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Introduction to NAHB Model GREEN Home Building Guidelines

The process of green building incorporates environmental considerations into every phase of the home building process.That means that during the design, construction, and operation of a home, energy and water efficiency, lot development, resource efficient building design and materials, indoor environmental quality, homeowner maintenance, and the home's overall impact on the environment are all taken into account.

Now to answer the question, "Why should we care about green building?" There are many compelling reasons for changing the way we build and operate homes. Although we cannot avoid affecting the environment when we build a house, green building can work toward minimizing that environmental impact.

These guidelines were designed with the mainstream home builder in mind. We recognize that many home building companies already incorporate some elements of green building into their current practices. However, the purpose of these guidelines is to highlight ways in which a mainstream home builder can effectively and holistically weave environmental concerns into a new home and to provide a tool for local associations to create a green home building program.

At the time these guidelines were created, there were 28 green home building programs in operation throughout the United States. These programs have done a great job of spreading the word about green home building. However, there are numerous other locales that are interested in green home building but have not had the resources to create a program from scratch. These guidelines are intended to serve as a tool kit for home builder associations to create new programs and to help those programs expand and flourish.

As noted above, during the process of building a green home, a builder takes numerous considerations into account simultaneously and consciously incorporates environmental issues into all decisions. These model green home building guidelines consist of a variety of distinct line items that a builder can choose from in creating a green home. For organizational purposes, we have grouped the line items into overarching sections, or guiding principles. Below are the guiding principles addressed in green home building:

Resource-efficient site design and development practices help reduce the environmental impacts and improve the energy performance of new housing. For instance, site design principles such as saving trees, constructing onsite storm water retention/infiltration features, and orienting houses to maximize passive solar heating and cooling are basic processes used in the design and construction of green homes.

Most successful green homes started with the consideration of the environment at the design phase-the time at which material selection occurs. Creating resource- efficient designs and using resource efficient materials can maximize function while optimizing the use of natural resources. For instance, engineeredwood products can help optimize resources by using materials in which more than 50% more of the log is converted into structural lumber than conventional dimensional lumber.

Resource efficiency is also about reducing job-site waste. Invariably, there are leftover materials from the construction process. Developing and implementing a construction waste management plan helps to reduce the quantity of landfill material. The average singlefamily home in the United States, at 2,320 ft2 (NAHB, 2003), is estimated to generate between 6,960 and 12,064 lbs. of construction waste. Thus, by creating an effective construction waste management plan and taking advantage of available recycling facilities and markets for recyclable materials, construction waste can be reduced by at least two-thirds, creating potential cost savings for builders and reducing the burden on landfill space.

Lastly, basing the selection of building materials on their environmental impact can be tricky. For instance, a product might be renewable, but on the other hand it takes a relatively great amount of energy to transport the product to a project's job site. One way to compare products is to look at a product's or a home's life-cycle environmental impacts through a process called life-cycle analysis (LCA). An LCA of a building product covers its environmental impacts "cradle to grave" through six basic steps:
1) Raw material acquisition,
2) Product manufacturing process,
3) Home building process,
4) Home maintenance and operation,
5) Home demolition, and
6) Product reuse, recycling, or disposal.
There are numerous reasons why building products are not commonly selected via LCAs. One of the issues is the availability of data- there is a lack of data to feed into tools that allow for an LCA on a product or system.

One such tool created by the National Institute of Standards and Technology (NIST) is the Building for Environmental and Economic Sustainability (BEES) software program. BEES has 10 impact categories: acid rain, ecological toxicity, eutrophication, global warming, human toxicity, indoor air quality, ozone depletion, resource depletion, smog, and solid waste. Since information is not available to conduct full LCAs on all available building products, we have instead included an LCA mind-set in creating the list of line items in the Resource Efficiency section. Our hope is that in the future the prescriptive line items in the guidelines will eventually be replaced with a full LCA approach for the home as a system and the components therein.

Energy consumption has far-reaching environmental impacts: from the mining of fossil-fuel energy sources to the environmental emissions from burning non-renewable energy sources. And each home consumes energy year after year, meaning that the environmental impacts associated with that use accrue over time. Therefore, energy efficiency is weighted heavily in a green building program.

Energy consumption occurs not only during the operation of a home but also during the construction of a home and, indirectly, in the production of the materials that go into the home. Although the energy used to heat and cool a home over its life far outweighs that to manufacture the materials and construct it, the large number of homes built (currently about 1.85 million per year) renders the energy used during the construction phase significant.

On average, a home built between 1990 and 2001 consumed about 12,800 kWh per year for space and water heating, cooling, and lights and appliances. Where natural gas is used, consumption averages 69,000 cubic feet per household annually. Total energy expenditures during a year cost these homeowners about $1,600. Energyefficiency improvements that make a home 20% more efficient-a conservative estimate for many green homes-could significantly reduce a homeowner's annual utility expenses.

No matter what the climate, energy efficiency is considered a priority in most existing green building guidelines/ programs. Moreover, as the cost to heat and cool a home becomes more unpredictable, it is advantageous to every homeowner to be "insulated" from inevitable utility bill increases. As with all aspects of these guidelines, the greatest improvements result from a "whole systems" approach. Energy performance does not end with increased R-values, the use of renewable energy, and/or more efficient HVAC equipment. Rather, there needs to be a balance between these features and careful window selection, building envelope air sealing, duct sealing, and proper placement of air and vapor barriers from foundation to attic to create a truly high-performance, energyefficient home that is less expensive to operate and more comfortable to live in than a conventionally constructed home.

Guiding Principle-WATER EFFICIENCY
The mean per capita indoor daily water use in today's homes is slightly over 64 gallons. Implementing water conservation measures can reduce usage to fewer than 45 gallons. For this reason, green homes are especially welcomed in areas affected by long- and short-term drought conditions.

The importance of water resources is becoming increasingly recognized, especially in the western third of the country. Choices between sending water to growing urban areas and making water available for irrigation highlight the issues surrounding the scarcity of this valuable resource.

Green homes often conserve water both indoors and out. More efficient water delivery systems indoors and native and drought-resistant landscaping choices outdoors can help prevent unnecessary waste of valuable water resources. Communities can obtain additional benefits when builders effectively use native species in landscaping. Current research and practice have shown that natural processes can be a successful means of filtering and removing contaminants from storm water and wastewater.

Healthy indoor environments attract many people to green building. After energy efficiency, the quality of a home's indoor air is often cited as the most important feature of green homes. Pam Sessions, president of Hedgewood Properties in Atlanta, said during the 2002 National Green Building Conference that the majority of people interested in green homes in the Atlanta market indicated that indoor air quality was their top issue of interest.

An increase in reported allergies and respiratory ailments and the use of chemicals that can off-gas from building materials have contributed to a heightened awareness of the air we breathe inside our homes. Even though there is no authoritative definition of healthy indoor air, there are measures that can mitigate the effects of potential contaminants including controlling the source, diluting the source, and capturing the source through filtration.

Improper or inadequate maintenance can defeat the designer's and builder's best efforts to create a resourceefficient home. For example, homeowners often fail to change air filters regularly or neglect to operate bath and kitchen exhaust fans to remove moist air. Many homeowners are unaware of the indoor environmental quality impact of using common substances in and around the house such as pesticides, fertilizers, and common cleaning agents. By providing homeowners with a manual that explains proper operation and maintenance procedures, offers alternatives to toxic cleaning substances and lawn and garden chemicals, and points out water-saving practices, a builder can help assure that the green home that was so carefully built will also be operated in an environmentally responsible manner.

Guiding Principle-GLOBAL IMPACT
There are some issues related to home building and land development that do not fit neatly into the context of the aforementioned guiding principles. For these items that are a by-product of home construction, we have added a separate principle-global impact. One example of an issue having global impact is the selection of paints that contain relatively low or no volatile organic compounds (VOCs). Although the VOC content of paint is often considered for indoor environmental reasons, the vast majority of VOCs are released by the time the paint is dry.

However, the release of VOCs from wet paint helps form ground-level ozone pollution. Therefore, the use of lowor no-VOC paints falls under the global impact principle because the environmental impact of using paints with relatively high VOC levels is greater on the global scale than it is on the indoor environment.

The process of green home building should not stop at the house. If a builder is also involved in the development of the community, site planning and land development can be part of the process. Therefore, information about low-impact site planning and land development is included in Appendix A. Considering the entire community and existing infrastructure in addition to the individual building( s) can amplify the benefits of green home building. For example, by improving a subdivision's storm water management plan and preserving natural resources through careful design and construction practices, a builder can influence not only the resource efficiency of each particular house but also the entire subdivision's overall environmental impact. Low-impact development (LID), which uses various land planning and design practices and technologies to simultaneously conserve and protect natural resources and reduce infrastructure costs, is one way to approach green development.

The previous section highlighted the environmental benefits of green building practices. However, green building is much more than just reducing a home's environmental footprint. Homeowners can also realize direct benefits by owning a green home. Here are some of the primary benefits that owners of green homes have experienced compared with owners of conventional homes:

  • Lower operating costs-Homeowners receive less expensive utility bills because of energy and water efficiency measures.
  • Increased comfort-Green homes have relatively even temperatures throughout the home, with fewer drafts and better humidity control.
  • Improved environmental quality-By following these guidelines, builders pay extra attention to construction details that control moisture, choose materials that contain fewer chemicals, and design air exchange/filtration systems that can contribute to a healthier indoor environment.
  • Enhanced durability and less maintenance-Green homes incorporate building materials and construction details that strive to increase the useful life of the individual components and the whole house. Longerlasting materials not only require fewer resources for replacement but also reduce maintenance and repair costs. Green homes have lawns that require less weeding and watering, building elements that require less maintenance, and more durable building components that reduce the time needed for upkeep.

It is important to note that a builder can do only so much when it comes to how the home will perform. Homeowners play a big role in the house performance and, therefore, should be instructed on how to operate the green home as it was intended.

GUIDELINES DEVELOPMENT PROCESS The NAHB Model Green Home Building Guidelines were developed through a public process that included the following major steps:

  1. 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.

  2. A review of the voluntary energy-efficiency programs endorsed by NAHB.

  3. 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).

  4. Input through an open process from numerous individuals in the NAHB Advisory Group and the Stakeholder Group.

  5. Applying certain criteria to each line item in order to give the line items point values.

Each line item in the guidelines has a point value attributed to it. Once the Stakeholder Group members finalized the list of line items for inclusion in the guidelines, the NAHB team looked at each line item through three different lenses: 1) environmental impact, 2) building science and best building practices, and 3) 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 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 are 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 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 the positive impact to the environment.

Building Science and Best Building Practices-Certain green building practices dramatically affect how a house operates. For example, the sealing of a home's building envelope has an impact on the home's HVAC system. 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.

Ease of Implementation-Some line items are easier to implement than others. The NAHB team compared each line item with current home building practices and estimated how difficult it would be for a builder to implement the line item relative to 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, NAHB assumed the home would be built in Baltimore, which is in Zone 4 of DOE's proposed climate zone map. The map can be viewed at the following web site: www.energycodes.gov/implement/pdfs/color_map_climate_zones_Mar03.pdf 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 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 probably place higher value on water efficiency measures.

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. A line item's point value is determined by 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 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 guideline may contain trade-offs and carry with it contradictory characteristics. For example, a recirculating hot water system can help 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.

The guidelines are organized by the guiding principles listed above. However, there are two underlying ideas that everyone should keep in mind before undertaking a green home project. First, environmental considerations should be incorporated into the project from the very beginning. It is much harder to weave green home concepts into a project after the house plans are finished. Second, the house should be looked at as a whole as the builder determines which of the green home guideline items to put into the house. For example, making a home's building envelope tighter through air sealing and quality building techniques can affect the way in which the builder designs the home's ventilation system. It is through such a forwardthinking process that builders can gain cost efficiencies.

PARTONE - Green Home - Building Checklist
Part One of these guidelines contains the checklist of line items. Each entry includes the line item title, the point value, and the items that should be provided by the builder to verify that the line item was implemented. The verification column assumes there is a green building program coordinator or other third party. However, the guidelines and point system can be used independently even if a formal green building program does not exist in a particular region.

It is again recommended that a builder first become familiar with the line items prior to designing a home to help introduce concepts that a builder can incorporate into the home's design, construction, and operation.

To help a builder holistically incorporate green building into homes, the NAHB team established different point levels to achieve for each guiding principle at each level of green building. The point system is described below.

There are three different levels of green building available to builders wishing to use these guidelines to rate their projects-Bronze, Silver, and Gold. At all levels, there are a minimum number of points required for each of the seven guiding principles to assure that all aspects of green building are addressed and that there is a balanced, whole-systems approach. After reaching the thresholds, an additional 100 points must be achieved by implementing any of the remaining line items. The table below outlines the various green building level thresholds.

Points Required for the Three Different Levels of Green Building
  Bronze Silver Gold TJB
Lot Design, Preparation, and Development 8 10 12 24
Resource Efficiency 44 60 77 94
Energy Efficiency 37 62 100 146
Water Efficiency 6 13 19 42
Indoor Environmental Quality 32 54 72 90
Operation,Maintenance, and Homeowner Education 7 7 9 8
Global Impact 3 5 6 9
Additional Points From Sections of Your Choice 100 100 100
Total Points 137 211 295 413

A reduction in the required points for a home without ductwork for the space heating and cooling systems reflects the fact that there are more points available for homes that do have ductwork. It is not intended as an indication of preference for one type of system over another.

To determine point values for each guiding principle, a builder simply adds the points for each line item applied to the home for each guiding principle. Comparing the project's points for the individual guiding principles with the chart above will determine whether the project is deemed a Bronze-, Silver-, or Gold-level green home.

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