By DAVID E. MARTIN February 7, 2013
In a city renowned for its cutting-edge renewable energy programs and green building, Las Casas Verdes in Austin, Texas, stands out as the state’s first true all-solar development. All of its homes include photovoltaic (PV), solar water-heating systems, are designed and oriented using passive design principles, and are sized to meet most or all of the occupants’ electricity needs with just a 3-to 6-kilowatt (kW) system. The development is also the state’s first urban community to use rainwater for limited indoor use.
Austin Energy has awarded a Five-Star energy rating to all of the Las Casas Verdes homes built so far; in fact, the home at 3115 Sacbe was the highest-rated home in the city at its completion. The U.S. Environmental Protection Agency has awarded Las Casas Verdes homes a Five Star Plus rating, the highest Energy Star rating possible.
So how did a lone architect/builder like me build Las Casas Verdes? It began with a dream and a little help from family and friends.
Getting to True Solar
I have always had a passion for building things, whether boats or buildings, and whatever I built, I strived for it to be more efficient, longer lasting and a better value than the rest.
I was raised during the 1960s in South Texas, where abundant sunshine, heat, humidity and Gulf breezes are a way of life. People know from experience that the comfort level of their home is vastly improved through proper ventilation, shading and the direction the house faces.
While attending Texas A&M University, I studied solar design principles along with traditional architectural courses. Long before “green” was fashionable, I incorporated these principles and available technology in many of my college designs, as well as a home for my parents.
I graduated A&M in 1979 with a degree in environmental design and apprenticed for six years with firms around the state before starting my own company, Martin Associates Architects, in 1985. During my career, I have designed residential, commercial, federal and municipal buildings throughout the country; however, I always yearned to build a practical, attractive and
affordable solar home.
I got that opportunity in 1995, when I designed and built a home for my three daughters and myself. The design utilized passive solar shading techniques, including large covered decks that shaded windows during the summer and created wonderful outside living areas. The home included a thermal chimney, with motorized windows working in tandem with an air-recirculation system.
Through the years I refined my designs to create what I consider a true solar home, one that incorporates active-solar photovoltaic panels, solar water heating and passive solar design technologies as standard features, not add-ons or after thoughts. True solar design achieved the efficiency I desired, and I soon envisioned a community of these homes — homes that would have a traditional look and feel, yet produce as much or more energy than they consume.
It was on a trip back to Austin to visit my children and grandsons in 2007 when I discovered a 20-lot subdivision that had been abandoned for years. Though overrun with weeds and construction debris, this property looked perfect for a small solar community. The site had been a ranchette, so most of the trees that would impede an efficient use of solar techniques had been cleared. The land sloped to the southeast with a prevailing breeze.
I knew that a project of this type was well out of the norm for most lenders, and since I’m not a wealthy developer, I turned to family and friends first to fund the project. They had always supported my idea of a solar community and were eager to help. Together we scraped up the money to purchase the property, and Las Casas Verdes, “The Green Homes,” was born. It would be the first subdivision in Texas and the southern United States comprised entirely of true solar homes — not tract homes with a few solar panels attached, but site-specific, custom- designed solar homes that middle-income families could afford.
The first order of business was installing the infrastructure. I spent my days doing everything from supervising installations to operating equipment. By night, I designed the model home and finalized the motif for homes throughout the subdivision. Soon we had streets and utilities for every home site. By late 2008 we selected lot No. 6 for the model home.
Designing Based on Sun Angles
The first thing to consider when building a true solar home is the property’s solar characteristics. In the northern hemisphere, the sun arcs across the southern sky, meaning south walls and roofs receive nearly all the sunlight. During the summer, when the sun is more directly overhead, properly sized overhangs, awnings and porches shade these southern walls and windows, reducing the home’s overall heat load. In Texas, if you can stop the sun from hitting a wall or window, you’ve won half the battle of heat control. During the winter, the sun arcs much closer to the southern horizon, allowing southern walls to heat up and enter the windows, thus warming the home. This sun-angle information is invaluable in the initial design, as it determines the home’s proper orientation on the property, overhang dimensions and correct window sizes and locations.
Another passive solar feature I include in every Las Casas Verdes home is a thermal chimney. This feature has been around for thousands of years and works off the simple principle that hot air rises. The thermal chimney is the tallest part of the home and acts as a chamber to capture this rising air.
During spring and fall months, occupants open the home’s motorized awning windows, allowing the air to rise through the chimney and exit the home. At the same time, the low awning windows around the home’s perimeter draw in cooler outside air. In the winter, these windows are closed and the warm air is directed back into the HVAC system via a high return air intake, where it is recirculated throughout the house. Capturing, evacuating or recirculating this air is the key to controlling interior temperature cost-effectively.
It is important to remember that South Texas has scorching hot summers with high humidity and eight months of cooling load. Our winters are short and wet, so the main battle we face is humidity management year round. Our model home incorporated an energy recovery ventilator in its air-conditioning system; while it is effective, we’re building our newest home with an Aprilaire dehumidifier. This, along with running in fan-only mode, should stretch our temperate season at least two or three more months a year by dehumidifying the humid air of early fall and late winter.
The thermal chimney tower and sun-control devices are two of the more visible passive-solar features I include in every home. But a “hidden” technique I use is possibly more important and works off the same principle as the thermal chimney.
Every home in Las Casas Verdes utilizes a thermal-break roof system detail. Slotted spacers are sandwiched between the interior insulation layer and the outer metal roof, creating a space that allows airflow between the eaves and ridge. As the metal roof heats up, the air in this space rises due to natural convection and exits through the continuous ridge vent, drawing in cool air from under the eaves. This convection circulation reduces the roof’s heat load and drastically improves the home’s thermal properties.
But passive solar is only part of the equation for a true solar home. The home must also produce its own power.
Adding Active Solar
Homes in Las Casas Verdes are equipped with two active-solar technologies.
We offer a 3-kilowatt (kW) photovoltaic system as a standard feature in every home. As we consider Las Casas Verdes to be a test bed, we have homes using DC-based Fronius inverters with REC panels or AC Enphase microinverter technology using Siliken panels. On sunny days, homes in Las Casas Verdes send power back to the electric grid. We have not implemented battery backup in a home, but we will, when a buyer requests it.
The second active-solar device in every home is a solar water-heating system. The system utilizes a recirculating pump that provides instant hot water to all taps in the home. Unlike point-of-use push-button systems that require the homeowner to wait for the system to flush, our system “stirs” the water in five-minute intervals in a whole-house loop. The homeowner can set the intervals to match the household’s schedule of use. One common misconception is that tankless equals instant hot water. Not only is that untrue, but without a loop and with high usage, tankless can cost more to operate than a standard water heater.
On repeated cloudy days, an electric, high-efficiency water heater element in the tank takes care of the home’s needs.
Creating a Tight Envelope
My plan from the start was to construct the homes using structural insulated panels, or SIPs. SIPs are individual wall panels, some with doors or windows, that lock together to form a wall. They have a superior insulation value, create far less waste and offer faster construction times than traditional construction. After national price studies, I decided to build my own SIP panels using traditional metal stud framing techniques and insulated sheathing for a thermal break. When steel cost rose, we shifted to finger-jointed, reclaimed 2×6 framing on 24-inch (61-cm) centers and in 2- to 4-foot- long (0.6- to 1.2-meter-long) modules. All my homes are designed around this module, so the same panel detail can be used throughout the home/subdivision. We still offer the metal-stud SIP panels as an upgrade.
All exterior doors are fiberglass and installed with additional thermal seals. All windows are Andersen 400 series double-pane low-E glass, 0.2 solar heat gain coefficient-rated awning-type, which can be opened even during a light rainstorm. All Las Casas Verdes homes incorporate the same sizes and detailing to insure consistent energy efficiency.
I also utilize prefabricated SIPs for the roof systems. Like the wall panels, the roof panels are built with a moisture-proof exterior siding, but they use metal studs to span the longer distances required for our high vaulted roofs, part of the passive air-movement design. In addition to the thermal-break detail, these panels have additional insulation and are secured with hurricane straps at every joist to create rigidity throughout the entire structure.
Putting Sustainability in the Details
I had always envisioned building a solar community using sustainable materials and practices, and that’s exactly how every home in Las Casas Verdes is constructed.
Each home is equipped with a 2,000-gallon (7,571-liter) rainwater-harvesting system. Rain- water is collected from the roof, pre-filtered and stored in an aboveground outdoor water tank. This water is piped to all outside hose bibs and used to flush the home’s commodes. While the system has an auto municipal backup valve, which maintains a small amount of rainwater in the tank until the next rain event, it can also be switched to full municipal water if needed for a longer drought period. The model was the first home in Texas to be tied to the city water grid utilizing pressurized rainwater as a second water source for commodes and irrigation. As of this December writing, Las Casas Verdes remains the only subdivision in the state with homes offering this dual plumbing system.
Other green and money-saving standard features I’m proud to incorporate into every Las Casas Verdes home include the following:
- 16 SEER or higher HVAC system with a natural gas furnace or heat pump.
- Geothermal systems as an upgrade
- Multispeed fans and variable-speed compressors.
- HVAC systems are multi-zoned via duct controllers to allow for heating or cooling only occupied rooms.
- Energy Star programmable thermostats with humidistat.
- James Hardie ColorPlus flame-resistant, recycled fiber-cement siding, which comes with
a 15-year warranty on the finish.
- A standing-seam “cool” metal roof manufactured by Central Texas Metal Roofing Supply in Austin.
- Energy Star-compliant appliances.
- All paints and stains are no-VOC. Even our cabinets are finished off site to keep the home no-VOC.
Refining for Greater Efficiency
We’ve built three residences since the model home was completed in 2009, and I’m very pleased with how Las Casas Verdes is coming together. The community offers a common garden area to help our residents save on grocery bills, as well as a park. It will soon feature a bio-filtration natural swimming pool. The property is on a city bus route. We are two blocks from one of Austin’s largest parks and only 3 miles from major shopping centers.
The model home obtained a HERS rating of 31 with the standard 3-kW PV system. Our homes under construction have just received a HERS rating of 9, with improved glazing, A/C efficiency and a larger, 6-kW PV system. We signed contracts for three new homes in December and are confident our next homes will be very close to a net-zero HERS rating, producing as much electricity as they
Our family is proud of the steps we’ve taken to demonstrate that with knowledge and careful planning, a builder, no matter how small, can make a big difference in how society uses energy. Soon we hope to build more Las Casas Verdes communities around Austin, in cities across Texas — and possibly, in a city near you.
David E. Martin is a native Texan. He is an architect, builder, developer and solar installer. He lived and practiced architecture in Austin until 1999 and then moved to Miami to take part in the BT Global Challenge around-the-world sailboat race. He and his wife, Rhonda, not only talk green, but lived it aboard their 42-foot ketch sailboat, “Rhonda vu,” for nine years, traveling the East Coast of the United States while practicing the conservation of water, fuel and electricity. They moved back to Austin in 2008 to begin work on this project. Both he and Rhonda believe that Las Casas Verdes and projects like it are a necessity for the future of this country and our nation’s families.