How do humidity and air movement affect the performance of breathable jackets?

Question

In what ways do humidity values or air motion affect the performance of breathable waterproof jackets?

Answer 1

Humidity and air circulation disturb the normal function of moisture-wicking jackets. When there is low humidity, the air has the ability to compress water vapor molecules thus making the jacket breathable. However, in tropical regions, for instance, rain forests, sweat can hardly escape from the jacket because the surrounding temperature is too humid for evaporation to occur at a high rate. Most importantly, if there is no airflow, there is nothing to evaporate off the skin. However, jackets are supposed to provide partial wind penetration because that is what they’re designed for. Components of jackets such as pit zips can create ventilation; however, one must question the need for a breathable jacket if the user must use ventilators.

Comments

In NZ, the places we hunt often average 200+ rain days per year, and it looks like the jungle from Jurassic Park (it literally is).
We wear wool. It stays warm when wet, and we don't even pretend to attempt to stay dry. Rain jackets are for cutting the wind basically.
It's also quiet when you brush again stuff unlike all the synthetics. If its a really really bad storm, an old school fisherman's yellow PVC rain jacket outperforms anything.

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I grew up scouting in the pacific northwest and our leaders use to say things like "No such thing as waterproof and breathable" or "Prepare to be wet and warm, because you can't always be dry". It was great reading this content that explains the science behind that old northwest wisdom.

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The beauty with all those membrane jackets is that they kind of work under different circumstances. A membrane jacket will protect you from the wind, it might give you a little bit more insulation when it's cold and it will keep the rain off if it's raining. So it's a multi purpose piece of equipment. However you need to know its limitations. This is transporting sweat caused by intense physical activity when it's wet outside - and worse wet and warm.

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I'm a former REI sales specialist in camping and outdoor clothing. I was geared up with all the best equipment and started out on a long Appalachian Trail thru hike and quickly realized that GoreTex and other similar fabrics were indeed useless in the rain soaked conditions on the trail in the humid south. The best thing was to just suck it up and expect to get wet and hope to dry out later on. Open up the vents and pit zips and flush as much air through the jacket as you can. The GoreTex shoes were also useless so again just expect to get wet, live with it and know that you can dry out later on. If you encounter a quick and short shower then you may be protected if the shower doesn't last too long. Often just wearing GoreTex outer wear gets one hot and sweaty and it basically begins raining on the inside.

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“When humidity is high, these jackets don’t seem to breathe at all.”

This was my experience with Gore-Tex hiking the West Highland Way in Scotland. I stayed dryer wearing shorts, sandals, and a large cheap poncho than my fellows hikers in full length head to toe waterproof gear.

(Note: Poncho goes OVER the backpack)

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I own an Outdoor Research Goretex rain jacket and can confirm-- it does a pretty good job keeping you from getting soaked by rain, but it's certainly not dry inside. I always feel clammy underneath especially in the arms. I'm in Iceland (cold maritime climate) and even when I use it as a wind shell, it can feel clammy inside on cold, humid days as well. It works best when you're not exerting yourself (just sitting or walking a bit), or as a wind shell on dryish days, but once you start doing work, it always ends up feeling damp inside if it's at all humid outside.

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Love this content and actually it explains how I’ve felt with my goretex jacket through my years of use.

1. Performance in heavy rain - meh, keeps water out. But yes with sweating or anything condensation sits in the jacket

2. Performance for high winds, alpine conditions surrounded by snow where you want to have breathable but shell like barrier to make sure any melting ice or snow is not getting you wet. I’ve been glad to have it

The experience above makes sense now with the content

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This content explains exactly my experience of 20+ years of hiking in the Scottish Highlands, one of the most humid places on Earth. GoreTex (and all those related fabrics) are basically useless. Got myself a light umbrella. Worked perfectly! 100% waterproof and 100% breathable!

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When a daily downpour broke in a Central American jungle, we broke out our Gortex the guide broke out an umbrella. Two hours later we were soaked from the inside out while the guide was wet only from the knees down. Rainwear is just another tool to be used at the right time. Best mitigation though, pit zips! Don’t buy a “waterproof” jacket without them.
Great content backed up with facts.

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Note that even at 100% humidity your clothes will continue to dry out on you when the air around you is colder than your skin as the boundary layer warms up and can absorb more water. In practice I find 100% humidity fine at 20ish degrees C as those translate to roughly 50% at ~33C temperature of your skin.

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Your content points out exactly what I learned from guides in Hawaii. I would ask them about the clothing they wear and the most experienced would tell me to forget about the claims of how breathable a fabric is, get pants and jackets with big zipper vents because air circulation is the cat's meow. The more you are moving, the more you sweat, but also the more you are forcing fresh air through the vents. Another huge difference was switching to a mesh ventilated backpack.

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Just going to share an experience that may be helpful to others. Hiking some years ago in N.H. in November. Some snow on the ground, snow in trees-melting and dripping, exceptionally wet. Temps 33-34F. After maybe 3 hours, we arrive at the shelter soaked to the bone and while fine hiking, begin to chill quickly once stopped (as expected). Instead of changing into some dryer clothing, realizing that the wet clothes won't dry well at all, I decided to just walk (w/o pack) at a speed that generated heat but not enough to sweat while opening my jacket zipper and vents accordingly. After about 20 minutes from the shelter I felt the idea was working and turned around to head back. Back at the shelter after about a forty minute leisurely walk, I was bone dry! We were higher now in elevation so trees were much shorter (less dripping) than 'in the forest.' Even the outer wear was dry (breezier up high). Probably, only my leather hiking boots were still somewhat wet. It was great learning experience in managing wet conditions.

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Where I find that goretex works is in cold weather paddling gear. You need the waterproof-ness when in the water. You're going to sweat while paddling. The breathing happens when you get out of the water - I'm frequently wet inside the garment while paddling (sweat) but am dry by the time I have my boat loaded.

I have adopted ponchos for when I'm hiking - very well ventilated. I use a belt sometimes to keep them from flapping.

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I live in the PNW, and most of the time it's not raining very hard. Good chance you will have intermittent rain or light rain, depending on the time of year. These jackets can work well here as the humidity won't be completely saturating.

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So skiing in the winter... with low humidity but melting snow on your coat... it should work well. That's basically the perfect use case for goretex if you are going to use it at all.

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Another point that I, for some reason, rarely hear in the WPB fabric debate is that sweat isn’t just water. It has a fairly high concentration of minerals, which also make it harder for the water to evaporate. And when after extensive use these minerals pile up on the inside of the goretex membrane, they can actually pull the water in through the goretex. Which is especially a problem with goretex shoes.

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Thanks for a good content.
2 comments from me:

1) your test setup might benefit from a wick in the water to help it evaporate - perhaps an offcut from a wicking baselayer product?

2) I absolutely agree that goretex and all the competitors don't wick well during rain,
but when the rain is frequently on/off all day long (Or all month long in Scotland), then you get the benefit between rain showers. It's still cold and it's still windy, so I'm not taking my wet jacket off to pack it in my bag. I just keep walking/riding and if it's a good breathable fabric I will dry off pretty inside the jacket quite quickly. Even when the outer layer has wetted out, a strong wind will dry that pretty quickly, and a good tight fitting wicking base and breathable intermediate layers really help get the moisture inside to evaporate.

I've also spent a summer riding in the southern half of the US. The weather is much more humid, much warmer, and more importantly, it usually rains very heavily for a short period, and then the sun comes out again for the rest of the day. In those conditions, it's easier to just get wet and then dry again, or put on a waterproof when the rain starts and take it off when the rain stops.

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Hi Steven, great content. I've found that Goretex works excellently as a ski jacket outer shell. I think skiing is an ideal environment for it. It's dry and cold outside and you're only producing a small amount of vapour inside. Plus, the airflow is quite high as you're moving fast. I tend to buy used Goretex items and reproof them. The older items were built to a much higher standard and I prefer the style of original Latok or Berghaus. They were very well designed. One of my biggest bugbears is laminated pockets etc which detach too easily. Also, my one key feature for an outdoor jacket that tells me it's been designed by someone who knows: Pocket zips must close downwards!

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Good content and analysis, BUT… 1) the big pit zips in my Montbell and Patagonia are great, and I regularly air out by fully unzipping and lifting up the jacket. It creates a fast cooling effect which is also reduces sweat. 2) when it gets hot, or I know it will be even just warm and rainy, I use a very thin alpaca or merino layer underneath and then the rain jacket. The jacket provides warmth, and can get muggy, so I do the regular airing out. 3) a big source of heat, sweat and lack of ventilation (air) is on the back, so I use a Vaucluse air ventilator with my frameless, hip-beltless pack. This extra air on the back helps with outside and inside moisture — less sopping wet back sticking to the pack, and less heat from the pack up against the back, which also means less sweat because less heat. I even stopped using a fanny pack and love the sense of freedom and air all around me. Improved visibility too when I look around on the ground. All this extra body exposure to air and fewer materials contacting me allow for extra ventilation, easy to open/close zip my jacket, I can undo the chest straps for a few minutes of super-ventilation as well. One thing to consider is the pros/cons of a goretex hiking PONCHO. Of course, it would have to have straps for wind, but the whole idea of the ancient poncho technology is that it allows for flexibility in ventilation.

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I worked in the extreme sportswear industry and we knew that the best that "breathable" fabrics could do under the ideal circumstances was about a 7% improvement. If the outside or inside got wet from precipitation or sweat, it was like a thick plastic sheet. But if you did not have "breathable", few people would buy. They were well trained consumers. Since we had an extreme customer base who knew when things were working and not, we were very early with the zip vents in pits, pockets, pant legs, .... And we had the flap vents also.

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Thank you for explaining this so well!  I have grown tired of explaining this to others and was too inexperienced to make my own content.

This was in part why I converted to using a rain cape for bicycling (better airflow) and when I go camping in our typical PNW winter I ensure wicking-synthetic or wool layers because I know it’s not if I get wet, but if I can stay warm when I do get wet.

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Ahh, thanks for that, it explains why my gortex lined textile motorcycle jacket works so well on my bike (lots of air movement) but is totally useless when I take a bushwalk at the end of the ride!  And I take exception to that Gortex ad which implies it's always dry down here in Oz!

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i have multiple arc'teryx jackets and i agree that they do not work well when hiking/trecking but if you ski it works amazing if your skiing in dry powder with little to no humidity.

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What you miss in this content is that, as long as the temperature inside the jacket is higher/warmer than outside the jacket, there is still a vapor pressure differential that should drive vapor from inside the jacket to outside the jacket, even when the outside is wetted out. The problem is that this differential is generally not sufficient to transfer the amount of sweat generated while hiking.

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Well, it is not always raining, is it? Sometimes it is cold and then I sweat. And if I wear Gore-Tex that sweat is actually ventilated out, while the jacket is keeping me warm.

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So this is a surprise to anyone? I've used them since the 70s, and they never worked perfect, but they were noticeably better than the hard-coated urethane jackets we used before. When you're backpacking, you only want one wind/rain shell. Sitting around in a rubber jacket to  keep the wind off sucked, so the so-called waterproof breathable jackets were way better for that application. A few of my friends had expensive ventile parkas which might have been the best compromise, but like today, they were very expensive.

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I wear a goretex jacket and pants for resort skiing. They have been 100% waterproof even during all day wet snow, with falling, sitting on the snow etc, until the seams go or they delaminate.  For resort skiing, in a gondola/bubble or a mountain restaurant, you can see the steam coming out through the jacket, and you don't have to keep farting around with the front zip, pit zips or taking the jacket off.  Similarly if it stops snowing, the moisture underneath comes out. So waterproof AND breathable is useful if it sometimes stops raining or snowing on you. I get that this is a hiking channel, but think it's worth a mention that the situation is different for skiing. ps all that stuff about needing a humidity gradient between inside and outside applies also if you're wearing just a pair of shorts or a wicking baselayer. Sweat will not evaporate when the humidity is high.

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Great content. I don't like Gore-Tex in heavy rain, but it works great for most of what I do, which is winter downhill skiing. It keeps me dry and warm because even when it is snowing the outside humidity is pretty low out here in the west. I have a buddy that bought a cheap waterproof (not breathable) jacket for skiing - bragging that it only cost about $90. Every day after skiing, he would take off the jacket and was pretty much soaked with sweat, while I was almost completely dry. Moisture was dripping off the inside of his waterproof jacket. For cold, dry conditions, fabrics like Gore-Tex are hard to beat. If I was working as a lift operator, I would definitely wear a big loose waterproof (not breathable) jacket - for that the expensive fabric would make no sense.