The North Face underwear thermo light tight with 

HyActive - The North Face base layer fabric technology. HyActive is a whole line of base layer fabrics that should be selected depending on the external temperature and intensity of use. HyActive fabric consists of ultra-light hollow fibers that provide effective heat storage without being overweight. The main objective of HyActive fabric is to remove moisture to the surface of the base layer, where it then evaporates. The fabric also has persistent antibacterial properties.

Fabric:97% Polypropylene/3% elastane