Apparatus and method for depositing water onto the surface of a substrate passed through the apparatus. The apparatus has a housing that defines an internal atmosphere confined in at least partial isolation from the ambient atmosphere. The housing has a moisture saturation portion (12), a deposition portion (36), and a passageway (16). The moisture saturation portion has a water supply (10) and a vapor reservoir. The water supply contains a quantity of water. The vapor reservoir holds a volume of the interior atmosphere in thermal and phase equilibrium with the water in the water supply. The deposition portion has a plenum wall (86) surrounding a central chamber. The plenum wall is substantially insulated from ambient temperature variations and has a pair of opposed substrate gaps (82). The gaps define a path (84) for the substrate through the chamber. The passageway (16) communicates with the vapor reservoir and the chamber. A primary heater (56) is disposed to heat the water in the water supply. A secondary heater (68) is disposed within the passageway between the vapor reservoir and the chamber. A fan (66) drives the internal atmosphere from the vapor reservoir through the passageway to the chamber. A controller (92) is operatively connected to the primary and secondary heaters. The controller senses wet and dry bulb temperatures of the internal atmosphere in the chamber. The controller actuates the primary heater at a range of wet bulb temperatures below a predetermined wet bulb limit temperature and actuates the secondary heater at a range of dry bulb temperatures below a predetermined dry bulb limit temperature. The dry bulb limit temperature is greater than the wet bulb limit temperature.