Overlooking Arizona’s expansive San Rafael Valley, at an elevation of 5,000 feet, the tiny house known as Casa Caldera doesn’t provide the easy escape of a typical vacation getaway. What this 1,060-square-foot, off-the-grid retreat does deliver are stunning views and the opportunity for an adventure that begins with getting there. Roughly half of the two-hour drive southeast from Tucson cuts through mountains and goes off-road, across spectacular high-desert grasslands, past grazing cattle, and up into forested slopes.

The earth-hued house was built with scoria, a porous cementitious material, also referred to as lava concrete, that takes its name from its primary component, a lightweight volcanic cinder, similar in color and makeup to the surrounding terrain. (Geologically, the region was once the site of a caldera, or large volcanic crater.) The building is not visible from the road. Tucked just below the crest of a hill and camouflaged by Emory oaks, Casa Caldera emerges from the landscape slowly, but only after you park your vehicle at an informal “carport” and activate the house’s sources of electricity and hot water—a self-contained portable solar system and a propane tank.

The enigmatic site and gradual approach to it were calculated by architect Cade Hayes and his partner Jesús Robles, principals at the Tucson-based design/build firm DUST, to heighten the anticipation of arrival. It also satisfies the client’s brief for a secure, discreet home that won’t disturb the viewshed of neighboring properties.

An adventurous Ohio-born winemaker who counts falconry as one of his hobbies, the 40-something owner purchased the land from family planning to sell the remains of a larger ranch. His budget was modest, his needs simple: a modern, self-sustaining refuge, with outdoor space, where he could commune with nature and plant a small vineyard for pleasure. Because the house would be left empty for long periods of time, he also wanted the ability to shut it down when he was away.

Given the remote location and budgetary limitations, the designers convinced the client that building with scoria would be the best option for the project. The construction process is similar to rammed earth, a system DUST had employed for its first house (Tucson Mountain Retreat, record, April 2013, page 72), but it is less expensive and has superior thermal properties due to its porosity. Additionally, because it is less dense, scoria can be easily reinforced with rebar as needed.

Hayes and Robles responded to the Southwest vernacular with a contemporary variation of a traditional Spanish Colonial Zaguán house. According to Hayes, this compact style, bisected by a central corridor (or zaguán) that leads from entrance to patio, would be easy to close up. “But we were also thinking about ventilation,” he says. Since the house would have no mechanical cooling or heating, insulation and cross-ventilation were key factors in the scheme. The zaguán creates a pressure differential. “So as long as you have access to cool breezes, it will pull the air through,” Hayes explains.

The one-story monolithic structure, slightly off axis to capture views, is sited to benefit from airflow and solar heat gain. The zaguán was configured with rugged weathering-steel bifold doors at either end, which can be left wide open to extend the living space in good weather or shuttered. The crew added a layer of foam insulation within the building’s 18-inch-thick scoria walls and installed five operable windows, set half-way back into the facade for shade. Thin, removable steel bars, horizontal rather than vertical, add an almost decorative layer of security without being obtrusive.

“The openings are small. We didn’t go for the architectural move of a skylight,” says Robles. “And we resisted any temptation to make design decisions just for an aesthetic. Everything is rooted to function.” Still, a west-facing corner window, positioned to bring in plenty of daylight, frames a mountainous panorama. Adjacent young trees will prevent glare when they are fully grown. To water them, DUST devised a central gutter for the standing seam metal roof that directs rainwater down through the building and out toward their roots. The house is fed by a natural well, but irrigation in this semi-arid clime can be an issue, so even the septic system was configured to filter cleansed water into the rocky soil around saplings.

The interior is split into three zones, about 270 square feet each. The 10-foot-wide zaguán is bordered by enclosed public and private wings and opens onto a west-facing concrete patio, a place to enjoy the amazing sunsets. Wood is the primary heat source: a fireplace for the kitchen/ dining/living room and a wood stove for the bed and bath area opposite, while a ceiling fan, transoms, and operable kicks (on a glass wall between the living and zaguán spaces) circulate the air.

The owner looks forward to spending months at a time at Casa Caldera. “My dream is to be there for several months in the fall. I can forget about the rest of the world, but it is only an hour and 40 minutes to an international airport,” he says. “It’s pretty special.”

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