Brothers Paul and Bryan Schaefer started developing UnderWarmer in the St. Louis area in 2015 as a simpler way to stay warm outdoors. Battery-powered heated jackets and vests were available, but the Schaefers took a different approach. They built air-activated heat packs into a lightweight compression shirt, using the same basic type of warming material found in disposable hand warmers.
The design placed eight heat packs around the torso, with four in front and four in back. Once the package was opened, exposure to air started the warming process. The close-fitting shirt held the heat near the body and could be worn beneath regular cold-weather clothing. The Schaefers designed it as a disposable product rather than another piece of electronic outdoor gear that had to be charged and maintained.
Paul and Bryan worked with St. Louis designer Mike Lawler on the garment itself. Developing the product meant finding a way to combine the warming packs with a shirt that could be manufactured, packaged and stored without activating them before use. The finished UnderWarmer was sealed until it was needed, then opened and worn as a base layer. The heat packs could provide warmth for about 12 hours.
UnderWarmer reached the market in 2016. The Schaefers initially aimed it at people spending long stretches outside, including hunters, anglers, campers and football fans. The shirt gave them a product that occupied a space between ordinary disposable hand warmers and the more costly battery-powered heated clothing already being sold for outdoor use.
The brothers also protected the appearance of the garment. They and Lawler applied for a design patent on the compression undershirt in 2017, and the patent was granted in 2019. By then, UnderWarmer had moved beyond the initial product-development stage and the Schaefers were working to build a market for the shirt.
Rather than develop a large line of clothing, they kept the business centered on the original body warmer. UnderWarmer began selling directly to consumers and later opened the product to wholesale buyers. The business also worked on building relationships with outdoor retailers in the United States and Canada, where hunting, fishing and other cold-weather activities offered a natural market for the shirt.
The outdoor market became an increasing focus. UnderWarmer joined Faire, where independent retailers could order the shirts wholesale, and began pursuing reseller accounts directly. In 2026, the business exhibited at the Archery Trade Association’s annual trade show, putting the product in front of retailers and other businesses in the hunting and archery industry.
By 2026, Paul and Bryan were still selling the same basic product they had developed a decade earlier: a compression-style shirt with built-in air-activated warming packs. The design had also gained a reuse option for shorter outings. A partially used UnderWarmer could be placed in an airtight bag to stop the reaction and then exposed to air again later, until the heat packs had exhausted their roughly 12 hours of heating time.
