What is the best insulated underwear for keeping you warm? Well, first we must understand how Thermal underwear works, then we can look for those fabrics that have the best characteristics we are looking for in our base layer.
To begin, insulated underwear are not designed to keep you warm, but to keep you from getting cold. So you need to think in reverse logic.
So base layers are designed to keep you warmer, but how, by removing sweat. So you want a fabric (base layer) that fits tight to the skin, making as much contact as possible. Next this fabric needs to be hydrophilic, absorbing sweat quickly and hydrophobic, dry as fast as possible.
Looking at the different fabrics that thermal underwear/insulated underwear/base layers are made of you can easily see by the properties of those fabrics which ones are the best for outdoor adventures in the heat or cold.
Cotton – absorbs sweat well, but dries why to slow.
Polypropylene – absorbs sweat well, but retains body odor after just a few hours.
Polyester – absorbs sweat well, dries fast, great mobility, does not hold up over time.
Merino wool – hydrophilic, dries faster than Cotton, natural antibacterial properties (no odors). This is definitely the best overall fabric on the market for Thermal Underwear (Base Layers).
See Best Thermal Underwear (Base Layers) for Women.