Abstract: An environmental room for controlling temperature and/or humidity levels at very close uniformity control levels, and at reduced energy requirements. Air flow of the system is divided such that only a portion of total air flow for the room is passed through the temperature and/or humidity air conditioning components, while the remainder of the air flow remains untreated. The treated and untreated air flow is then homogeneously mixed to obtain air having the required temperature and/or humidity levels. By passing only a portion of total air flow through the temperature and/or humidity control components, the required size and capacity of these components can be reduced, and energy requirements and costs for the environmental room can also be reduced. In addition, previously unattainable tolerance and uniformity levels are achieved.