Layered bands of crushed limestone and cement make the golden-hued walls of a Southeast Side home look as if it’s been there for a century.
In fact, the house was made possible through advances in construction science and technology, and will be completed in less than a month.
The rammed earth house is a prototype, built by San Antonio Affordable Housing (SAAH) as a model for future affordable housing with low maintenance costs.
Made of crushed limestone, water and 3% Portland cement, the exterior walls of the house on San Salvador Avenue are 18 inches of thick, solid rock, making them durable, energy efficient and environmentally responsible.
Rammed earth is the term for a wall system made of compacted gravel, sand and clay that is extremely strong and durable.
It is not unlike how homes were built before lumber and other materials became more widely available at the turn of the 20th century.
Though rarely used today, the building technique is especially suitable for the climate in South Texas, said David Komet, a passive house consultant with Urban Earth who’s helping out with the project.
“This is actually considered a future material, even though it’s an historic material,” he said. “More and more people are starting to design with it because it makes sense.”
Komet previously used rammed earth when he developed a multi-family property at 3050 Eisenhauer Rd. in San Antonio. A single family residence recently for sale in south Bexar County is also constructed using the rock and cement method.
But the San Salvador house represents the first time rammed earth’s been used on an affordable home prototype.
“Nobody is doing this, no one’s modeling it,” said Jaime Damron, program manager at the City of San Antonio, who is leading the SAAH project.
Damron wanted to be the first to try it and worked with architect Michael Clancy and Senior Principal Jim Bailey of Alamo Architects to kick off the project.
Established in 1995, SAAH is a component of the city’s Urban Renewal Agency in the Office of Urban Redevelopment. The nonprofit develops affordable housing, improves existing affordable housing stock and works to improve the community through housing and community programs.
The organization’s goal was to come up with recommendations on how to promote and build climate resilient housing as infill development by determining the real costs, challenges and red tape involved in rammed earth construction.
“The main cost that drives affordable housing expenses over time is maintenance and electricity costs — those are things we cannot control,” Damron said.
Situated next door to another SAAH-built and energy-efficient home, blue with white trim, the rammed earth house has thick walls that help regulate interior temperatures, keeping the home warm in winter and cool in summer.
The striated stone walls work with the mechanical air conditioning system to dehumidify the air in the room, making it feel cooler. Plus, the rammed earth house is pest-proof, rot resistant and requires no painting or siding.
In a recent air exchange test, the 1,211-square-foot San Salvador house came in at three air changes an hour, which would translate to utility costs of about $300 a year for air conditioning.
The average home has five to seven air exchanges an hour per code.
“Half of our experience inside of any space, temperature-wise, is what’s in the air and what’s radiant, so if the walls are cooler, it pulls heat off your body,” Komet said. “So it’s not just pretty, there’s a function to this, and that’s why people used to build like that.”
SAAH projects like the rammed earth house and others are modeled using data that builders can use to make decisions on how the home is built and what materials are used to create energy cost savings.
Throughout the project, a research team led by Antonio Martinez-Molina and the UT San Antonio Klesse College of Engineering’s building performance lab has been assisting with testing and analysis.
Martinez-Molina is associate professor of architecture at the Antoinette Westphal College of Media Arts & Design at Drexel University in Philadelphia, and continues to support the research through UT San Antonio.
His work also supported SAAH’s construction of the neighboring traditional wood-frame house, built previously using energy-efficient standards and materials.
“Our team supported the project by evaluating and optimizing the environmental and energy design and performance of the rammed earth prototype and comparing it with other common residential construction systems in San Antonio,” Martinez-Molina said.
The team used advanced building energy simulations and life cycle assessment tools, he said, to analyze and refine the building design to reduce energy losses, improve energy efficiency, enhance indoor comfort and lower the overall carbon impact and operational costs over the building’s lifespan.
The downside to rammed earth construction is the higher upfront cost — generally about 20% to 30% more than a traditional wood-frame house.
The total construction budget for the SAAH prototype, including site improvements, is $246,000, or $202 per square foot.
The house will be marketed for $260,000 with eligible buyers having 120% of the area median income, or $115,900 annually for a family of four.
For the build, SAAH provided a loan of $125,000, Damron said, which is repaid at closing.
The city supplemented that amount with a $30,000 development incentive and fee waivers. The San Antonio Water System contributed an infrastructure grant for the sewer system.
In addition to rammed earth, other energy-efficient materials and techniques have been used in building the house, including thermally insulated panels in the ceiling, reflective roofing material and rigid foam insulation between the foundation and walls.
The house also has an energy recovery ventilator, a whole-house ventilation system that continuously replaces stale indoor air with fresh outdoor air.
Projects like the San Salvador house also require a skilled and knowledgeable builder since it’s never been done on that scale, Damron said.
Modern Earth Construction owner Carlos De Luna has been building rammed earth structures for almost two decades, about two or three a year, he said, mostly on private ranches and in Austin. It’s been a long learning process, he said.
“It took us years to figure out how to build a straight wall without a belly or without it being crooked or things like that, so after three or four different innovations of our forming system we developed this,” De Luna said gesturing at the walls during a recent site visit.
For the walls in a standard wood frame house, 27 pieces are commonly needed and seven contractors touch that wall in the process, he said.
By contrast, in rammed earth construction, it takes one contractor and three products — soil, cement and water — materials that are readily available in the region.
“Yes, it’s a little time-consuming and a little more effort, but it’s because we’re in our infancy,” he said. “I’m sure that if we had built four, five, 10, the prices would change, the cost would come down, the efficiency would increase, our technology would get smarter, wiser, quicker.”
These days, rammed earth construction is more commonly found in the private sector, and typically for people who can afford the extra cost, Damron said. Thus, the lessons learned and knowledge can remain tightly held.
When she sought input from specialty materials companies, “I had to pry information and beg for it,” she said. “Eventually, they sent us some very, very minimal, very vague photos to show what was being done.”
A report released this fall will describe the house in detail, along with all the lessons learned, and be made available to the public and the industry.
Such reports can be used to inform the city’s affordable housing bond, which is likely to be included again in the 2026 bond program given the council’s early support for it.
“All of the modeling we did with UTSA’s building performance lab gives us information as a community for what’s worth investing in our climate,” Damron said.
Komet has volunteered his time as a consultant for the project.
“This is for my kids and my grandkids,” he said. “I’m a big believer in ‘we all row together’ as a community. It’s important to me that the city invest wisely for a future that we’re all going to benefit from.”
