Breathability is two separate properties sold under one word. Air permeability is how easily air moves through the cloth. Moisture management is how readily the fabric takes up sweat and lets it go again. A fabric can be strong at one and weak at the other, which is why "breathable" on a label tells you almost nothing on its own.
Every innerwear brand in the country uses the word, including us. Almost none of them say which of the two things they mean, and the two behave very differently on a humid afternoon. This is what is underneath the claim, and how to judge it yourself.
The short answer
— Breathability is two properties: air permeability and moisture management.
— Air permeability is measured as the volume of air passing through a set area at a set pressure — ISO 9237 and ASTM D737.
— Moisture management starts with moisture regain. Cotton holds about 8.5% of its dry weight in water; polyester holds about 0.4%.
— Knit structure changes breathability as much as the fibre does, which is why "100% cotton" on its own tells you very little.
Breathability is two properties, not one
The word covers two different things: how much air passes through the fabric, and how the fabric handles moisture. They are measured separately, and a fabric can score well on one and badly on the other.
An open polyester mesh has high air permeability — you can practically see through it. It still feels clammy after an hour in heat, because the fibre itself takes up almost no moisture, so sweat sits on the skin instead of moving into the cloth.
A dense cotton interlock is the other way round. The fibre absorbs readily, but the knit is tight enough that air moves through it slowly. It manages moisture well and ventilates poorly.
Neither of those is "breathable" or "not breathable". They are good at different halves of the same word. What you want for daily wear in heat is a fabric that does reasonably well at both, and knowing that is most of what this article is for.
What does air permeability measure?
Air permeability measures the volume of air that passes through a fixed area of fabric at a fixed pressure difference. It is a laboratory number with an agreed method behind it, not a marketing adjective.
Two standards are in common use and they differ in their settings, so the numbers are not directly comparable between them.
- ISO 9237, the international standard, tests 20 cm² at a pressure difference of 100 Pa and reports the result in litres per square metre per second [1]
- ASTM D737, used mainly in the United States, tests 38.5 cm² at 125 Pa and reports in cubic feet per minute per square foot [2]
What moves the number is mostly structure rather than fibre: how open the knit is, how fine the yarn is, how heavy the fabric is, and what finishing has been applied. A tight finish can close up a fabric that was perfectly open off the machine.
What does moisture management measure?
Moisture management covers how much water a fibre will take up, how fast it moves that water through the cloth, and how quickly it releases it. The starting number is moisture regain — the weight of water a fibre holds, as a percentage of its dry weight.
The standard commercial figures explain most of what people notice when they wear the two fibres [3]:
- Cotton — about 8.5%
- Viscose, and the modal fibres derived from it — about 11%
- Polyester — about 0.4%
That gap is not marginal. Cotton can take up roughly twenty times as much moisture as polyester before it feels wet, which is why a cotton garment stays comfortable through a long warm day and a synthetic one often does not — even though the synthetic dries faster once it is off you.
The other half is wicking: how quickly moisture spreads through the fabric to a larger surface area, where it can evaporate. Polyester is good at this and cotton is moderate. Fast wicking and low absorption is the combination that suits sport, where the priority is getting sweat off the skin and out of the garment quickly. Higher absorption suits a long ordinary day, where the priority is the garment holding moisture without you feeling it.
Why does knit structure matter as much as the fibre?
Because air permeability is largely decided by the spaces between the yarns, not by what the yarns are made of. The same cotton, knitted two different ways, gives two different fabrics.
Three structural variables do most of the work.
- Knit type — a single jersey is more open than an interlock made from the same yarn, so it ventilates better and feels lighter
- Fabric weight — lower GSM means less material between you and the air, and generally more airflow
- Yarn fineness — finer yarn in the same construction leaves a smoother, more even surface and changes how the fabric sits against skin
This is why "100% cotton" on a label is not a breathability claim. It tells you the fibre, which sets the moisture behaviour, and says nothing at all about the structure, which sets the airflow.
The two halves side by side
| Air permeability | Moisture management | |
|---|---|---|
| What it measures | Air volume through the fabric | Water taken up, moved and released |
| How it is tested | ISO 9237 · ASTM D737 | Moisture regain, wicking rate, drying time |
| Mostly set by | Knit openness, GSM, finishing | Fibre type |
| Cotton | Moderate — depends on the knit | High — 8.5% regain |
| Polyester | Can be high in an open knit | Low — 0.4% regain |
| How it fails | Fabric feels stifling, heat builds | Fabric feels clammy, sweat sits on skin |
Why does humidity change the answer?
Because evaporation depends on the difference between the moisture in the fabric and the moisture already in the air. When the air is close to saturated, everything dries slowly — so the fabric's capacity to hold moisture matters more than its speed at moving it.
This is the point at which advice written for temperate climates stops applying. In dry heat, a fast-wicking fabric wins: it moves sweat to the surface and the air takes it away almost immediately. In humid heat, the air cannot take much away, so a fabric that wicks quickly and holds little just delivers the moisture back to your skin.
A fibre with high moisture regain has somewhere to put the sweat while it waits. That is most of why cotton remains the sensible default across an Indian year, and why the same fabric would be an odd choice for a marathon.
How can you judge breathability without a lab?
Four checks, none of which need equipment: hold the fabric to light, breathe through it, weigh it in your hand, and read the composition for the fibre rather than the adjective.
1. Hold it up to light
An open knit passes visible light through the gaps between yarns, and those same gaps are what air moves through. A fabric that blocks light almost completely is dense, whatever it is made of.
2. Breathe through it
Hold the fabric against your palm and breathe out through it. The difference between an open jersey and a tight interlock is obvious in about a second, and it is the closest thing to an air permeability test you can run in a shop.
3. Weigh it in your hand
Lower GSM generally means better airflow and faster drying. Heavier fabric feels more substantial and usually lasts longer. Neither is better in the abstract — but for peak summer, lighter is the safer bet.
4. Read the fibre, not the adjective
The composition tells you the moisture behaviour, which is the half of breathability you cannot see. "Breathable" tells you nothing, because there is no threshold a fabric has to clear before a brand is allowed to print it.
One caution worth carrying: softness is not breathability. A heavily finished fabric can feel beautiful in the shop and ventilate poorly, and the finish that makes it feel that way often washes out within a few cycles. What you are feeling at that moment is the finish, not the cloth.
How we build ours
Our CotCloud Trunks are cotton because of the moisture half — in a climate where the air is often already carrying a great deal of water, the capacity to absorb matters more than the speed of wicking. The knit is chosen to keep the airflow half reasonable rather than pushing the fabric as dense as it could go.
The four checks above work on ours the same as on anything else.
CotCloud is Vault & Co's name for the whole system, not just the fabric: super-combed cotton knit with Lycra at thread level, a wide flat zero-pressure waistband stitched directly to the body, flat seams, and a mid-rise no-ride-up trunk cut, developed in-house. Read what CotCloud is, or see our breathable cotton trunks for men (2-pack).
Frequently asked questions
What does breathable fabric actually mean?
Breathable covers two separate properties. Air permeability is how much air passes through the fabric, measured in a lab under ISO 9237 or ASTM D737. Moisture management is how much sweat the fibre absorbs and how quickly it releases it. A fabric can be good at one and poor at the other, and the word on a label does not say which.
Is cotton the most breathable fabric?
Cotton is not the most air-permeable fabric, but it has high moisture regain — about 8.5% of its dry weight in water. In hot, humid conditions that absorption capacity matters more than raw airflow, which is why cotton stays comfortable across a long day better than most alternatives.
Does 100% cotton mean a garment will be breathable?
No. The fibre sets how the fabric handles moisture; the knit structure sets how much air passes through. Two garments both labelled 100% cotton can behave very differently depending on knit type, fabric weight and finishing.
What is moisture regain?
Moisture regain is the weight of water a fibre holds, expressed as a percentage of its dry weight. Standard figures are about 8.5% for cotton, about 11% for viscose and modal, and about 0.4% for polyester. It is the single number that explains most of the difference in how those fibres feel in heat.
Why does polyester feel hot if it dries quickly?
Polyester dries quickly because it barely absorbs water in the first place — about 0.4% moisture regain. The sweat has nowhere to go into the fabric, so it stays on the skin. Quick drying and staying comfortable while worn are two different properties.
Does a thinner fabric always breathe better?
Usually, but not always. Lower fabric weight generally means more airflow, though a thin fabric in a tight knit with a heavy finish can still ventilate poorly. Knit openness matters more than thickness on its own.
What is the best underwear fabric for sweaty men in humid weather?
In humid air, absorption matters more than wicking speed, because sweat cannot evaporate quickly whatever the fabric does. Cotton takes up moisture and lets it go without feeling clammy; dense synthetics move it but hold it against the skin. A mid-weight cotton with a small amount of elastane and an open knit is the practical answer for most of India's climate.
References
- [1] ISO 9237 — Textiles: Determination of the permeability of fabrics to air. Tests 20 cm² at a 100 Pa pressure difference, reported in litres per square metre per second.
- [2] ASTM D737 — Standard Test Method for Air Permeability of Textile Fabrics. Tests 38.5 cm² at 125 Pa, reported in cubic feet per minute per square foot.
- [3] Standard commercial moisture regain values for textile fibres (ASTM D1909 tables): cotton 8.5%, viscose 11.0%, polyester 0.4%.