Abstract: Method and apparatus to control temperature and humidity in drying a material, for example green lumber, including a chamber to receive the lumber in stacked relation with air flow space between individual lumber pieces, a refrigeration system having a refrigerant compressor, evaporator and condenser where the condenser is disposed within the chamber, blower means to circulate air from the condenser over and through a stack of lumber, conduit means to communicate with the chamber for emission of air passing over the stack of lumber where the evaporator means is disposed to selectively receive the air flowing to the first conduit, solar cell means to receive radiant heat and having an inlet communicating with the first conduit and an outlet communicating with the chamber, third chamber means communicating with the first conduit and the chamber, damper means to selectively proportion air from the first conduit to the second and third conduits, controller means responsive to the temperature of the chamber to ope