Insecticide dust formulations are considered as more effective for controlling the common bed bug, Cimex lectularius (L) (Hemiptera: Cimicidae), than residual sprays. Various environmental factors may affect the efficacy of insecticide dusts. In this study, we evaluated the effect of moisture on the efficacy of three insecticide dusts against C. lectularius. Moisture was created using two methods: applying steam to insecticide dust-treated tiles and aging insecticide dust-treated tiles in chambers with various levels of relative humidity (RH). In the steam treatment, three insecticides including Cimexa (92.1% amorphous silica gel), Alpine (0.25% dinotefuran, 95% diatomaceous earth), and Tempo (1% cyfluthrin) were evaluated. Exposure to steam significantly reduced the efficacy of all three insecticide dusts. Among the three insecticides, the efficacy of Cimexa was significantly higher than Alpine and Tempo. In the different RH treatments, Cimexa treated tiles that were aged under different (52, 75, and 100%) RH conditions for 1 and 2 mo caused significantly lower mortality to C. lectularius than nonaged Cimexa. We conclude that both, a short period of exposure to steam and long-period aging in a moist environment, can significantly reduce the efficacy of insecticide dusts. Moisture needs to be considered when applying insecticide dusts for controlling bed bug infestations.