In colder weather it is best to layer your clothing. That way you can put on or take off what you need as the temperature rises or falls. However, more importantly this layer of clothing, called the Core or Foundation layer must be able to wick away sweat quickly so you feel dry no matter how hard you exert yourself in cold weather. Look for underlayers that have that breathability factor. This is especially important if you are working out, downhill skiing or taking a long hike in cold weather. If you intend to get sweaty then make sure that the clothing can help keep the moisture out of your clothes.
Some modern day core layers are absolutely amazing, not only in their construction and fabric that makes them feel like a second skin but in the way they are now treated to so sweat does not build up and make them smell. This is especially important if you do not have a change of clothes and you have a natural tendency to sweat a lot.
Core layers usually include a long-sleeved top and modern long johns although you can buy cleverly constructed one-piece foundation layer garments to keep you warm.
These undergarments mostly come in Lycra or Cotton blends but the height of fashion are those made out of Merino or a fabric called Smart Wool. Smart wool is a type of soft Merino that has the natural ability to keep the moisture away from the skin and the fabric also naturally inhibits odor. Flatlock seams help these slenderizing garments fit nicely under your insulating and top layers of clothing. However this core garments made out of these materials tend to be a lot more expensive than cotton or Lycra styles running mostly at over $100 or more per top or bottom.
Yet another feature of these clothes is a treatment of the fabric with “clothing sunblock.” This UPF 50 protection blocks the sun, which is good if you are mountain climbing or skiing in an extremely sunny place.
Insulated base layer pants and tops are very sexy and many women wear these as part of their every daywear or work out wear. Companies that make these gorgeous garments include Mountain Hard Wear, The North Face, Ice breaker, Patagonia and Spyde.