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For rain pants, there are both breathable pants that let sweat out and non-breathable ones that do not. The problem with the non-breathable pants is that your sweat can build up inside and then you will get wet underneath the pants anyways.

As non-breathable pants are still sold, what are reasons why some people might choose to use them? Is it simply a matter of cost?

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As you are aware waterproof breathable fabrics can "wet out" reducing them to simply waterproof fabrics. That does not make wetted out waterproof breathable the same as waterproof non-breathable fabric. Non-breathable fabrics tend to be cheaper, stronger, lighter, and in some cases more water resistant (i.e., a higher mm H₂O rating) than corresponding breathable fabrics. Compare the differences between the high end fabrics used in Z-packs non-breathable kilt (1.0 oz/yd², 20,000 mm H₂O, and 15.5 N puncture strength) and breathable rain pants (1.92 oz/yd², 20,000 mm H₂O, and "don't look at it funny" strength).

If weight or durability matters and either sweat is not a factor or wetting out is a given (e.g., sailing or prolonged wet weather), a non-breathable fabric might be desirable. Sweat is heavily activity dependent and good ventilation can often mitigate the need for breathability. For really cold weather activities the effect of sweat can be eliminated through the use of a vapor barrier, but you will still want a waterproof shell to keep snow-melt from soaking your insulation.

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    Totally agree, I'd even go farther and say that breathability is overrated: If you sweat a lot even the best breathable fabric doesn't have the throughput to keep you dry, and in heavy rain/high humidity the throughput is even a lot smaller. I don't think I have every been "dry" after a day of rain in the mountains with my breathable rain clothes - I have definitely been less wet and warmer than without any rain-gear at all. And in intermittently rainy conditions zips to easily put on/take off is much more effective at reducing moisture.
    – imsodin
    May 20, 2019 at 18:48
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You seem to have a specific outdoor activity in mind, like trekking or climbing. But what about water sports? A sailing ship deck would be the most obvious place. Sometimes you won't move much, but you wish to stay 100% dry.

In my experience waterproof breathable fabrics are only water resistant. Also notice that fabrics like GoreTex deteriorate with time and lose their water repellent qualities. A simple plastic trousers won't have this problem.

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    @StrongBad that might be the equivalent of a wave coming inboard. May 20, 2019 at 19:26
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    @StrongBad: as said above. We have different uses in mind. You won't be sweating much in a sailing ship under cold rain. May 20, 2019 at 19:57
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    @StrongBad This sounds like a big misunderstanding of "psi" (to start with, it's a relative unit, and says nothing about area uniformity too), together with dubious sources. To use an "evil" interpretation of your information, this means a water tank with Goretex pants bottom can have water 40 meter high and still leak nothing (unlikely), while a professional footballer kicking a ball at a ship hull will break it (very unlikely). ... In reality, they have likely different references values, the Goretex value is just for very short pointwise pressure, etc.
    – deviantfan
    May 21, 2019 at 6:18
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    My experience entirely. Breathable = not really watertight. May 22, 2019 at 9:37
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    @GabrielC.: you are right on this one. Added the word 'sometimes' and changed the meaning completely. Sailing can be strenuous but sometimes you are on deck as watchkeeper. May 22, 2019 at 9:49
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There are several uses, but regular strenuous outdoor leisure activities aren't really among them.

Breathable gear only goes so far - there comes a point when the best gear you can afford will result in getting very sweaty. This point is a function of temperature, humidity, price and activity level. If you're consistently going to pass that point, non breathable gear is cheaper (much cheaper in the long term as membranes degrade) and still keeps the wind off.

They're often used by those working outdoors doing fairly light duties, such as directing traffic or otherwise mainly standing. These are often high visibility. More strenuous outdoor workers may well use heavy duty poorly breathable versions.

They can be kept in a vehicle as emergency wear to keep in case of breaking down or worse. These may never be worn so it's not worth investing in something expensive. They also seem more waterproof than at least cheap breathables if you're actually sitting or kneeling in water fixing something. A similar use is car camping, when you may need to go out in the rain to tighten ropes or pegs, but would avoid going out in the rain otherwise.

In the UK climate, and many other places, it's often worth carrying rain gear for even fairly gentle walks that take you more than a few minutes from shelter. You'll be a lot drier and warmer in cheap waterproofs than none at all, which is why compact ones are widely sold in national park shops etc. (I keep a pair in the van that were an emergency purchase in such a shop)

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    +1 for mentioning that a lot of outdoor rainware is for non-aerobic work. Also, in my own experience, the breathable fabrics are much less effective at blocking wind than the non-breathable ones. So for standing watch in really cold stormy conditions, go for the heavy raingear.
    – CCTO
    May 21, 2019 at 15:43
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    @CCTO I wonder if that wind blocking difference (that I've also noticed) is due to the actual fabric, or trying to get a reasonable amount of ventilation in the breathable gear to augment the breathability, as breathability alone can't keep up.
    – Chris H
    May 22, 2019 at 7:09
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    good question. In principle I suppose you can't expect sweat to evaporate and dissipate (humid air leaving) without making up the air from outside, so breathable and air-tight are pretty much opposites. In mine, I don't perceive a draft coming through openings, it feels more like the wind is coming right through the garment. So I think it's a property of the fabric. Caveat: my experience is with mid-range products, I've never had any of the premium technical gear.
    – CCTO
    May 22, 2019 at 17:14
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The non-breathable pants are usually considerably cheaper than the breathable ones. That can make them a better choice for activities that stand a good chance of ripping holes in the material, glissading down icy slopes for example. Far better to rip your $25 non-breathable pants than your $125 Gore-tex pants.

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The non-breathable cheaper wet-weather protection is useful for when you must sometimes go outdoors in heavy rain to do essential jobs, but not for very long.

Long enough that you would otherwise get soaked, but not long enough for the lack of ventilation to be a problem.

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Non-breathable waterproof garments are basically polyurethane plastic at the surface. This has one very big benefit in that dirt doesn't stick to it much, and what sticks will usually come off with just water.

So they're good for uses such as:

  • Trekking in muddy conditions. Just hang them overnight to dry and yesterday's muck falls off.

  • Children's use. A high-tech fabric will not stand up to daily washes for very long.

  • Agricultural and other jobs where you'd get excrement on your clothes.

After all, getting sweaty is a discomfort but it can be tolerated.

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To ride a motorcycle, for example, you will never chose breathable fabric. If you have to wear rain pants, temperature is such that sweat is almost never an issue and you want your garment to be as waterproof as possible. Also, you want to be sure not only water but also mud, grease, dust, etc. is kept off your under-layers, hence the use of a fabric as less porous as possible.

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As a cyclist, I expect to get wet if I'm riding hard in the rain. But if I'm wearing something waterproof -- breathable or not -- the (cold) rainwater will be kept separate from the (warm) sweat, and I'll be warmer.

And if I'm not riding hard, I'll be dry longer in the non-breathable stuff, because water vapor can pass through the breathable layers in either direction.

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  • Breathable doesn't necessarily mean it let's water through. There are products out there that let air in, sweat out and prevent water from coming in.
    – JJJ
    May 22, 2019 at 3:13
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Aside from the practical differences aside, i.e. breathable lets sweat out, but tends to only keep rain out temporarily, breathable fabrics are typically made of 'forever chemicals' which might cause cancer, in very small doses, and stay in your body for a long time. 'forever chemicals' don't tend to break down in the environment. See for example https://www.theguardian.com/environment/2022/jan/26/water-resistant-products-toxic-pfas-study for an article about water resistant products and PFAS ("PFAS" means "Per- and polyfluoroalkyl substances").

So, one possible reason to use non-breathable waterproof pants is to not absorb PFAS into one's own body, or distribute them into the environment around oneself. This doesn't mean one cannot obtain breathable waterproof pants that don't contain PFAS; nor that everyone will choose to take this path, but it's one possible reason why someone might choose to use non-breathable waterproof pants.

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