Thermal underwear, or thermals, may be the bullied little brother of the garment world, but take one camping trip in November and you will realize just how useful it is. Choosing the best thermal underwear can be daunting, particularly if you’re unfamiliar with the varieties and options available.
Thermals of any fabric come in different weights across the spectrum, and each weight offers different advantages. Lightweight is more conducive to moderate temperatures, and is also preferred for a high level of aerobic activity. Midweight is the most versatile, and can be used for the biggest variety of sports and recreational activities. Expedition weight is preferred for frigid temperatures, when more warmth is needed.
Cotton is not a good choice for thermal underwear, because the material does not insulate--in fact, cotton keeps your body cooler. According to AdventureSportsOnline.com, "The primary intent of a base layer/underwear is to manage moisture (wick or transport perspiration away from your skin) while adding a degree of insulation," so using any material that doesn't provide that insulation means you're just wearing an extra layer of clothing.
Synthetic materials, including polypropylene and polyester, offer cheaper alternatives to wool underwear and are more economical options for children, who will likely outgrow thermals after a few uses.
Polypropylene is, according to AdventureSportsOnline, a hydrophobic material, which means it does not absorb moisture (perspiration) from the body. Because of this, the body can retain moisture, and damp skin quickly becomes cold skin. It can still work as insulation, though, as long as a wool or fleece item is layered on top to do the work of wicking moisture away from your body.
Polyester, an easier to care for and more UV-resistant material, does not hold heat as well as polypropylene. It is, however, easier to care for and easier to wash, and has a higher melting point than polypropylene, meaning it is less likely to melt to your skin or to melt in the dryer.
Wool thermals, the most well-known and well-loved thermals, are enjoying a surge in popularity after being low on the totem pole for a few years. Merino wool is the most popular choice for thermal underwear, because it is not as itchy as other wools.
Because of the expense, wool is often not the first choice for children's thermals--the likelihood that a child will outgrow them before they have "paid for themselves" is too high. Bear in mind, though, that wool does not hold odor in the same way synthetics do, so making sure your son or daughter is not the "smelly kid" at the campout may be worth the extra expense.
Wool/polyester and wool/polypropylene blends are popular because they offer the advantages of both wool and synthetic materials. AdventureSportsOnline cautions "the one drawback to this approach is that because of the wool, the underwear takes longer to dry." Keep this in mind when packing for a camping trip, and remember the extended drying time when laundering the thermals.