Project: Tonalacalli custom home, Austin, Texas. Size: 3,082 square feet. Cost: $200 per square foot. Completed: June 2008. Certifications: LEED-Platinum, Austin Energy Green Building Program 5-Star rating. HERS Rating: 40. Architect: Environment Associates, Houston. General Contractor: Custom Building, Austin.
Austin, Texas’ first LEED-Platinum residence began with a simple request from the owners: a healthy house that would live in harmony with the land. The couple—he’s a geophysicist, she’s a biologist who grew up in Mexico—wanted to experience the low-impact pleasures of minimal energy bills, clean indoor air, and total reliance on rainfall for their household water needs. For a 34-year green building veteran like architect LaVerne Williams, AIA, LEED AP, it was a dream commission. Not only were the owners committed to making their home as environmentally friendly as possible, it was also an opportunity to work in a city whose builders are exceptionally well versed in sustainable materials and methods, thanks to Austin Energy’s venerable Green Building Program.
To Houston-based Williams, that difference is measurable. “The same house in Houston will cost twice as much as in Austin, because Austin’s electric utility is city-owned and they’ve been pushing the public to build green for almost two decades,” he says. “There’s an industry trained to deliver.”
Austin also delivers a mild climate and plenty of sunshine and rainfall. Williams’ design is a simple, common-sense response to the site’s microclimate using modern materials and technologies.
The house—dubbed Tonalacalli, which means “house of sun and water” in the ancient Nahuatl dialect of Central Mexico—is situated on 10 acres just outside city limits. Heating and cooling loads were minimized by orienting the house east-west, bathing the long south-facing front in sunlight while avoiding late-day heat gain from the west. Deep, shaded porches capture cooling breezes and add outdoor living space to the 3,082-square-foot house.
The need for air conditioning is delayed by two months, Williams says, thanks to north-facing operable clerestory windows in the master suite and the open-to-below second floor. When the windows are open, hot air is swept up through the clerestories and replaced with cooler air from the bottom of the house. “You have this Venturi effect creating the air flow, and also the heat stack effect with hot air rising,” Williams says. “It’s like having a whole-house fan.”
The house’s thermal mass also helps maintain comfortable indoor temperatures year-round. First-floor walls were built with 8-inch-thick aerated autoclaved concrete (AAC), a lightweight structural material with tiny insulating air pockets. In winter, the stained-concrete floor and AAC walls absorb and store the sun’s heat, moderating temperature swings by radiating it slowly into the home. The reverse occurs in warm weather, when the thermal mass slowly releases cooler nighttime temperatures throughout the day. This delays the need for air conditioning until nighttime, when the cooling load is lighter. Throughout the house, Energy Star–rated appliances and compact fluorescent bulbs also reduce electricity use.