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.
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.
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.