’m often asked how often you should reproof a waterproof jacket, and the answer is not that simple. If it’s getting dirty and the pores are clogging up then wash it and reproof it. If it’s no longer ‘beading’ and water is soaking into the material, then reproof it. In normal use I reproof my waterproofs every couple of months. (But I use mine quite a lot).
Thanks To Russ Mills from Mountaintrails for permission to share
