The best waterproofing agent among the options available in the United States is typically polyurethane. Polyurethane is favored for its versatility, durability, and ability to form a seamless, flexible barrier against water. According to a study published in the Journal of Coatings Technology and Research, polyurethane coatings have shown to have a higher resistance to water penetration compared to other types of waterproofing agents, such as acrylics and epoxies. This is due to the chemical structure of polyurethane, which allows it to expand and contract with the substrate, thus maintaining a tight seal even under extreme weather conditions. Additionally, polyurethane coatings have a longer lifespan, requiring less frequent maintenance compared to other alternatives.