In Austin's historic Castle Hill District, where charming late 19th-century homes line walkable streets just minutes from downtown bars and restaurants, an extraordinary architectural achievement has quietly emerged. The largest certified Passive House project in the American South now sits on an unassuming lot, featuring a newly constructed 5,000-square-foot duplex alongside a beautifully restored 100-year-old bungalow.
Architect Trey Farmer of Forge Craft Architecture & Design has made Passive House construction his signature specialty, implementing rigorous building standards that emphasize airtight construction, high-performance windows, and innovative techniques that can reduce energy consumption by up to 90 percent. His own Austin residence, a 1914 Craftsman home, was among the first Passive House Institute US (PHIUS)-certified projects in the region, achieving remarkable results with average monthly energy bills of just $10. "We're aligned with our clients and our builders," Farmer explains. "We're being sought out for it. The market has caught up with us a bit."
The project underwent significant changes during development, originally designed as a custom home for a young family before evolving into three separate long-term rental units, each spacious enough to accommodate families. "We think of Passive House from a sustainability standpoint but also in terms of density," Farmer notes. The challenging topography actually became an asset, as the sloping terrain allowed the duplex garages to be offset by six feet, creating five staggered, finished levels. "There's a lot of density without it feeling dense," he adds.
The restoration of the 1924 bungalow showcased sustainable design principles throughout, with Farmer's wife, interior designer Adrienne Farmer, leading the materials and finish selections to align with environmental standards. The renovation featured reclaimed shiplap for an airy interior atmosphere and formaldehyde-free cabinetry, demonstrating how historic preservation and modern sustainability practices can work together seamlessly.
The new duplex, comprising a 3,500-square-foot unit and a 1,600-square-foot unit, features wood-clad interiors with skylights strategically placed throughout, carefully integrated into what Farmer describes as an extremely challenging building site. "It was crumbling and unsafe," he recalls of the original conditions. "It's almost twenty feet of grade change on a fifty-foot-wide lot—feels like you're in San Francisco, not Texas."
The construction process required innovative solutions to overcome significant obstacles, beginning with Austin's notorious clay soil conditions that created muddy construction challenges. "We drove seventy-eight steel piles into the ground until friction stopped them—basically like a hairbrush in mud—then tied them together with concrete footers and floating foundations," Farmer explains. The excavation timeline was further complicated when construction crews encountered a natural spring, requiring them to "dam the spring long enough to pour the retaining walls."
Farmer applied lessons learned from his own Passive House experience to refine the construction approach, maintaining simple forms and framing while "insulating like crazy." The German-born Passive House building standard focuses on dramatically reducing energy consumption through impermeable construction methods and highly efficient ventilation systems. A key breakthrough came with AeroBarrier application, a mist-based sealant applied during pressurization that transformed the home from "very airtight to extremely airtight," according to Farmer. While the house was already five times more airtight than building code requirements, Passive House standards demand ten times greater airtightness.
Despite budget constraints that required cutting $1 million from the original design, the team achieved Passive House certification by spending just two-and-a-half percent extra—approximately $75,000—to meet the demanding standards. These standards include comprehensive stormwater reuse systems, advanced heat pump technology, and net-zero-ready infrastructure. "You can't hear any outdoor noise," Farmer observes. "The comfort and air quality are unmatched." The project incorporates premium materials including Marvin windows, metal roofing, Zip-R sheathing, and blown-in recycled newspaper cellulose insulation.
Looking toward the future of sustainable construction, Farmer sees tremendous potential in this approach. "Passive House seems like a solvable problem to me—that's what attracted me to it," he reflects. "We're in a good spot to use single-family homes like this as a sandbox for what's possible." The project demonstrates how energy efficiency and architectural heritage can not only coexist but actually enhance each other, setting new standards for sustainable development in historic neighborhoods across the South.
