An improved numerical model is developed for coupled heat and moisture transport in fire protective suits exposed to flash fire. This model is combined with Pennes' bioheat transfer model and subsequently, second-degree burn time is estimated using Henriques' burn integral. Natural convection is considered inside the airgap present between the multilayer clothing ensemble and the skin. Comparisons of temperature and moisture distribution within the multilayer clothing, airgap, and the skin during the exposure are presented considering combined heat and moisture transport and only heat transport. The effect of moisture transport on the protective performance of the fire protective suits is shown. The impact of both horizontal and vertical airgap orientations on second-degree burn time is studied. The effect of temperature-dependent thermophysical properties, relative humidity, fiber regain, different exposure conditions, and fabric combinations for the fire protective suits on burn time is analyzed.