Abstract: An environmental control apparatus controls humidity and temperature in a closed environment such as may exist in outdoor electrical switchgear boxes and the like. Temperature and humidity sensors are coupled to a controller which compares the sensed conditions to low temperatures and a high humidity setpoint. If either the temperature is below the temperature setpoint or the humidity is above the humidity setpoint a heater is energized until the respective condition is corrected. A high temperature cutout is provided to prevent high humidity conditions from causing the heater to raise the temperature to excessive levels. Typically, the controller engages the heater at temperatures of below 40.degree. F., humidity levels of above 65.degree. b, with a high temperature cutout at 140.degree. F.