by Rich Michal
With the help of Onset Computer Corporation and its HOBO series of data loggers, I will monitor the home’s actual performance over the next year. I will then analyze the actual operating results and compare them with the projected performance results that I generated during the home’s design using computer energy simulation modeling.
Passive Solar Residence Criteria & Design Concepts
Orientation & Shape
The first and most critical design decision in creating a passive solar residence relates to orientation and shape. While it can be argued that other decisions relating to passive solar energy design are important to the home’s ultimate performance, this fundamental design decision determines the home’s placement on the lot and its basic shape. It is so critical because, unlike other design decisions, it is one that cannot be later corrected or compensated for once the home is built.
In the northern hemisphere, in order to maximize a home’s exposure for beneficial passive heat gain from the low-angle winter sun, a home’s southern exposure should be maximized, resulting in a long east-west axis. Similarly, to minimize unwanted heat gain and glare from the east and especially that of the late afternoon summer sun from the west, the home’s exposure along the eastern and western facades should be minimized, resulting in a short north-south axis.
For the passive solar residence, we accomplished this criteria by first choosing a shape for the footprint of our home that oriented the long axis east and west, increasing the southern exposure while at the same time decreasing the length and exposure of the home’s east and west facades.
Accomplishing the orientation proved to be challenging given the relatively small lot sizes in Civano and the orientation of the lot itself. Our lot is approximately 6,000 square feet and rectangular—similar in shape to the state of Utah. It is oriented with its short southern axis (72.5 feet in length) not facing due south, but instead rotated 27.5 degrees off of due south.
To compensate for the lot’s shape and orientation, in laying out the home’s front facade we simply cut the diagonal from the southeast corner of the lot due west to the opposite property line. By not adhering to the property line in determining the home’s orientation, we were able to help ensure the home’s true south orientation and also increase the length of the home’s southern facade.
Size & Function: Outdoor Living
The easiest way to minimize energy consumption associated with the conditioning of interior space is to simply reduce the amount of interior or conditioned square footage. Fortunately, in this region of the Desert Southwest, year-round outdoor living is possible, allowing us to carve out 1,000 square feet of functional space as unconditioned outside living spaces. Instead of building and conditioning 3,000 square feet of living space for our five-bedroom, three-bathroom house, we reduced the building footprint to only 2,068 square feet, with only 1,750 square feet of conditioned space. The functional outdoor living spaces include an outdoor dining porch, front porch/sunspace, laundry room (now located in the garage), cool tower, and outdoor “cowboy” shower.
Fenestration & Shading
Hand-in-hand with orientation and shape are the decisions relating to the location, sizing, and types of windows. In order to maximize the beneficial solar gain from the south and minimize unwanted gain and glare from the east and west, not only do the facades need to be lengthened and shortened respectively, but the appropriate sizes and types of windows need to be placed within the facades. The passive solar residence, for example, has only two west-facing windows for two of the three children’s bedrooms. To reduce solar heat gain through these windows we installed low-e, dual-glazed Andersen windows.
We also implemented shading strategies to minimize solar heat gain. Along the home’s west facade we extended the rainwater harvesting system to carry water from the flat-roofed garage to a 500-gallon cistern. The rainwater harvesting aqueduct is supported by a series of concrete masonry unit (CMU) pillars from the garage at the northwest corner to the rainwater collection cistern at the southwest corner of the home. The pillars, along with a series of trellises placed between each, support deciduous vines that provide shade for the two west-facing windows.
Half of the residence’s windows were placed along the south facade. This was accomplished by not only placing windows within the south facade to optimize beneficial winter solar heat gain, but also by adding a bank of south-facing clerestory windows. To minimize the amount of unwanted solar heat gain through the windows during the summer, each was carefully located to ensure that the high summer sun was shaded by the roof overhangs. Additionally, French doors within the south facade are recessed an additional 18 inches within two-foot-thick rammed earth walls.
Thermal Mass & Insulation
Once good beneficial solar access (the source) is provided, thermal mass systems (the storage) can be integrated into the design. It is important that these systems be integrated so they have access to solar gain in the winter but are shaded from solar gain in the summer. Thermal mass and insulation systems work by absorbing heat from occupants (and not heat from the sun) in the summer. If the interior thermal mass is not adequately shaded in the summer, the system will be compromised and other cooling systems will be required to compensate for the additional heat stored in, and subsequently released by, the thermal mass. Conversely, during the winter, if thermal mass does not have access to solar gain, then the mass will instead absorb heat from the occupants, making them cooler and compromising their thermal comfort.
In addition to Central Arizona Block Company CMU walls and exposed concrete floors, we built four two-foot-thick rammed-earth walls. Three walls were integrated into the south facade, where they have good solar access. The fourth was placed opposite a rammed-earth wall along the north facade of the master bedroom. This decision was made for aesthetic reasons—for consistency of materials and appearance in the master bedroom—with the full awareness that this would compromise the thermal performance within this space. While in the summer this additional mass in the master bedroom will help keep it cooler, during the winter the mass, without solar access, will work against the passive heating by absorbing heat from the occupants and making the space colder and less comfortable. It was a small sacrifice we were willing to make.
Along the other facades and roof, insulation strategies were utilized to minimize unwanted heat gain and loss. Two roof systems were incorporated into the design of the residence. The first system covering the center third to half of the home is structurally insulated panels, or SIPs. The SIPs panels are 9.5-inch-thick polystyrene foam insulation sandwiched between two pieces of orchestrated strand board (OSB) plywood. The panels are four feet wide and can span lengths as long as 24 feet. The system serves not only as the insulation and roof sheathing but also as the underlying structure, allowing us to provide higher vaulted ceilings and greater spatial volumes in the family room, kitchen, and master bedroom without requiring additional structural members.
The other roof insulation system, covering the west wing over the girls’ three bedrooms and bathroom, and the east wing of the guest bedroom and bath, is comprised of steel roof trusses with a one-inch polyurethane thermal break and blown-in cellulose batt insulation. At the girls’ bedrooms, for example, the polyurethane foam insulation was attached to the underside of the steel roof trusses to reduce thermal bridging from the steel trusses and drywall ceilings. The drywall ceiling serves as the separation between conditioned space and unconditioned space. Above the drywall ceiling in the unconditioned attic space, cellulose insulation from recycled newspapers was blown in to provide a minimum R-32 insulation.
The exterior CMU walls are covered with a layer of 2-inch extruded polystyrene Dow “blue board” insulation with a value of R-10. The wall insulation is subsequently covered with lath and plaster. The combined system of thermal mass on the inside and insulation on the outside allows the thermal mass to perform within the space while at the same time allowing the insulation to reduce heat gain and loss from the outside.
A common misconception regarding thermal mass is that the outside or exterior face is the important face for thermal storage. In fact, it is the interior or inside four to six inches of the thermal mass that provides the most thermal storage. For this reason all of the interior thermal mass is left exposed or un-insulated.
The exterior face of the rammed earth walls, on the other hand, were not insulated so that the aesthetic quality of the natural rammed earth walls could remain exposed. The slight energy savings associated with insulating these 2-foot-thick walls would not justify the amount of resources that would have to be expended—nor would it look as good.
Ventilation & Passive Cooling
In order for the thermal mass to work effectively during the summer, the mass must be ventilated or recharged each night. In an effort to enhance the natural ventilation and provide greater control of thermal comfort within individual functional spaces, twelve Hunter ceiling fans were incorporated into the home’s design. The fans were placed in each of the five bedrooms (master, guest, and the girls’ bedrooms), the family room, kitchen, south porch/sunspace, north dining porch, and the laundry room in the garage.
Two additional fans were incorporated into the passive evaporative cooling system. The primary component of the evaporative cooling system is the 30-foot-high cool tower located within the center of the home, between the family room/kitchen and the hallway to the girls’ bedrooms and bathroom. The inside of the cool tower has a five-foot by five-foot square cross section shaft to allow the placement of six-inch-thick, four-foot-high by five-foot-wide Glacier Corp. cellulose pads at the top of the tower on each of the four faces of the tower. The pads are located at the top of the shaft, allowing a 16-foot fall to the top of the door openings at the bottom of the shaft. When the pads are saturated with water from the water supply lines located above the pads, the air drawn through the pads from the outside is also saturated. This moist, cool, and heavy air then falls down the shafts and into the living spaces through the door openings at the bottom.
The tower was designed and sized to generate approximately 4,800 cubic feet per minute (cfm) of airflow, which should effectively distribute the cool, moist air throughout the immediately adjacent living spaces. One of the ceiling fans was placed within the cool tower to enhance the operation of the cool tower, especially during periods of higher outdoor humidity.
The master bedroom and bathroom are the only interior living spaces within the main house that are not immediately adjacent to the cool tower. (The guest bedroom shares a common wall with the main house, but is only accessible via a separate exterior entry.) To draw the evaporative-cooled air through the family room and into the master bedroom, a second tower, or “exhaust tower,” was incorporated into the design at the master bathroom.
This 20-foot-high tower, with a cross section of eight feet by 12 feet, has ten two-foot by two-foot operable Andersen awning windows located within its north, south, and east facades—three each on the north and south and four on the east. The windows were designed to be the highest windows in the main house: the tower acts as an exhaust chimney for the hot air. As hot air is drawn out, the cooler, moist air from the cool tower is drawn into and through the master bedroom. The twelfth and final ceiling fan is located in the master bathroom/exhaust tower to enhance its effectiveness.
Non-Passive/Active Solar Systems
The only active or non-passive solar system included in the passive solar residence’s design is the solar water heating system—an active closed-loop, drain-back system with two four-foot by eight-foot Sun Earth flat plate collectors and one 120-gallon solar/electric hot water tank. The flat plate collectors are mounted on a steel frame above the stair landing of the garage’s rooftop patio. By elevating these panels, we were able to ensure a true south orientation, free of obstruction, while at the same time providing some shade for the rooftop patio. This location also allowed us to install the solar/electric water heater tank immediately below the panel, inside the garage, providing optimal slope for the drain-back system and mitigating heat loss and the potential for system pipe freezing.
While the home’s passive energy conservation systems primarily rely on low-tech strategies such as orientation, size, and shading, the water conservation systems rely more on technology. According to the American Water Works Research Foundation, the highest single source of water consumption in the average household is the toilet. For this reason we selected an Australian ultra low-flow, dual-flush toilet manufactured by Caroma. The dual-flush toilets utilize either 1.6 or 0.8 gallons per flush, which based upon average usage significantly reduces the water consumption associated with the conveyance of waste.
Other water conservation technologies incorporated into the home’s design include motion-sensor faucets at the bathroom vanities, a New Zealand-manufactured Fisher & Paykel energy and water conservation dishwasher, and a hot water re-circulating loop to minimize the amount of water wasted while waiting for water to reach desired temperature.
The low-tech water conservation strategies include separate rainwater and graywater harvesting systems. The rainwater harvesting system collects water from the flat roof of the garage and stores it in a 500-gallon corrugated, galvanized steel cistern for use in landscape irrigation.
The graywater harvesting system is associated with the outdoor “cowboy” shower, located off of the master bathroom. Water from the shower flows from the floor drain through underground piping to a French drain in the back yard. The open French drain is filled with river rock, providing the appearance of a natural streambed and facilitating the distribution of the water along the trench to a salvaged mesquite tree planted in the back yard. In addition to being a native drought-tolerant plant, the velvet mesquite was chosen because of its tolerance for graywater and its rapid growth and shading characteristics.
Results So Far
According to Tucson Electric Power Company, the average Tucson residential customer’s annual electric bills total $1,403, or $116.92 per month. Our average total monthly electric consumption—including heating, cooling, water heating, lighting, appliances, and all plug loads—has been 76.9 KWh for an average of $65.75 per month, a 56% savings.
Similarly, Tucson Water reports that the average single-family residence uses 120 gallons of water per person per day. Our average water consumption has been less than 35 gallons per person per day—less than one-third of the Tucson average.
From an objective standpoint, we won’t be able to isolate and determine the true effectiveness of the heating and cooling energy conservation strategies and systems of the passive solar residence for at least a full year, after we have had a chance to collect and analyze the year-round operating results. We will then be able to determine whether the home is able to perform at the level to which it was designed and modeled: $113 per year for total heating and cooling. Subjectively however, my family continues to enjoy the unique spatial, functional, aesthetic, and sustainable qualities of our passive solar home.
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