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How They Pulled It Off: A Record-Breaking Passive House Behind a Refreshed Bungalow
This article details the development of a 5,000-square-foot duplex, the largest certified Passive House project in the South, located in Austin's historic Castle Hill District. Architect Trey Farmer of Forge Craft Architecture + Design, known for his commitment to Passive House standards, spearheaded this challenging project. The property includes a newly constructed duplex and a renovated 1924 bungalow, demonstrating how energy efficiency can coexist with architectural heritage on a compact, sloping lot. The Passive House standard, characterized by airtight construction and high-performance windows, aims to reduce energy consumption by up to 90 percent. Farmer's previous work, including his own Austin home, achieved Passive House Institute US (PHIUS) certification, leading to minimal energy bills.
The project initially planned as a custom home, evolved into three long-term rental units designed to accommodate families, emphasizing both sustainability and density. The duplex's garages are strategically offset by six feet to manage the slope, creating five staggered, finished levels that maximize living space without a sense of overcrowding. The renovation of the original 1924 bungalow involved interior designer Adrienne Farmer, who selected sustainable materials such as reclaimed shiplap and formaldehyde-free cabinetry. The duplex units, measuring 3,500 square feet and 1,600 square feet respectively, feature wood-clad interiors and skylights, thoughtfully integrated into the challenging site, which presented a significant twenty-foot grade change over a fifty-foot-wide lot.
Overcoming significant site challenges was central to the project's success. The construction team contended with Austin's clay soil, necessitating the installation of seventy-eight steel piles for foundational stability. Further complications arose with the discovery of a natural spring during excavation, requiring temporary damming for the concrete retaining walls. The Passive House precision was achieved by simplifying the form and framing and extensively insulating the structures. AeroBarrier, a mist-based sealant, was used during pressurization to ensure an extremely airtight building envelope, exceeding typical code requirements by a factor of ten.
Despite an initial budget cut of $1 million during the design phase, the team managed to achieve Passive House standards with only a 2.5% increase in costs, approximately $75,000. This additional investment covered features such as full stormwater reuse systems, heat pump systems, and net-zero ready infrastructure. The project utilized high-quality materials including Marvin windows, a metal roof, Zip-R sheathing, and blown-in recycled newspaper cellulose insulation. These elements contribute to superior indoor comfort, air quality, and effective noise reduction. Farmer views Passive House construction as a practical solution, using such single-family homes as a testing ground for future possibilities in sustainable building.
The project showcases a successful integration of historical preservation with cutting-edge sustainable building practices, creating highly efficient and comfortable living spaces within a dense urban environment. The detailed account of the construction challenges and the innovative solutions employed provides valuable insights into the complexities of achieving advanced energy performance in residential architecture.
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