The LEED for Homes program was in the pilot phase when used within this study, and was officially launched in November 2007 at the Greenbuild conference in Chicago. The format of the rating system is loosely structured on the LEED program for commercial projects (LEED-NC) that has been used throughout the United States since 2001. LEED for Homes rates residences in the following eight resource categories:

The NAHB Green Home Building Guidelines were under further development when used within this study, and may change significantly with the next release. The system rates residences in the following seven categories:

Residence A — 2,056 square feet. Passive Solar, masonry/rammed earth construction earned the following overall scores and certification levels:

Residence B — 2,169 square feet. Zero Net Energy custom solar home; CMU insulated (exterior) construction:

Residence C — 1,600 square feet. Large-scale production model meeting minimum energy code requirements:

Residence D — 988 square feet. Owner designed/built rammed earth custom home; based on permaculture principles:

Residence E — 4,071 square feet. Large-scale premium production model with features exceeding minimum energy code:

Residence A — 21.4 percent better than code at time of submittal (2003 IECC) — (thermal mass)
Residence B — 58.2 percent better than code at time of submittal (2003 IECC)
Residence C — 0.6 percent better than code at time of submittal (2003 IECC)
Residence D — 4.2 percent better than code at time of submittal (2003 IECC) — (thermal mass)
Residence E — 27.1 percent better than code at time of submittal (2003 IECC)

Pima County experiences many warm, sunny days and cool to cold nights resulting in large diurnal temperature swings. The range of temperatures that occur throughout the year — from minus 20 degrees Fahrenheit on winter nights to more than 110 degrees on summer afternoons — require adequate provisions for heating and cooling in all residential construction. The storage capacity of high thermal mass construction assemblies, such as adobe, rammed earth and masonry block construction, allow structures to absorb heat during the day and release it at night, tempering internal temperatures. This approach has been used in desert regions for thousands of years, and yet there is limited data available on the characteristics of thermal mass material. This study found that those residences employing thermal mass did not rate well with basic software checking programs such as REScheck. While REScheck does a good job of evaluating conventional building assemblies, the benefits of thermal mass are not assessed adequately within the program’s capabilities.

There are other available energy software packages tailored to the residential market that allow inputs for thermal mass materials and passive solar design such that the benefits are fairly reflected in building’s scores. Unconventional building materials may have numerous environmental benefits, but exceed the capabilities of many software programs that have been developed primarily for wood-framed, cavity-insulated building assemblies.

It is important to recognize that this study was not undertaken to rate the residences themselves, but instead to assess the approaches taken towards rating a variety of construction techniques by several green building rating systems. When documentation was unavailable, assumptions were made that may not have reflected the actual as-built conditions. In an operational green building program, client education and design charrettes would contribute to ensuring complete submittals.

Size adjustment criteria are important factors to fairly assess a home’s energy and material requirements. A doubling of house size results in a 15 to 50 percent increase in energy consumption, and an increase in materials use from 40 percent to 90 percent depending on design. In the period between 1982 and 2004, the typical new single-family home in the United States grew some 40 percent larger from 1,690 square feet to 2,366 square feet, while the average number of occupants per household was reduced from 3.3 to 2.6 persons. Whether the appropriate neutral measure for a rating system is the national average home size or the average home size for the local jurisdiction requires vetting. Many new homes under construction in the Pima County area exceed the national average and encouraging well-designed, smaller homes is a desirable outcome of employing sizing criteria. In the final version of the rating system, we based our sizing criteria on the average for the area (Pima County’s average for 2006 was 2,300 square feet — very close to the national average of 2,366 square feet).

Allowances for appropriate use of appropriate natural materials and methods should be included to accommodate owner-builders and developers wishing to employ these elements. Finding a user-friendly energy assessment tool, or providing basic energy modeling for applicants will encourage those wanting to use building assemblies which may not have gained popularity with large-scale developers. Additionally, a number of regionally technologies were not accommodated in the NAHB and LEED systems, except through the award of innovation points. Of all the rating systems, the Scottsdale program excelled at designating point awards for region-specific methods such as cooling towers (used in the Residence A of the evaluated homes in the study), and methods to reduce heat-island effect. The Scottsdale system was also the only programs that recognized the low impact of evaporative cooling systems.

The limited treatment of thermal mass performance, as mentioned above, is endemic to all national model energy codes which are directed at sealed insulated buildings and do not provide for the type of passive thermal and evaporative cooling/ventilation which can perform efficiently in desert regions. Passive systems, along with emphasis on cooling degree days instead of heating degree days will need to be further developed, not just within a green building program, but in future amendments to adopted energy codes.

Wise use of water, water conservation and water harvesting techniques were included in many of the measures identified in existing green building rating systems, although the Scottsdale rating system provided the widest coverage of these elements. Encouraging energy conservation and renewable energy can have a significant overall effect on a community’s water consumption. The magnitude of water use for energy generation is well documented, with an average of 0.75 gallons of water consumed for each kWh of thermoelectrically generated electricity. Production of electrical power results in one of the largest uses of water in the arid west, where most electricity is derived from coal-fired thermoelectric power plants with steam-driven turbine generators. In 2004, the water withdrawals of coal and gas electrical generating plants in the eight states comprising the interior west (Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona and New Mexico) were more than 650 million gallons per day, enough to meet the needs of nearly four million people. Conversely, there is also a significant energy use component in providing potable water to consumers. Large amounts of energy are consumed when pumping groundwater at its source, delivering it over long distances to users, and returning it to wastewater facilities for treatment. This interdependent relationship of energy and water provides compelling rationale for examining both energy efficiency and water conservation credits when evaluating rating system As a result of this evaluation, Pima County was able to assess the range of energy and water related credits in the programs we evaluated, and expand upon them. After reviewing these rating systems, we felt that including as many water efficiency and energy efficiency credits as possible in our program would best serve our region. Some of these measures include low-water (Xeriscaping) landscape techniques, cool roofing materials, rainwater harvesting systems, ultra-low flow fixtures, passive solar design, appropriate glazing for each façade (to block or allow solar gain as necessary), and heat island reduction strategies.

Another variable across the rating systems is balance between categories. By requiring a minimum number of credits in every category, NAHB ensures that a home design will not underemphasize one aspect of sustainable construction. LEED for Homes requires minimums credits in four of the eight categories in addition to meeting prerequisites. The Scottsdale Green Building Program allows applicants to choose credits at will from among the categories once the mandatory measures are met. As a result of this evaluation, we decided a compromise approach that required a reasonable but attainable number of credits in each category would provide the most balanced approach for our rating system.

Finally, two other elements must be considered during program design and development — whether a program should be voluntary or mandatory, and what costs and benefits there are to builders associated with builder participation. Pima County decided early on that a voluntary, incentive-based program would be implemented. There would be no costs to participate, and the review and inspection of green building elements would be tied to the usual plan review and inspections required by code. In this way, we sought to minimize the duplicate submittals, additional cost and external documentation that a separate certification process can create. By providing green building training to our existing review and inspection staff, who already possess excellent analysis and verification skills, the green building certification process is fully integrated into the review and inspection process already required of builders to ascertain code compliance.