Fire-retardant performance was imparted to the existing wood-fiber insulation boards (WIB) via internal and external treatment with silica- and phosphorus-based fire-retardants. The combustion and smoke characteristics were investigated using a cone calorimeter. Based on combustion for 600 s, the weight loss and shrinkage of WIBs decreased due to fire-retardant treatment. The time to ignition was delayed to more than 400 s on the WIBs treated internal and external fire- retardant (WIB-IEs), whereas that of WIB with only internal treatment (WIB-I) was 5 s. The overall heat release rate (HRR), HRRpeak, and total heat release (THR) of WIB-IE specimens decreased, and the fire resistance standard Class II was satisfied. The WIB-IE2 showed higher fire resistance performance, with a HRRmean level of 6.7 kW/m2 and a THR of 1.3 MJ/m2. The WIB-I showed extremely low total smoke release (TSR) compared to the external fire-retardant treated specimen. However, the externally treated WIB-IEs had an increased TSR of 165 to 256 m2/m2 due to the increase in incomplete combustion caused by the fire-retardant. After fire-retardant treatment, CO2 generation decreased because the rate of complete combustion decreased, but CO emission increased slightly. Therefore, silica- and phosphorus-based fire-retardants by internal and external treatments were suitable for WIBs.