April 2005
Guidelines Development Process
At the 2004 International Builders Show, the National Association of Home Builders (NAHB), with the assistance of the NAHB Research Center, released Model Green Home Building Guidelines, which would provide a practical nationally recognized baseline for determining minimum thresholds for resource-efficient, cost-effective home building.
The NAHB Green Home Building Guidelines, consisting of a Checklist and a User Guide, was developed through a public process that included the following major steps:
- An extensive review of the existing local green home builder programs - primarily home builder association programs, but also including several public sector and non-profit programs. All but three of the 28 existing programs are voluntary and market-driven.
- A review of the voluntary energy efficiency programs endorsed by the National Association of Home Builders (NAHB).
- A review of the leading life cycle analysis (LCA) tools available for use by residential design and construction professionals in North America (e.g., BEES, ATHENA).
- Input through an open process from numerous individuals on the NAHB Advisory Group and the Stakeholder Group.
- Additional input from the general public by providing draft versions of the guidelines on the NAHB Research Center website along with a Commenting Form.
- Applying certain criteria to each line item in the Checklist in order to give each line item a point value.
- Creating a User Guide that provides additional information on the intent of each line item in the Checklist.
Each line item in the Checklist has a point value attributed to it. Once the Stakeholder Group members finalized the list of line items for inclusion in the Checklist, the NAHB Research Center team looked at each line item through three different lenses:
- Environmental Impact,
- Building Science and Best Building Practices, and
- Ease of Implementation.
The team used publicly available information, experiential data, and other data inputs to assign each line item points via these three criteria. Each line item's final point total was then calculated by weighting the criteria. Environmental Impact received the greatest weight, followed by Building Science and Best Building Practices, with Ease of Implementation receiving the least weight.
Environmental Impact — The main purpose of these guidelines is to provide a framework for builders to reduce a home's environmental impact. We assessed how each line item helped make a home more energy efficient, improved indoor environmental quality, and so on. Assigning a value to each line item is an inexact science since all of the necessary data is not available. In addition, some line items had impacts that spanned multiple principles and, in some cases, the impact was positive for one guiding principle while negative for another. With that as background, the NAHB Research Center team took into account all of the above considerations and available data to assess the environmental impact of implementing each line item. Using qualitative and quantitative information, the team assigned value to each line item based on its positive impact to the environment.
Building Science and Best Building Practices — Certain green building practices dramatically impact how a house operates. For example, the sealing of a home's building envelope has an impact on how a home's HVAC system functions. In addition, some measures such as proper flashing details and installation of weather barriers enhance durability, minimize the possibility of indoor environmental problems, and are considered "best building practices." Line items that help a home perform effectively as a system for the long-term were assigned a higher point value than those line items that did not greatly impact the home's performance.
Ease of Implementation — Some line items are easier to implement than others. The NAHB Research Center team compared each line item to current home building practices and estimated how difficult it would be for a builder to implement the line item relative to primarily cost and time. For instance, would it take longer to install a new technology? Would subcontractors need to be educated on the use of a new product? Would a new technology cost more to buy? A line item will have a positive environmental impact only if it is implemented. Line items that were relatively easy to implement (and therefore more likely to be implemented) were assigned a greater point value than the items that are more difficult to implement.
Green Programs and Homes Differ Across the Country
When assigning points to the line items, the NAHB Research Center assumed the home would be built in Baltimore, MD, which is in Zone 4 of DOE's proposed climate zone map (PDF 396 KB). Download Acrobat Reader.
For associations located outside of Zone 4 that are interested in creating a green building program, point values can be customized for some line items most affected by climate conditions. For example, an association in Florida, you will likely want to increase the point values attributed to installing an energy efficient air conditioning system and decrease the point value associated with installing a high efficiency heating system. Similarly, in the southwestern United States associations would likely place higher value on water efficiency measures. A thermometer symbol in the User Guide identifies line items that most likely will see point value changes due to climatic differences across the country.
Additional factors can lead to the decision to alter point values for a certain location, such as the availability of materials, the recycling marketplace, and the existence of rebate programs. Determining a line item's point value is accomplished by way of consensus among the members of the green home building program's development committee. This is primarily a qualitative process and some acknowledgment of the decision-making process should be clearly stated in the program.
Various Levels of "Green"
Homebuilders differ in their relative knowledge and comfort level with green building concepts. Some builders have been building green for years, while others are being introduced to the ideas for the first time. Recognizing this broad range of knowledge, the NAHB Research Center team established various thresholds to delineate different levels of green building effort.
The first step was to identify practices that should be part of any home building project. The first level of green building, Bronze, includes additional line items that in the end show that a builder paid special attention to a project's environmental impact. The next two levels of green home building, Silver and Gold, include additional line items that place increasingly greater emphasis on the home's environmental impact. The "How to Use the Guidelines" section of this document outlines how to score a home to determine if it meets or exceeds any of the green home building levels noted above.
The Uncertainties of Green Building
It should be noted that although many green building programs have been in existence for 10 years or more, the concept and practice of green building is not clearly defined and straightforward. Many gray areas remain in identifying and quantifying the precise environmental impact for each particular line item. For example, there is very little publicly available information regarding manufacturing processes that document energy consumption, impact on natural resources, or CO2 emissions for each building material.
In addition, a particular line item may contain tradeoffs and carry with it contradictory characteristics. For example, a recirculating hot water system can help save conserve water, but may use a relatively large amount of energy in its operation. Although the guidelines in their current form are based on experiential evidence and the latest independent scientific research available, they still may leave many questions unanswered due to the lack of scientific and quantitative data.
Finally, assigning a particular degree of importance to different criteria undoubtedly involves a certain amount of personal or local value judgment. Life Cycle Assessment (LCA) tools are beginning to sort out such questions, but the tools still remain in their infancy. Therefore, this set of green home building guidelines should be viewed as a dynamic document that will change and evolve as new information becomes available, improvements are made to existing techniques and technologies, and new research tools are developed.
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