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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 better 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.


The U.S. Military chooses polypropylene long underwear as their cold weather base layer of clothing and for good reason. Military Thermals long johns keeps you warmer and dryer. Even at expedition weight these long underwear are thin, light and do not have the bulk of other materials such as wool that would be required to provide the same warmth. This combination of thermal efficient fabric and weight, gives you more freedom of movement while maximizing warmth. Polypropylene long underwear is ideal for under work clothing and for outdoor sporting activities like hiking, biking, fishing and camping just got more enjoyable when you are warm.


The polypropylene fabric used to produce our long underwear is a thermoplastic polymer used in a variety of applications, including as a base layer in the first generation of the ECWCS (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 gloves, socks, boots, etc.

The original Gen 1 Extended Cold Weather Clothing System (ECWCS) was upgraded to the second generation to improve limitations of the parka fabric which did not breathe well and as such created limitations for the system as a whole in the field. The second generation, Gen 2 ECWCS included two different layers made with Polartec® fabrics: the Polartec Classic 300 shirt and the Polartec Classic 200 overalls. The parka was improved with vents to help deal with the condensation problems created by the waterproof parka material.

Third Generation, Gen 3, Gen III, ECWCS was a substantial change from the prior two generations, featuring 7 new layers of insulation. Three of these new layers are Polartec fabrics; two layers of Polartec Power Dry® and a layer of Polartec Thermal Pro High Loft®. With these changes brought additional expense to the system as a whole, specifically to the lighter weight, higher cost Polartec base layers.

These new Gen 3 Polartec base layers do preform their function better within the confines of the system for which they were designed. Do they perform equally as well outside of the ECWCS? We think they do not. The original Gen 1 polypropylene thermal underwear is in our opinion the best performing and most comfortable product to be worn for everyday use as thermal underwear outside the confines of a military survival system. Many of our armed service men and women still prefer Gen 1 polypropylene and many military procurement specialist purchase our product to serve this demand.


In recent years polypropylene fabric has been exceedingly difficult to find; impossible if you were searching for a made in the USA product. The immediate solution was a Chinese produced alternative. Frankly, it’s not a bad option and one that even we were forced to embrace while searching for an alternative solution. 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.


Military Thermals is proud to produce our own brand of genuine US Mil-Spec Gen 1 thermal underwear at our offices in Fort Wayne, IN. Military Thermals brand polypropylene thermal underwear are produced with 100% materials made in the USA and with 100% US labor. Military Thermals are 100% made with pride in the USA!

Polypropylene is highly colorfast and is widely used in manufacturing carpets, rugs and mats to be used at home as well as thermal long johns. Polypropylene is widely used in ropes, distinctive because they are light enough to float in water.For equal mass and construction, polypropylene rope is similar in strength to polyester rope. Polypropylene costs less than most other synthetic fibers.

Polypro is a term which is generally used equally for polyester and polypropylene 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. If you have questions, please feel free to reach out.

Learn more about caring for your polypropylene thermal long underwear. If cared for properly you’re long johns will provide years of warmth and comfort.