Techniques for controlling moisture content of build material in a three dimensional printer are provided. An example system includes a build material vessel to contain the build material and receive a flow of air, and a humidifier to adjust the humidity of the air flowing into the build material vessel. The system also includes one or more sensors to determine a dew point of the air flowing into the build material vessel. The humidity level of the air flowing into the hopper is controlled by the humidifier to maintain the dew point at a level that will prevent condensation of water inside the build material vessel.