Theo Prudon is a professor at the Graduate School of Architecture, Planning, and Preservation at Columbia University and the president of the modern conservation organization Docomomo U.S.

“I see two main pressures exerting themselves on the future of preservation: density and temporality. Around the world, most of the population is along the coasts, and as populations increase, we’re going to be putting more pressure on places that are not developed or are underdeveloped. That means that preserving low-density sites will be increasingly difficult.

“As for temporality, a postmodern building comes from the era of the five-year lease, not the 99-year lease, and that economic reality affects the building’s physical reality. It’s just not as well made, yet it’s far more complex than something from the 19th century. Complex, less-well-made buildings are tougher and more expensive to save, so the time to find more creative ways to use them will come sooner than ever.”

Sharon Park is the associate director of Architectural History and Historic Preservation, a division of the Office of Planning and Project Management at the Smithsonian Institution. “The green movement is interested in retaining usable buildings and not sending them to the landfill, which is great. But to do that, its supporters push renovation, not restoration or rehabilitation. I want to preserve the historical character of a building—–like a brutalist building or maybe a 1950s modern office building now slated to become apartments—–but I get nervous when we lose the historic details of a great old building so that a developer can score an extra LEED point for reuse. Preservation and reuse aren’t the same thing.

“Here in Washington, DC, a lovely modern apartment building was recently outfitted with new, more energy-efficient windows, and that’s great, but that building has lost some of its really wonderful, sleek lines. You could say I’m being overly precious, but it’s not the same. Maybe that’s the price we have to pay to revitalize our communities.”

Peter Alspach is a mechanical engineer and an associate at Arup. “One of the challenges I see for buildings that have not gotten some historical designation or been added to the National Register—–I guess you’d just call them old buildings—–comes from present energy-code requirements and upgrades. Look at a building from the 1940s or ’50s,essentially designed to function without air-conditioning, and on paper it looks like it should be an energy pig, but in practice it’s actually pretty good, because the building has good bones and makes use of daylight and natural ventilation. But you may not see that if you just base upgrade scenarios on how many panes the windows have or wall construction. If you’re forced to make particular upgrades, instead of looking at the whole energy picture, saving the building can become cost-prohibitive for the owner.

“If we move to a system where we take the whole building’s energy performance into account, we can get a really high-performing building out of something that might otherwise have been torn down. Lots of people are starting to do it this way now, but this method needs to be sealed up in actual city codes and ordinances across the country.”