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Conventional stage controllers (CSC) for interior environment control of agricultural facilities are prevalent and well suited for slowly varying loads, smaller facilities with few stages of control, and in cases where discrete proportional control is deemed adequate. Ad hoc implementation schemes for using the same CSC over a range in size of building heating and ventilating systems, from one heating and cooling stage to many, are practiced in the industry. A fuzzy logic controller (FLC) was developed to satisfy a broad spectrum of installation sizes without any modification. Principles for designing an environment controller that can mimic CSC behavior over a broad range of system size were applied. By adjusting a single additional input this controller provides users with a trade-off between energy use and control precision. Simulations were conducted using the same FLC in a greenhouse and a broiler house; these two examples were selected for their considerable difference in magnitude of energy transfer and loads. Disturbances investigated included: variations in outside temperature, internal heat load (or solar load), and step changes in set point temperature. FLC system responses are compared with a representative CSC for stability, overshoot and mean square error from set point temperature. The FLC was able to keep the root-mean-square errors to 1.0 to 4.0C, depending on different energy use settings. The FLC provided useful improvements in performance over a CSC, and is readily implemented in modern electronic controllers with floating point arithmetic capability.