Firefighters’ protective clothing (FPC) is essential for protection against thermal and physical threats.  FPC must be comfortable, enable heat transfer from the wearer to the environment and should not restrict motion. The application of a cooling vest under protective clothing may prevent overheating by cooling at the microenvironment level while working in a hot environment. In this study, the passive system, in the form of a non-woven fleece material, was investigated. This system was distributed across the surface of a vest on the upper front chest and back. In this passive system, hydro-crystals swell on contact with cold water, lowering body temperature or holding the body temperature at a normal level. The thermal insulation of the wet and dry cooling vest was tested using a thermal manikin. Results show that the thermal resistance of the system dropped significantly due to replacement of dry air with moist air with higher thermal conductivity. This reduction was proportional to the amount of moisture present within the system.