Polypropylene thermal underwear is the best base layer to keep you warm and dry because it has the lowest thermal conductivity of any traditional long underwear fabric. Thermal conductivity is the measurement used to determine the rate at which heat (from your body) is transferred through the fabric to the outside where it is lost. A lower thermal transfer rate makes the best long johns because less heat will be lost over a longer period of time. Polypropylene has the lowest body heat or thermal conductivity of any traditional apparel fabric including silk, polyester and wool. If cared for properly you’re long johns will provide years of warmth and comfort.
In recent years polypropylene fabric has been exceedingly difficult to find; impossible if you were searching for a Made in the USA product. Many brands you may be familiar with like the Bass Pro RedHead E.C.W.C.S. claim to be “military fleece thermal” and they are not. These products and others like them are produced with polyester, not genuine polypropylene. Polyester is stiff and not nearly as soft and comfortable as polypropylene. You really can feel the difference. Polypro is a term which is generally used equally for polyester andpolypropylene long johns; depending on what is most fitting for the user’s purpose. The trouble with just a generic term is it’s not really a fabric. If taking the time to buy long johns to keep yourself or someone close to you toasty and warm this Winter, be sure to make sure you know what you’re buying.
The polypropylene fabric used to produce our long underwear is a thermoplastic polymer used in a variety of applications, including as a ECWCS base layer (Extended Cold Weather Clothing System). The ECWCS was developed by the United States Army as a protective clothing system. The system used a triple layering system to created an insulated platform to maintain a soldier’s mobility and survivability in extreme environments. The entire system consisted of 22 total items from base layer to exterior parka including glovessocks, boots, etc. The original polypropylene thermal base layer is, in our opinion, the best performing and most comfortable product to be worn for everyday us as thermal underwear outside the confines of a military survival system.