Southwestern Smarts: Passive Solar in Santa Fe

sw Passive Solar

South-facing windows and Trombe walls heat the home in winter and provide abundant daylighting. But because of the orientation of the house and good insulation, it has little solar gain in the summer. As shown in this summer photo, the south wall is completely shaded.

By MARK W. CHALOM, Photos by MATTHEW CHALOM

Richard and Susan Bechtold arrived in Santa Fe, N.M., in 2001. They came to me and described the dream home they wanted to build during the next decade — one that not only would conserve resources but also would recycle them in a holistic design. Over time, we had the opportunity to change the analytical methodology and modify the design as new information, materials and approaches evolved. As the architect, I was able to coordinate an interdisciplinary team of design professionals and help select a suitable site.

Six years later, our planning resulted in an artistic, sunlit, healthy home with a small carbon footprint, one that interacts with and responds to its environment.

Open Passive Solar Living Space

In 2008, the Bechtold home won the Green Home of the Year Award from the New Mexico Green Homebuilders Association and Su Casa Magazine. We also won their Water Efficiency Award. (See bit.ly/OnD0E9.)

Now that the home has been occupied for nearly five years, it’s a good time to evaluate, finetune and discuss improvements.

 

Preserving, Conserving Resources

I was fortunate to be in on the site selection. We used the site’s 5 percent slope to the south to our advantage, cutting the building into the ground on the north and rising slightly on the south. By building on the highest part of the site, closest to the road, we have minimal impact to the natural vegetation and drainage of the site.

We cut the semi-permeable driveway to follow the terrain and meander around existing trees. Extra runoff is directed to tree clusters. Only one tree was removed in siting the house; 90 percent of the site will be kept in its natural state.

passive solar design

One key to efficiency is a flexible design. The now-large studio room can easily be divided into two complete bedrooms with a short partition wall.

For the building, we began with a flexible design, just 2,360 square feet to conserve resources.

The now-large studio room can easily be divided into two complete bedrooms with a short partition wall. Rooms are placed to maximize the use of passive solar as well as views of the surrounding mountains. We used many natural local materials, with little processing and shipping. Local adobes with mud plaster, vigas (peeled tree trunks), bricks, milled timbers, gravel and locally built cabinets and doors convey the Southwestern style, while supporting the local economy and labor force and maintaining a low carbon footprint. Other materials, like engineered timbers, low VOC paints and no carpets, minimize the environmental impact.

kitchen Plumbing

The kitchen plumbing goes through the adobe wall and runs inside the frame wall. The electricians learned quickly to run wires between the two walls and not bother drilling studs.

Our team studied water efficiency carefully, using water-saving fixtures and Energy Star appliances throughout. Rainwater collection was engineered to balance the rainfall, usage, storage and distribution system. An underground 10,000-gallon tank provides storage. Rainwater offsets non-potable water uses such as toilet flushing, floor washing and car washing. The exterior hose bibs and the utility sink provide rainwater directly from this tank.

All the wastewater and the kitchen sink disposal flow to an anaerobic digester that turns the blackwater into clean effluent for landscaping. The computer-controlled distribution system provides nutrients and moisture where and when needed. The utility sink has a two-drain system: One goes to the digester and the other flows to an external, closed-loop evaporation pond to keep any detrimental products out of the digester system. These systems help offset the water needed to sustain this home.

Icynene

Icynene foam is sprayed between the studs, and excess foam is cut off prior to sheathing. The two red-covered hose bibs are for community water and collected rainwater

Our reliance on collected rainwater and processed effluent for all non-potable uses reduces the Bechtolds’ metered water usage dramatically. The average person in Santa Fe uses 50 gallons of domestic water per day — less than the national average. By comparison, Richard and Susan’s metered water usage has been 14 to 15 gallons per person daily.

 

Designing and Monitoring for Efficiency

The Bechtold residence is the most advanced passive solar adobe home I have designed in my 35 years in architecture. It is full of natural light and air. Because the solar heat is radiant and the home is massive, air temperatures are not as critical to comfort; that allows this home open ventilation, even during the heating seasons. As this home is built to be very tight, an exhaust fan on a timer allows for minimum ventilation when needed.

insulation valuesSanta Fe is at 7,000-foot elevation. The number of heating degree days is 6,001, and the number of cooling degree days at base 65 is 573. Heating is still our primary design strategy.

In the earliest stages of designing this house, we used the sensitivity curves in the Builder Guideline Design Tool to analyze the passive solar contribution. Six years later we were able to do detailed analysis of the house design with Balcomb’s ENERGY-10 software. Our team built 3-D computer models to study the shading of the overhangs and the site integration.

By applying these analyses, we optimized the blending of passive solar apertures, insulation and thermal mass. The construction combines adobe with an airtight, highly insulated frame shell, for a high-mass house with low overall heat loss. With a balanced solar aperture of both Trombe wall and direct gain, the house is comfortably warm day and night with no backup heat, even through short storm periods.

adobe Construction

The construction combines adobe with an airtight, highly insulated frame shell, for a high-mass house with low overall heat loss. The house is comfortably warm day and night with no backup heat.

Adobe is simply a sun-dried earthen brick made much like they were 6,000 years ago — from natural local materials with little external energy. Adding a selective surface film to an adobe Trombe wall brings it into the 21st century, improving its efficiency. We used Krosol, with a solar absorbance of 0.95 and a thermal emittance of 0.08.

We were 60 percent through construction when we learned that the state of New Mexico had modified the LEED certification program to be more regional and offered a $10,000 tax incentive for LEED houses. We reviewed the requirements against our design and realized that we were way beyond the highest category given, just by adhering to my usual design criteria. All we needed was a HERS (Home Energy Rating System) rating. The HERS program was in its early stages and had no basis in a design like this one; it didn’t acknowledge thermal mass, Trombe walls and selective surface, and the program mentioned nothing about passive solar contribution. We had a hard time passing this analysis because of our south-facing glass. I felt the green movement had gone backwards 20 years.

icyneneFoam

Icynene foam insulation forms an airtight seal. Insulation contributes to the very small heating demand — just 268 gallons of propane annually compared to 1,000 to 1,200 gallons for similar homes in Santa Fe.

Despite a rating program out of sync with the regional design requirements, we had cooperative clients and the hard data needed to prove the validity of our decisions. Susan and Richard Bechtold participated in the design decisions needed to reach a high level of sustainability and are agreeable to having this home used for long-term monitoring and ongoing modifications. As an engineer, Richard is interested in understanding, monitoring and troubleshooting all the home energy and water systems. A wonderful laboratory, the house has already been presented at two major technical conferences and showcased on various sustainable home tours.

For accurate energy consumption, all thermostats were left at 70°F (21°C) 24/7. The propane tank, used for spacing heating and domestic hot water, was filled annually, giving us a yearly usage through our boiler.

clerestory windows

South-facing clerestory windows in the studio room charge the adobe mass.

The boiler is rated at 95 percent and then corrected for altitude. Most of the domestic hot water needs are satisfied by a small active solar collector system. It takes a few years for a passive solar adobe home to dry out and charge its mass. After a few years our annual propane usage dropped from 347 gallons (1,314 liters) to 268 gallons (1,014 liters), an improvement I attribute to drying and charging the home.

By comparison, well-built homes of this size in Santa Fe that were built since 1990 average 1,000 to 1,200 gallons (3,785 to 4,542 liters) of propane a year. One of these homes is reported to stay cold, another has thermostats set at 65°F (18°C), and a third has solar water heating. Another neighbor has been working to keep his propane usage to 826 gallons per year by using setbacks and turning off heating zones. A third-party design/build firm found energy usage at the Bechtold house to be almost twice as efficient as at the best-performing house the firm has surveyed.

Fine-Tuning with Active Solar Systems

Three years ago we retrofitted the house with a photovoltaic system to provide most of its electricity. It averages 87 percent of the home’s usage over 12 months, including electric cooking and clothes washing and drying.

skylight

An interior hall skylight borrows daylight from a clerestory window that opens to the bathroom beyond this door.

We used low-voltage lighting throughout the house, along with Solatube skylights and an innovative skylight in the hallway. All rooms except the utility room have south-facing windows for heat and daylight. These systems and educated, interactive owners make this home close to net-zero energy, with a low carbon footprint.

After two summers of record-high temperatures in New Mexico, night flush ventilation of the thermal mass has proven inadequate during extreme times. Cooling degree days were 66 percent higher than average, outdoor temperature reached over 100 and indoor temperature peaked at 82. One option we’re examining, and which the house is designed to integrate, is night sky radiant cooling (NSRC).

Through NSRC, heat is transferred from the warm floor to the cool night sky. Our strategy is to install a bank of glazed collectors on the living room roof and run cold fluid through the radiant floor slab at night. In 2006, Solar Logic LLC CTO Bristol Stickney and I completed a two-year research study testing and analyzing various unglazed collectors for night sky radiation in our clear, dry, high-altitude climate. We found night sky radiation to have extremely effective potential (see sites.google.com/site/nsrc4nm). Bristol has been adding NSRC to the active collector systems he has designed in this region and monitoring them.

Our preliminary calculations showed that by adding 320 square feet (30 square meters) of unglazed collectors, we can easily reduce the temperature of a 2,300-square-foot (214-square- meter) slab 3-1/2 degrees every night and that this effect would be cumulative. That would offset the one month our projected system fell short of needed loads. Any other month we could be ice-skating.

Assessing, Modeling Passive Solar Successes

Trombe Wall

Selective surface foil was applied to the Trombe wall. It has an absorption of 0.95 and emissivity of 0.08. The installers took care to guarantee full adherence for best conductivity.

One reason this house achieves near net-zero-energy use in Santa Fe is because it was designed for its site, region and microclimate.

Regional design comes from the evolution of vernacular architecture. Local strategies, materials and approaches must be understood and applied to guarantee the most direct design for any building.

Remember, heating and cooling systems are relatively new in the history of building. It was always a matter of understanding what a climate had to offer and exploiting that for human comfort. It took our ability to match a building’s geometry, orientation, materials, siting and landscaping with its climate to harness this free comfort. It is up to us to understand what has come before us and why.

radiant tubing

Solar-heated fluid is piped through the slab floor to provide radiant heat. A night sky radiant cooling system the Bechtolds are considering would run cold fluid through this system to cool the house.

Post-construction evaluation and monitoring is crucial. Although my conclusions are valid, my methods and tools were unscientific. Passive solar design has always been a trickle-up technology. Starting at the grassroots level, universities and national labs provided validation and necessary design tools.

Now is the time for these organizations to provide more analysis of post-construction data to judge techniques, materials, design approaches and, of course, computer modeling. There are many well-designed buildings that have been operating for years utilizing various design approaches. We stand to benefit richly as these too are monitored, evaluated and compared.

 

Mark Chalom

Mark Chalom

Mark Chalom (solarch@rt66.com, markchalom.com), architect, specializes in blending handcrafted traditional and contemporary styles with sustainable technologies. Chalom’s experience spans both design and construction of environmentally appropriate buildings.

Since 1975, he has taken his architecture beyond passive solar to incorporate the total structure, site, materials, systems, water and landscaping and their interaction with each other and the environment. With sensitivity for the historic cultures, he has blended these new technologies with the traditional forms and materials that have evolved in Northern New Mexico out of common sense and appropriateness. Chalom is known for his expertise in the development of the “Solar Adobe” based on the past but adaptive and relevant to the future.

In 2012 Chalom was awarded the Equinox Lifetime Achievement Award from the Bridgers family in honor of the 50th anniversary of the Bridgers & Paxton solar building in Albuquerque, N.M. Chalom was also awarded the American Solar Energy Society’s prestigious Passive Solar Pioneer Award in 2012.

 

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