Breathability: an explanation of its importance, mechanisms, and limitations

Outdoor fabrics are frequently described as being “breathable,” and this is (except in one case) a desirable characteristic. My observation based on clinics and online writings is that the concept of breathability is generally understood, but usually superficially. Further, there is some confusion about how it relates to “ventilation” as well as some unrealistic expectations about the performance of breathable fabrics. In this post I will try to explain what I know.

Screenshot from Columbia.com marketing their waterproof/breathable fabric, Omni-Dry. Their emphasis on breathability is common.

Breathability is the layman’s term for moisture vapor transmission rate (MVTR), which is the measure of how quickly (or slowly, if at all) moisture passes through a fabric or other substance. It is usually measured in g/m²/day, or the mass of moisture that passes through a square meter of fabric in 24 hours.

Not surprinsgly, breathability is an oversimplification of MVTR: whereas MVTR measures a degree, breathability is used as an absolute. Specifically, if the MVTR of a fabric is greater than zero, the fabric can be described as “breathable,” even if realistically it is not. (There is no industry standard for breathability.) So, for example, even a body suit made of painter’s plastic could be described as “breathable” if the suit had a few needle-sized vents in it, because in theory moisture could pass through these vents.

To describe a fabric’s breathability, the task is left to adverbs like “extremely,” “very,” and “ultra,” which all seem susceptible to exaggeration by writers of product marketing copy.

Why breathability is (usually) a good thing

The cotton knit t-shirt I am currently wearing is breathable. If it was non-breathable – e.g. suppose it was made of latex – then it would trap my perspiration. With time, the shirt would feel clammy, especially if I began exerting myself like by running 100-meter sprints between sentences.

A clammy shirt obviously would be uncomfortable, but the repercussions can be more significant on a backpacking trip. For example:

  • If my base layer clothing or rain gear is insufficiently breathable, I will get wet from the inside due to trapped perspiration. Since water is significantly more thermally conductive than air, wet clothing can be significantly colder than dry clothing.
  • If the outer shell of my sleeping bag is insufficiently breathable, moisture will get trapped inside my sleeping bag, wetting the insulation. Even synthetic insulations – which are sometimes falsely described as being “warm when wet” – would be compromised in this scenario; the effect on goose down insulation would probably be more significant.
  • If my fully-enclosed shelter  (e.g. a “tent”) is insufficiently breathable, moisture vapor resulting from respiration, perspiration, and drying equipment will likely cause condensation to collect on the shelter walls, potentially soaking me and my gear.

The one instance when breathability of fabrics may be undesirable is in extreme cold temperatures. Read more about vapor barrier liners for an explanation.

After a long day of skiing across the Alaska Range, I crawled into my shelter and removed my soaking-wet "waterproof" boots, resulting in visible vapor emanating from my socks. If I had been using a fully-enclosed shelter that didn't breathe well, this vapor would have collected on the inside of the shelter as condensation. However, I was using a mid-shaped tarp that excellent ventilation, so condensation was less of a problem.

How fabrics breathe

There are two ways that moisture can pass through a fabric:

1. Ventilation. A porous fabric allows the direct passage of air through it – in other words, it vents – and this air may carry moisture in the form of vapor. Fabrics with many and/or large pores (e.g. bug netting and base layer fabrics) breathe better than fabrics with few and/or small holes (e.g. eVent or Omni-Dry raingear fabric).

2. Solid state diffusion. Some non-porous fabrics permit the transmission of moisture. Latex, painter’s plastic, and Cuben Fiber will not, for example. But there are many varieties of “waterproof-breathable” fabrics (in fact, most, including the Gore-Tex family) that feature a membrane partly made of non-porous polyurethane. Polyurethane normally repels water (i.e. it is hydrophobic) but it can be chemically altered to absorb water (i.e. hydrophilic). Then, this type of fabric acts like a dish sponge: water vapor is absorbed out of the air; it moves through the sponge in a solid state towards the other side; and when it reaches the other side it can evaporate again.

Screenshot from eVent website, which includes images of porous membrane versus a non-porous membrane (e.g. eVent versus Gore-Tex), plus an explanation of the difference.

To fully understand how fabrics breathe, I should make two other points:

1. For a fabric to noticeably breathe, there must be a humidity differential between the two sides of the fabric. For example, if the humidity inside a rain jacket is 90 percent and the humidity outside is 20 percent, then the water vapor inside will naturally want to pass through the fabric to the outside. However, if the humidity is 90 percent inside and outside the jacket, then the fabric’s breathability will not be noticeable, even though it can still pass moisture.

2. Water can move both directions through a fabric. For example, if the humidity inside a rain jacket is a mere 30 percent and the humidity outside is 80 percent, then moisture will actually move inwards. This would be a strange situation, but there is one instance when this can happen, as explained in the next section.

Why breathable fabrics fail

As a general comment, I think we expect too much of our gear. Every piece of gear has limitations, and nothing is a complete panacea for Nature’s challenges. Gear manufactures seem less willing to acknowledge this reality, either because it’s better for business to ignore them, or because they are oblivious to them (which is an understandable but sad result when non-users design, market and sell gear).

The breathability of fabrics seems more overstated than average. Every time I see a “Guaranteed to keep you dry” hang tag or read product copy for “waterproof” shoes, I dream about taking that company’s marketing department for a short trip in Alaska, the Pacific Northwest, or the Appalachians. If their experience is anything like mine have been, they might be more realistic about the performance of their products when they return.

Fabric breathability can halt or slow for three reasons:

1. Moisture may not move through the fabric fast enough. If this occurs, you will get wet from the inside via trapped perspiration. In dry environments, most “breathable” fabrics work well. In semi-humid environments, waterproof-breathable fabrics struggle. In very humid environments, nothing is going to keep you dry, sorry.

2. The “outside” humidity is too high. If the outside air is nearly saturated with water vapor already, there is simply no capacity for it to absorb additional vapor generated by you. When you perspire, it remains next-to-skin, unable to evaporate.

3. The fabric can “wet out,” or become saturated with water. When this occurs, usually due to the failure of a durable water repellant (DWR) finish, the outside humidity is essentially 100 percent. Moisture on the inside of the fabric cannot pass through these saturated spots. And, in fact, if humidity inside the jacket is less than 100 percent, then moisture can transmit into the jacket from these saturated spots, since it’s actually less humid inside than outside the fabric. This scenario explains why it’s important to regularly restore DWR finishes of rainwear, like by using ReviveX Synthetic Fabric Cleaner.

When a supposedly breathable fabric "wets out" (i.e. when the exterior fabric becomes saturated with water) its breathability slows because moisture will not pass through the fabric in these spots. I have found that moisture can actually enter the fabric at this point, since it is more humid outside the jacket than inside.

 

24 Responses to Breathability: an explanation of its importance, mechanisms, and limitations

  1. Tomas May 1, 2012 at 9:04 am #

    Very interesting stuff! Personally I’ve kind of given up on giving a shit about whether a fabri is gore tex, event or some noname breathable fabric. Zips work, and that’s about the only thing that I look for anymore. The breathable fabrics may work to a greater or lesser extent, but there are other factors I think are more important.

  2. Tim Miner May 1, 2012 at 12:49 pm #

    Great post Andrew. It’s always good to understand the “science” behind the gear we use. It’s the job of the marketing departments to paint the use picture for the customer. Most of the time, they do that with a scenario that may only exist under the most perfect conditions for the product. It may be a risky proposition for us as consumers to think that the architects and marketers of a product are working a concerted effort. I’m not sure the right hand knows what the left hand is doing sometimes.

    That said, your post will help would be customers understand what to look for and what questions to ask. It’s certainly more info than most of us had when we bought our last piece of breathable gear.

  3. Rhodri Lewis May 1, 2012 at 2:02 pm #

    Hi Andrew. Nice article and a subject close to my heart. I came across a very small company in Norway making mesh base layers. I now represent the company in the UK as well as other Norwegian brands. Nothing on the market today “breathes” and keeps you warmer and drier (NOTE: not DRY!) than this technology. I don’t want to advertise blatantly on this blog, but if you are interested in trying some kit, let me know. Regards

  4. Chris Zeller May 1, 2012 at 2:47 pm #

    Hi Andrew,

    I like how you nuance this with “overstated”. While I agree that there are many ways that you can still get wet in waterproof breathables, I still much prefer today’s products to the old coated nylon and rubber rainwear we used 20-30 years ago or still use in cheaper garments today. I feel much more comfortable over a broader range of conditions than ever before. These fabrics are “state of the art”, while not perfect they really are the best you can get.

    The best solution I have found is active ventalation and drying the gear when possible in combination with synthetic insulation for prolonged wet conditions. Pit zips really help here. I also like jackets with mesh pockets that add additional ventalation.

  5. Stephen Akrill May 2, 2012 at 7:54 am #

    Andrew refers to the fabric label “waterproof-breathable” in his new book as being an “oxymoron”, a sentiment with which I can concur.

    A few years ago I developed a test of a garment’s waterproof qualities in response to the bold claims and aggressive advertising of the Gore-Tex company, following a series of jackets that had failed me in cold wet British conditions, and challenged them.

    After an exchange of correspondence an air ticket duly arrived, and I was invited to their Edinburgh factory to demonstrate it.

    My test was very simple, employing a water-soluble food dye used in Indian restaurant cuisine as an indicator. I dubbed it my “Tandoori-test”, as it utilised the bright red colour found in the much beloved chicken dish.

    It involved soaking an item overnight to achieve the kind of fabric condition witnessed following a typical day’s backpacking in Scottish weather and a night in a tent’s porch, and then towel drying it and applying a small puddle of the red water onto its surface, with a piece of kitchen tissue placed underneath to catch anything passing through.

    The result was always the same; the red water traversed the material and stanied the paper.

    At the factory I was greeted by a number of industrial chemists who were clearly disbelievers, confident in their material’s propriety and ability to withstand any challenge. I was shown a range of sophisticated tests they claimed clearly demonstrated their fabric’s imperviousness, which supported the various patents and awards they held. After seeing my test however their faces and attitude changed; all of the pre-prepared samples failed as I expected.

    In fact little was said and I returned home. Subsequently I received a letter stating their experts had considered what they had witnessed, and had concluded that water was not actually passing through the fabric, but only the dye, the solid part of which had “sublimed” across the membrane and recombined with the dampness on the other side.

    Afterwards I attempted to get the story published in the TGO magazine, as I thought people would like to judge for themselves, but they would not run it. Seemingly there was a conflict of interest, as they heavily advertise the product and were obviously dependent upon revenues.

    Over the years I have continued to employ my test with many other so-called waterproof-breathables, and I can say that they all fail – actually and intuitively in order of breathability. The more “breathable” the less “waterproof” is the truth of the matter. As Andrew said, the term waterproof-breathable is an oxymoron. It is a physical impossibility to have both.

    All such garments are actually porous to water in varying degrees. In sustained precipitation it is only a matter of time before it comes through. The inevitable is simply delayed, which is usually enough for most people, and a reasonable trade-off in clement conditions.

    (Anyone attempting the test for themselves shouldn’t be afraid of permanently staining their item, as being water-soluble, a further soak removes any residue.)

  6. Matt S May 2, 2012 at 7:38 pm #

    Andrew,

    Try one more for us… Polartec’s new Neoshell… It will have it’s limitation like all the rest, it can’t control humidity, it will wet out like the rest, but is there any advantage to it? Their marketing is certainly saying as such.

    • Andrew Skurka May 2, 2012 at 9:36 pm #

      You answered your own question. The bottom line is this: when it’s wet outside, expect to get wet. Some fabrics may be more waterproof or more breathable than others — and certainly compared to their ancestors — but they are not “magic” and they ultimately will not keep you dry when it’s wet.

  7. Jason L May 6, 2012 at 2:10 am #

    Have you had a chance to try UK’s parmora fabric brand by nilwax since your book. I’ve been reading up about it and found a couple places that ship to the US…

    • Andrew Skurka May 7, 2012 at 5:41 pm #

      No, but I want to. Hopefully a Paramo employee reads this blog…

  8. Jason L May 8, 2012 at 3:44 am #

    I know you showed some likes, favorites, and dislikes in your book have you thought of posting reviews of gear on this site. Stuff you use, would like to use, and wish was produced.

  9. Steve May 8, 2012 at 2:11 pm #

    After several years being regularly disappointed by “waterproof/breathable” garments in a range of environmental conditions (note: dry/cold environments were great for wp/b fabric, but I don’t spend a whole lot of time around the Arctic Circle anymore), I tend to follow the direction some in the US military are leaning–that is, a majority of the time I wear a wind/water resistant outer layer that is quite breathable, and pull out a waterproof/non-breathable shell when conditions hit their worst. Staying completely dry is a myth IMO, so I can live with a little dampness with the resistant outerwear. When the wind is hawking and blowing snow or rain sideways, then the coated shell (which, btw, is much more pocketbook friendly) comes out, and heat-generating activity is reduced (can’t see where you’re going, anyhow, so why keep pushing on?), minimizing sweat inside the garment.

  10. Andrew Lichtman May 9, 2012 at 10:44 am #

    Nice overview. Do you think you should mention the problem of dew point? No matter how breathable the fabric, it will wet inside if the temperature inside is at or below the dew point. A common user error is to invite this condensation by wearing too much clothing under the rain shell.

    • Andrew Skurka May 9, 2012 at 10:53 am #

      I could have mentioned the issue of dew point, but I feared the post was already getting long. I rarely have experienced dew point-caused problems with rain gear, but it’s definitely been an issue for sleeping bags and insulated clothing while in cold-and-wet conditions. If I fear the dew point will be within my layering system, I do at least one of the following things: use VBL’s; use synthetic insulation; dry my gear in the sun or at a laundry mat on a daily or regular basis.

      • Andrew Lichtman May 9, 2012 at 5:08 pm #

        You may have suffered from this special case describe by David Chenault:  “During very wet and cool conditions, evaporation from the sodden exterior fabric can even lower the dew point just inside the shell enough that condensation forms inside the jacket, just as it does on the inside wall of a tent during especially unsavory conditions.”

        In this case, one alternative to VBLs is to wear the rain shell as Twight’s “second skin,” closer to your body, even if that means it may be covered at times with insulation (synthetic insulation which dries quickly from body heat, since it WILL get wet). Twight’s insight is that this way you can keep your baselyer drier than with any other system!

  11. Marnix August 28, 2012 at 2:39 pm #

    Hi,
    it’s very clear to me that there is no “waterproof” (i was foolish enough to buy gor-tex hiking boots with the thought of keeping my feet dry. I noticed that, even after an hour of walking through wet grass, a few hours later the inside of the shoes where wet.)

    “Waterproof” only means a delay for getting wet, right?
    So my question is: how long woeld that delay be?
    could you maybe give an example for how long your raincoat delays you for getting wet?
    for example if it would rain non-stop a third inch per day

    just asking this because i’m curious about what a realistic expectation would be for using a raincoat in prolonged wetconditions, and i mean other the getting wet ;)
    grtz,

    • Andrew Skurka August 28, 2012 at 8:20 pm #

      The length of time before a waterproof shell wets through is highly variable, probably depending most on your rate of perspiration, the state of the DWR finish, and how much abrasion the jacket incurs while wet. For example, if you are sweating a lot, if you have never restored the DWR on an old jacket, and you are bushwhacking, then you’ll get damp pretty fast (30 minutes?), and wet soon thereafter (1 hour?). If the opposite conditions are true, the shell can keep you dry much longer, maybe a half-day.

  12. Jay C September 20, 2012 at 11:34 pm #

    While most of this makes sense, I am not sure I buy this reasoning:

    “And, in fact, if humidity inside the jacket is less than 100 percent, then moisture can transmit into the jacket from these saturated spots, since it’s actually less humid inside than outside the fabric. ”

    I spend a fair bit of time in drysuits and other float gear packrafting, and often that time is spent with at least some (or most) of me submerged in water, and in my experience, so long as the dry suits fabric is not damaged (thorn holes, abrasions, etc) I do not get wet unless i start sweating. Since the fabric is underwater, it is “wetted out”, and since I am dry inside the suit, there it is a lot less humid inside the suit then outside, so by your logic I should notice leakage, but I don’t. Maybe it is just me.. perhaps growing up in SE Alaska damaged my ability to notice when I am wet :) Obviously if I start sweating I get wet, because as you pointed out, wetting out fabric can’t pass moisture vapor.

    Clearly all bets are off once the fabric is damaged – thorns, abrasions, etc, Maybe thats your point?

    I should say that I agree wholehearted that if you are exercising in the rain you are going to get wet one way or the other, regardless of the claims of the manufactures.

    Thanks for taking the time to write this up!

    • Andrew Skurka September 22, 2012 at 9:33 pm #

      Once the fabric “wets out,” moisture should be able to pass inwards if the humidity is less than 100% inside. This may not be in the form of leakage, but simply an increase in the humidity.

  13. Alan Holzman March 17, 2013 at 5:48 pm #

    I learned several years ago that there is no such thing as truly waterproof hiking shoes/boots. On the Overland Track in Tasmania it rained every day. My feet (wearing new “waterproof” boots) were wet from day one. When I tell salespeople at outdoor gear shops that I don’t feel that any shoe is truly, 100% waterproof they think I am misinformed. Oh well, I guess they don’t spend much time hiking in wet conditions. Thanks for making this issue public.

    • Andrew Skurka March 17, 2013 at 7:24 pm #

      I think many sales associates are severely deficient in personal backpacking experience. They can be trained well, but that’s not a replacement for first-hand learning.

  14. Nicholai December 3, 2013 at 1:22 pm #

    Thanks for the very detailed article. I am curious though if you have a direct experience with any of the “air permeable” shells fabrics like the polartec neoshell and could comment on their “breath-ability”, and their effectiveness of moving air across the shell when saturated.

    I am gearing up for a 45 day sheep hunt in the brooks range next fall and I am considering the raingear that I would like to bring. I have been using PU laminated hardshells for the last few years with acceptable results, but as you state there are limitations to its “breath-ability”. My thought with the air permeable fabrics is that even in humid climates if relatively cold; warm, water saturated air could exit the shell carrying moisture with it, and cool outside air would replace it. Even if the outside air is 100% humid; as it warms up within the jacket its humidity will decrease, allowing it to absorb more moisture and vent out of the jacket.

    Do you have any experience with this, or do you think that the permeability rate is too slow to have any noticeable effect?

    • Andrew Skurka December 3, 2013 at 4:48 pm #

      Wow, a 45-day sheep hunt in the Brooks. I’m a novice hunter, but even I know that this is a life-list hunt trip.

      The breathability of both types of WP/B fabrics — monolithic (solid state diffusion) and air permeable (direct venting) — is dependent on the quality of the DWR, which if degraded will cause the face fabric to become saturated, and moisture will start entering the jacket soon thereafter. In my experience, water only comes in — the humidity does not circulate around and exit elsewhere, at least to the point where you can feel it.

      Assuming the DWR is in good condition, there are pros and cons of both types of shells. Specifically, monolithic shells are more wind-resistant, while air-permeable shells are more breathable. I suppose that depending on your level of exertion, it may make sense to go with one over another.

      I’m not sure what months your hunt is, but if it were in that September/October window I would consider abandoning WP/B technology entirely. I can just imagine sitting on a mountainside in the rain waiting for some sheep to show themselves. The last thing I’d want is water seeping through my jacket and being unable to offset it with the generation of body heat.

  15. Dogwood January 11, 2014 at 8:48 pm #

    “Some non-porous fabrics permit the transmission of moisture. Latex, painter’s plastic, and Cuben Fiber will not, for example. But there are many varieties of “waterproof-breathable” fabrics (in fact, most, including the Gore-Tex family) that feature a membrane partly made of non-porous polyurethane. ”

    This also had previously been my assertion of cuben – WP /non-breathable. Unless I missed it somewhere, no mention is made about the new so called “breathable” Cuben(CT3, perhaps CT5) from Cubic Tech that a couple of the UL cottage gear manufacturers such as Joe Valesco is using over at Zpacks for some of his apparel pieces. I was skeptical at first as I too have seen the terms breathability and waterproof been overused ad nauseum. I’ve always been a big eVent fan when seeking an optimal statistical balance between what are generally consider true WP and breathabilty measurements such as in a true WP rain jacket fabric but when I saw the hydrostatic head and MVTR stats Joe relayed to me that he received from Cubic Tech it seems to me, upon initial review, that this cuben is indeed waterproof/breathable.

    I’m new to this type of cuben and have no real world demoing of it but I wanted your, and your audiences, input upon it. THX.

  16. Anthony Sharp May 22, 2014 at 7:07 am #

    I’m curious if anyone has any suggestions on other non-breathable waterproof shell layer materials besides “Latex, painter’s plastic, and Cuben Fiber”. I’m a scrawny 140 pound 6 foot tall guy so I’ve been fine (sweat buildup wise) with my PU coated Campmor rain jacket with pit zips but I’m in the market for something slightly different. Have any of you bought or made a sil-nylon rain jacket?

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