When to Use breathable cloth？

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.

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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.

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.

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.

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The answer may surprise you! 

It is a common misconception that all Polyester is non breathable. In fact, some of the most breathable, moisture-wicking fabrics are in fact polyester! How can this be?

PET material, another name for Polyester in its raw form is not breathable at all. But Polyester fabric can be constructed in a way that allows breathability.

In the past decade new manufacturing techniques have designed knit and woven fabrics to effectively move moisture along the yarn and through the fabric to allow moisture to escape. Evaporation is a result of air contacting those moist fibers, which not only keeps you dryer, but also cooler. So, when air and moisture move freely it makes the fabric breathable.

The two factors that most affect comfort levels are climate (temperature )and activity of the wearer. We don’t notice how breathable our clothing is when we are sitting around watching TV…but try running on a hot day and you’ll soon discover if your shirt is breathable!

If you love details and want to read about standard testing methods, the Hohenstein test is explained.

Nylon is a synthetic material made from polymers, which allows air to pass through its fibers. However, along with other synthetic fabrics it must be constructed (knit or woven)to enhance breathability.

Regardless of fabric content it is the thickness and mechanical construction of the fabric that most affects breathability. 

If Nylon is not knit to enhance breathability it will be hot and inefficient! 

But, the same is true with any fabric, even linen or cotton.  

A light, loosely woven natural fabric would be the hot-weather choice of those who are not highly active. But you would not find anyone wearing heavy, densely knit cotton in the same weather or they would overheat quickly!

So What’s the real story?

There is much misinformation on the internet about breathable fabric.

Many people believe that only natural fabrics are breathable, and that polyester is hot and uncomfortable.

The truth is this...it depends! Natural fabrics are lovely and breathable when they are loosely woven or knit. But, when a person is active or sweating, Cotton or Linen can be very uncomfortable as it does not dry quickly. Also, loosely knit fabrics are not very sun protective, as the harmful rays can get through the openings of the weave.

Polyesters, especially poorly designed fabrics, can be hot and sticky...

However, well designed polyester or nylon knits can enhance moisture movement, dry faster, and offer high sun protection. Many also boast superior antimicrobial treatments for odor control.

Make an educated choice based on what you intend to be doing while wearing the garments you make.

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