First, about the term
In the English-speaking environment, this type of product is called underwear (what is under the clothes) or first / base lair (first / base layer). In our country, the term thermal linen has caught on and often misleads the buyer. After all, the question most often asked by buyers is "Thermo, does it heat?".
The spectrum of application of the functional base layer is extremely wide - it is any type of activity that requires the help of the body in the processes of thermoregulation, removal of moisture from the body in winter, demi-season and summer periods. A competent sales consultant, depending on the period of the year, the type of activity and the build of the buyer, will offer the most optimal version of functional underwear in terms of composition, zoning, cut and level of compression.
Components
At the moment, manufacturers of functional underwear produce products from synthetic yarn, merino wool, or mixes of these materials. Sometimes silk, cotton or wool of other origins, such as camel or alpaca, can be found in the composition, but these are rather exotic exceptions. As for synthetic materials, it should not be forgotten that, in addition to the chemical composition, the shape of the fibers is also of great importance. Many developments are registered as trademarks and used in descriptions as proper names - Dryarn® based on polypropylene, Coolmax® based on polyester, etc.
If you compare the characteristics of synthetic fibers and merino wool, then each of the materials has its own set of advantages.
Zonality
Different parts of our body react differently to physical exertion, unevenly releasing heat and sweat. Zones that need cooling sweat intensively - back, chest, axillary and inguinal zones. In turn, some parts of the body at low temperatures require additional thermal insulation - elbow and knee joints, abdominal area (solar plexus - a large cluster of nerve nodes), shoulders, lumbar region.
In accordance with the zonal structure of our body, thermal underwear should be designed. The material that covers the areas that require additional cooling should be thinner or have an air exchange structure. The material in the area of the knee joints, shoulders or other areas that require additional protection against hypothermia can be thicker or specially structured for better thermal insulation.
Cut and compression
Functional thermal underwear can be low, medium or high compression. And each of these options has its own set of advantages.
Merino wool yarn has low elasticity and, accordingly, underwear will have low compression. This allows you to wear it for a long time without feeling discomfort, including and not to change clothes during the night. High- and medium-compression synthetic underwear, irritating the nerve receptors on the skin, keeps the muscular apparatus ready for active actions, improves blood flow and the supply of oxygen and nutrients to the muscles. Suitable for active sports.