In this study, we evaluated the long-term antifungal effectiveness of 3 types of interior building materials (gypsum board [GB], cement board [CB], and softwood plywood [S-PW]) impregnated with thermally reduced silver nanoparticles supported by titanium dioxide (AgNPs/TiO2) under 95% relative humidity for 4 weeks. AgNPs/TiO2 was synthesized at 2 thermal reduction temperatures (TRTs, 120 and 200°C) with 2 different AgNP weight percentages (2 and 5 wt%). Four different silver-loading levels (SLLs, 0.025, 0.05, and 0.5 μg/cm2 and the critical concentration required to inhibit fungal growth on agar plates) and 3 fungal species (Aspergillus niger, Penicillium spinulosum, and Stachybotrys chartarum) were used in the experiments. Higher temperature reduced more ionic Ag+ to metallic Ag0 and increased the dispersion of Ag on TiO2 surface. The 200°C thermally reduced AgNPs/TiO2 demonstrated excellent antifungal efficiency: Mold growth was almost completely inhibited for 28 days at the low SLL of 0.5 μg/cm2. Additionally, AgNPs/TiO2 exhibited higher antifungal activity on GB and CB than on S-PW. The stepwise regression results indicated that the TRT of AgNPs/TiO2 (β = −0.739 to −0.51), the SLL (β = −0.477 to −0.269), and the Ag0 level in the AgNPs (β = −0.379 to −0.136) were the major factors influencing antifungal activity and TRT might be the most significant one.