The distribution of air gaps and moisture in thermal protective clothing has a large and complicated impact on thermal protective performance. The effect of air gap size on the thermal protective performance of flame-resistant fabrics with different moisture content was investigated under intense exposures. The air gap sizes from 0 to 24 mm were analyzed using an air gap height regulation device. Fabrics with different moisture content were prepared, and the thermal protective performance was evaluated. The results showed that the effect of air gaps was influenced by the amount of moisture added to the fabric. It was also determined that the moisture in the fabric significantly increased the thermal protective performance (P < 0.05). The positive effect of moisture was enhanced by the amount of moisture if the air gap size was less than 12 mm; the effect of moisture varied for air gaps larger than 12 mm. The mechanisms associated with heat and mass transfer in moist fabric were discussed. The results suggest that protective clothing design should consider the combined effects of air gap and moisture. Based on the current study, air gaps of 9–12 mm seem to achieve maximum thermal protection.
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