The purpose of this research was to evaluate clothing ventilation designs for their ability to reduce heat stress incurred during firefighting activities. Ventilation applications were implemented on structural turnout suits, including both active and passive systems, to determine the benefit of ventilation towards heat loss. A total of five different designs were evaluated on a sweating manikin for thermal and evaporative resistance. From these measurements, a predicted total heat loss (THL) was calculated for each design compared back to a control suit with no ventilation. A large improvement in heat loss was measured, specifically, with the maximum open ventilation design. Overall, ventilation designs in structural firefighter turnout gear show a statistically significant increase in heat loss improvement for the wearer.