Can thermal underwear be worn close to the body? Can it be worn outside?

Buy thermal underwear are said to wear outside, in the end thermal underwear is close to wear good. Very tangled! Thermal clothing, refers to close-fitting clothing, as long as a thermal clothing can replace the bloated knit sweater, lock the heat, and maintain the body is not bloated, loved by the majority of consumers, but thermal clothing can be worn outside, there is a need for close-fitting, the material requirements are relatively high, the version of the type and warm function is demanding, the fabric must be pure cotton, no static electricity, soft sweat absorption, It should be breathable, but also to lock the heat, I have bought a few pieces of thermal underwear that can be worn outside in a treasure, the quality is very good, the following examples hope to help you