Located in the heart of Pittsburgh, the Phipps Conservatory and Botanical Gardens has “the greenest garden in the world,” according to director Richard Piacentini. He could be describing the Phipps' Thai-style forest filled with winding pathways and diverse vegetation that he created—or the new Tropical Forest Conservatory that encloses it. The 12,000-square-foot sustainable building, designed by IKM Architects, overcomes the problems the Phipps faced maintaining the interior temperature and humidity in its beautiful but inefficient Lord & Burnham conservatory built in 1893. The new design is engineered to get the most out of natural sunlight: On a glazed roof, light is often hard to control and gets reflected away in the winter, when it is needed most. The tall south wall (shown right) admits the most direct sunlight, allowing for better control and better penetration into the space. Roof glazing provides plants with much-needed sunlight on overcast days, but is angled down and away from the direct sun to avoid oversaturation.

J. Patrick Rand is a professor at the North Carolina State University School of Architecture.

HEATING AND COOLING: Outside air is pulled through six 24-inch-diameter concrete tubes that are 300 feet long and buried deep in the earth, where the temperature is 55 degrees Fahrenheit year round. In the winter, the tubes deliver air that is passively warmed along this path before entering the interior space. In summer, the same tubes direct passively cooled air to the spots where visitors are most likely to linger.