Hygroscopic fibres and why they feel dry
Short answer: Hygroscopic means that a material absorbs moisture from the air and from the skin. That property decides whether a fabric feels dry or clammy against the body.
Absorption rather than transport
A hygroscopic material pulls the moisture into the fiber itself rather than moving it along the surface. A synthetic material does the opposite and carries the moisture out to the outside of the thread. The two methods feel very different, even at the same amount of moisture. See facts about bamboo.
What happens inside the fiber
The moisture binds chemically to the fiber's inner structure, and that binding is why the surface stays relatively dry even though the fiber as a whole holds water. That is why a hygroscopic fabric can absorb quite a bit of moisture without you feeling it on the outside.
This is why cotton and viscose feel dry
Because the moisture sits inside the fiber rather than on the surface, the fabric feels dry against the skin even though it has actually absorbed a fair amount of water. See bamboo underwear for men as an example of a fabric with this property.
How synthetic fibers behave
A synthetic material such as polyester is barely hygroscopic at all. The moisture therefore sits on the surface and between the threads rather than inside the fiber itself, which is why such a fabric can feel clammy against the skin sooner during heavy sweating.
The limit of what the fiber can hold
Once the fiber is saturated with moisture, it cannot absorb more, and only then does the fabric start to feel wet on the outside. That is why natural materials lose out during very heavy sweating, because the saturation point is reached and the excess then sits on the outside of the fiber.
Evaporation cools
Moisture that is later released from the fiber into the air carries heat away with it as it evaporates. That is the same mechanism you know from sweat evaporating directly off the skin, and it is the reason for the cooling feeling after activity. See bamboo underwear for women.
Climate and how it feels in practice
In humid or warm weather this property matters most, because the body sweats more and there is more moisture to move away from the skin. In dry, cool weather the difference between the materials is less noticeable day to day.
Fabric softener blocks it
A film of fabric softener sits on the outside of the fiber and stops moisture from getting in. The effect therefore fades gradually if softener is used at every wash. See washing and care for how to avoid that.
What you can do yourself
Wash without fabric softener, and let the fabric air dry or tumble dry on low heat, so the fiber's ability to absorb moisture stays as intact as possible over time. That is the simplest way to keep the property, without needing any special equipment.
Frequently asked questions
What does hygroscopic mean?
That a material absorbs moisture from the air and from the skin into the fiber itself, instead of letting it sit on the surface of the thread or fabric.
Are synthetic fabrics hygroscopic?
Only to a very small degree. They instead move the moisture along the surface and between the threads, which can feel different against the skin than a hygroscopic material.
When does the effect stop working?
When the fiber is saturated with water, it cannot absorb more. The fabric then starts to feel wet on the outside, instead of dry despite the absorbed moisture.
You can find the full range at Bamboo-Clothing.com.
