What it’s made of, and why it works

Impetus' commitment is certified by bluesign®. Its quality system is ISO 9001 certified, and its underwear is OEKO-TEX® STANDARD 100 certified [D2, D3].

According to Impetus, all its production facilities treat their own water, control their gas emissions and use biomass to heat water. Solar panels cover a large part of the industrial site, 70% of the company car fleet is electric or plug-in hybrid.

The mechanism, in two or three sentences.

Seamless means the garment is knitted to shape on a circular knitting machine, with few or no seams. Cut & sew means the fabric is knitted first, then cut to the pattern and sewn together.

Our Stealth range is seamless. All our other styles are cut and sewn, to patterns made in 3D.

Technical, in our world, means the pattern and the construction: where the seams sit, how the waistband is built, where the fabric is allowed to stretch and where it is not.

Functional means fit: the underwear stays where you left it, squats, long strides and the saddle included.

The fabric is made from fibre that does not absorb sweat but conducts it to the outer layers, where it evaporates. Polyamide is hydrophobic: sweat travels along the surface of the fibre and evaporates on the outside of the fabric. Cotton absorbs it and holds it against the skin.

Moisture travels along the surface of the fibre without soaking in. Liquid moves from wet towards dry, towards where the capillary channels are narrowest and the surface area greatest. In practice this is a property of hydrophobic fibres only.

Water vapour moves from higher vapour pressure to lower, and vapour pressure rises with temperature. Next to the skin it is warm and humid, further out cooler and drier, so the vapour pushes outwards. Evaporation binds heat, which cools the skin at the same time.

Not ventilation, but the rate at which water vapour passes through a fabric, measured as MVTR in grams per square metre per 24 hours. Under 5,000 is a modest figure, around 10,000 is considered good, over 20,000 is enough for running and the mountains, and the best membranes reach about 40,000. Any figure above zero earns the word, so ask for the number.

Synthetic fibres such as polypropylene, polyester and polyamide (ECONYL® among them) are hydrophobic: they do not absorb moisture. Natural fibres are mostly hydrophilic: they absorb moisture into themselves.

When cotton binds moisture, air is pushed out, and air is the insulating layer that keeps a person warm. Water conducts heat at about 0.6 W/(m·K), air at about 0.026, so wet fabric moves heat off the skin about 23 times faster than air [1, 2].

Cotton binds the most moisture; linen dries faster and feels cooler. Bamboo viscose is regenerated cellulose and binds as much moisture as cotton, though it is often sold as moisture-wicking. Of the natural fibres, merino manages moisture best, by a different mechanism: it binds moisture as vapour and stays warm even when wet.

A plastic bag keeps you wet because nothing crosses a solid film. Smoothshell™ is a knit, not a film: water vapour moves out and air moves in. Synthetic is not the problem. Sealed is. In technical underwear the pattern and the knit matter as much as the fibre: the best hydrophobic fibre fails if the knit or the cut is done poorly.