Passive house construction is a standard that was developed in the U.S. in the 1970s and refined in Germany. “It relies on the quality of the building envelope to reduce energy use in the building,” explains Andreas Benzing, president of New York Passive House, a trade organization and educational nonprofit.
To qualify as passive, a building needs to be airtight and have continuous insulation, triple-paned windows, and a system for guaranteeing high air quality at all times. The building should also eliminate thermal bridging, a phenomenon that occurs when extreme temperatures reach the building’s interior, as in cold air transferring from the exterior, through window frames, to the building interior.
Achieving passive house certification for an existing building is difficult, but it is possible to retrofit a century-old building using passive house elements. There are two key factors to consider. “The passive house elements that are the most important are air-sealing and insulation,” observes Ryan Cassidy, director of properties at RiseBoro Community Partnership, a leader in affordable housing development and supportive community services. “Everything flows from those two things. Once you air-seal and insulate, then you can have really small, efficient systems.” But it’s not as easy as it sounds. “A retrofit poses a lot of problems because it’s a little harder to do work on the interior of apartment units,” says Cassidy. “That forces you to work on the outside of the building. The worst part is meeting the air-tightness standards, because the building is already built. You’re going to have to address the flaws in the current facade assembly from the outside.”
Many boards would probably balk at the thought of insulating and re-cladding the exterior of the entire building, but Cassidy points out that the effort might be worth it. “You could [renovate a] building that may not meet the passive house standard for air tightness,” he says, “but if it met the energy requirements, you would still have a building that would be saving 50 to 60 percent of what the typical building would be using.”
The population density found in co-ops and condos is, in this case, a benefit. “One of the advantages of multi-family passive house is that there’s a really big heat load in the building,” Cassidy says. “You have a density that you don’t normally get in a single-family residence. You can actually just do a double-pane window, and the window performance will be adequate. What you really need is for the window frames to not provide a big thermal bridge.” While there are arguments for nontraditional materials such as fiberglass and polyvinyl, passive house architect Chris Benedict says her projects often use aluminum frames with a core material – usually a type of plastic – that reduces thermal bridging. What’s the price for a retrofit? “The cost might not be as much as you think,” says Benzing. “If you do a new roof and you don’t insulate to a sufficient level, then you basically waste your money over the long run. The savings you would get out of insulating is probably much bigger over 30 years.”
Cassidy agrees. “If you’re in a masonry building that’s part of the Local Law cycle,” he says, “you’re likely spending a few hundred thousand dollars every 10 years re-pointing or correcting issues. If you redo your building with a highly insulated material, it gives you a huge thermal benefit on the operations side. You can’t really look at these projects without considering the savings in operations.”