UFFI stands for Urea-Formaldehyde Foam Insulation, a type of thermal insulation material that was commonly used in the 1970s and early 1980s to insulate homes, particularly in North America. UFFI is a foam insulation that was injected into the walls of homes to improve energy efficiency by reducing heat loss. However, its use was largely discontinued due to health concerns related to the release of formaldehyde gas.