While many engineers and architects throughout the United States have used ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, as the basis of their building design, there remains some confusion about the proper application and requirements of the standard, especially surrounding the Energy Cost Budget Method (ECBM). As ASHRAE prepares to release the 2004 edition of 90.1, let's take a quick review of the standard.


Standard 90.1 originated in 1975 in response to the energy crisis of the early 1970s. It was revised in 1980 (taking longer than the usual ASHRAE standards cycle of the time), but became more prominent in building designs with the co-sponsorship of the Illuminating Engineering Society of North America (IESNA) in the 1989 edition and as a result of its adoption within the building codes of many regions of the country.

In 1999, ASHRAE placed the standard on continuous maintenance in conjunction with the American National Standards Institute (ANSI). The U.S. Department of Energy (DOE) also reviewed the 1999 edition and determined that it was in the country's best interests to require all states to implement a state energy code requirement that met or exceeded Standard 90.1-1999 by July 2004 (see sidebar).

With this increased interest in Standard 90.1-1999, the U.S. Green Buildings Council, through its Leadership in Energy and Environmental Design (LEED) rating system, created the Green Building Design Program, which provides up to 10 points (out of the 26 needed for basic LEED certification) in the LEED-NC credit for optimizing energy performance (Credit EA-1).

Chapter 11 of Standard 90.1 deals with the ECBM. This may be the least understood portion of the standard. ECBM is a critical requirement of LEED-NC Credit EA-1. Standard 90.1 now requires (per addendum P) that the energy simulation program utilized be tested in accordance with ANSI/ASHRAE Standard 140-2001, Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs. Therefore, any energy simulation program utilized should have this test documentation.

As with most things in life, the best way to learn is to do. The purpose of this article is to provide a place to start and a “big” picture to aid engineers in utilizing ECBM in design practice. A review of the standard's user's manual, which is available from ASHRAE, is recommended to clarify the procedures to properly implement the ECBM calculation.


The ECBM is a comparative modeling system in which two models of the same building are created and compared.

  • The first model is the “Proposed Design Model” (PDM) (also referred to as the “Design Energy Cost”). This is the building as designed. It is exactly what the construction documents (or contract drawings and specifications) show. The building occupancy, envelope, areas (floor, windows, walls, roof, and skylights), and MEP systems (HVAC, lighting, power and, plumbing systems) are included in this model.

  • The second model is the “Budget Building Design” (BBD) (also referred to as the “Energy Cost Budget”). Just like the PDM, this is the building the engineer would like to construct; however, this model uses 90.1's prescriptive (minimum) requirements for component performances. This model has exactly the same floor and envelope dimensions as the construction documents show. The envelope components, while dimensionally the same, have the code minimum performance values (i.e., insulation of walls, roof, and glazing). The building occupancy and MEP systems are also included in this model. However, the MEP systems can be modified to reflect the code minimum component efficiencies (with the same components). Controls for the lighting and HVAC systems should be the minimum required. The standard does acknowledge that some components, systems, or operational modes may not be properly simulated by an otherwise approved program. Therefore, 90.1 allows:

“Where no simulation program is available that adequately models a design, material, or device, the authority having jurisdiction may approve an exceptional calculation method to be used to demonstrate compliance with Section 11.”

If the design calls for a demand ventilation control strategy, one would have to get the authority having jurisdiction's (AHJ) approval for an exceptional calculation method to show the energy savings. It would then also be up to the LEED submittal review process to also approve for any additional Credit EA-1 points. Securing AHJ approval is a recommended step in the LEED review process.

It is important to understand that Standard 90.1 has specific requirements for the comparative models in terms of items that must be identical, such as the occupancy schedule, conditioned areas, temperature setpoints, building orientation, minimum outdoor air ventilation rates, etc. Likewise, LEED Credit EA-1 has Energy Modeling Protocol (EMP) requirements that can differ from those in the standard. It is very helpful to refer to the LEED credit interpretations for Credit EA-1, which are available to all registered project teams, to better understand EMP. Both LEED Credit EA-1 and 90.1 require the same simulation program, weather data, and energy rates be used for both models.

In addition, both the standard and LEED are most interested in the regulated (vs. non-regulated) energy, although only the LEED credit uses these terms directly. Non-regulated loads are “…energy used primarily to provide industrial, manufacturing, or commercial processes,” according to LEED-NC.

Note that the applicable non-regulated loads are included in the models and then subtracted from the overall building energy usage. This is done to properly model the MEP systems' actual performance (i.e., part loading) if, for example, there is a non-regulated load that impacts an air-handling system or a chiller plant.


The good way to understand the ECBM calculation as it relates to a LEED Credit EA-1 building is provided in this example.

A new two story office building located in St. Louis with 16,900 sq ft per floor (total of 33,800 sq ft) was modeled using a commercially available energy simulation program.1 The occupancy is 100 sq ft per person. A common variable-air-volume rooftop air-handling unit with zone reheat was the selected HVAC system.

The budget model inputs:

  • Glazing: U = 0.57 Btu/hr ft2 F; solar heat gain coefficient (SHGC) = 0.39.

  • Walls: U = 0.077 Btu/hr ft2 F (R-13).

  • Roof: U = 0.053 Btu/hr ft2 F (R-19).

  • Lighting: 1.3 w per square foot.

  • Miscellaneous power: 0.75 w per square foot.

  • Air-cooled rooftop unit (RTU) with gas heating and 30 percent minimum zone airflow.

  • Gas-fired heating: 80-percent efficient.

When the utility costs were calculated, the annual energy costs were found to be $23,112 for electricity and $1,172 for natural gas (for a total of $24,284). This then became the base case (BBD), against which the annual energy cost savings were compared for improved energy performance (PDM).

One of the first conclusions evident from the BBD was that the heating cost was only about 7 percent of the total energy cost. Therefore, the heating was not going to achieve target 15 percent total annual energy cost savings goal for one point in Credit EA-1, even reduced to zero cost. Therefore, the key to this endeavor was to reduce the cooling costs.

It is a good idea to start with the most straightforward energy reduction opportunities that will maximize energy savings. This includes changes to lighting, which have a doubling effect because electrical energy is used to both power the lights and to run the air conditioning, which cools the heat that is shed by that same light.

It is also important to focus on the building envelope. For this example, the lighting was revised to 1.2 w per square foot, and the insulation values of the walls, roof, and glazing, were improved to R-19, R-30, and U = 0.34 Btu/hr ft2 F, respectively. The SHGC was also reduced to a 0.32 value with an actual glazing selection.

These modifications resulted in an 8 percent reduction in the annual energy cost for the building.


The first option to consider when optimizing the HVAC system in this case study should be a water-cooled chiller and the associated air-handling unit. Water-cooled chillers use significantly less kilowatts per ton when compared to air-cooled chillers. However, this turned out not to be the optimum approach, as our model indicated that this system needed 6-percent more energy dollars than the base, which totals 14 percent in the wrong direction. So there is a good reason why this building type (mid-size office building) in this climate (St. Louis) has more air-cooled equipment than water-cooled chillers. (Note that LEED EMP does allow the use of a water-cooled system in lieu of an air-cooled system in the budget model for systems less than 150 tons.)

Additional HVAC system options were then considered. These included improving the motor efficiencies by going to premium efficiency electric motors in lieu of the high efficiency motors. Next, friction losses in the ductwork were reduced to require only 3-in. static pressure for the supply-air fan in lieu of 4 in. in the base model. A water-cooled RTU selection was considered. This actually reduced the compressor size because the needed performance could be obtained with the next size smaller, also reducing the first cost impact to less than a 2-year payback. These modifications resulted in a 22.4 percent annual energy savings (as compared to the base) and two points in Credit EA-1.

The goal of the project team was to earn three points for Credit EA-1 so additional options were then considered. Standard 90.1 does not require economizer in the St. Louis climate, but it was worthwhile to determine if economizers would help in this instance. The energy simulation model was run again, making this revision and the result indicated a total annual energy cost of $18,341 (26.3 percent savings). The three points for Credit EA-1 were now possible by using economizers.

The proposed model inputs (final):

  • Glazing: U = 0.34 Btu/hr ft2 F; SHGC = 0.32.

  • Walls: U = 0.053 Btu/hr ft2 F (R-19).

  • Roof: U = 0.033 Btu/hr ft2 F (R-30).

  • Lighting: 1.2 w per square foot.

  • Miscellaneous power: 0.75 w per square foot.

  • Water-cooled RTU with gas heating and 30 percent minimum zone airflow.

  • Gas-fired heating: 80 percent efficient.

  • Air-side economizer.


The conclusion that is evident from this example is that to effectively improve the energy performance of a building, the engineer should always begin with the basic parameters of the energy-consuming systems utilized. The first costs involved do not have to be major. Look for the low-hanging fruit obtained by the progressive use of basic engineering principles, rather than searching first for more exotic solutions that will likely not be cost beneficial to the owner. This approach will also be helpful in identifying the specific impact of a particular component and clearly define the cost-benefit relationship of each.


The author wishes to thank Scott Hunke of Trane in St. Louis for his assistance in the modeling and equipment selections for this case study. He also wishes to thank Jordan Heiman, Al Black, both former members of the Standard 90.1 committee, and Jerry Williams of McClure Engineering for their time and input.


  1. Trane Trace 700

For HPAC Engineering feature articles dating back to January 1992, visit www.hpac.com.

Peter W. McDonnell, PE, is a senior mechanical engineer for McClure Engineering Associates in St. Louis. He is a LEED Accredited Professional and a past adjunct professor of construction engineering at the University of Central Florida. He has spoken several times on ANSI/ASHRAE/IESNA Standard 90.1, including at HPAC Engineering's Engineering Green Buildings Conference in July 2004. He can be reached at pmcdonnell@mcclureeng.com.

U.S. Commercial Energy Codes

On July 15, 2002, the U.S .Department of Energy (DOE), under the directive of the Energy Conservation and Production Act (ECPA), issued a determination that ANSI/ASHRAE/IESNA Standard 90.1-1999 would save more energy in commercial buildings than the 1989 edition of the standard. “As a result of this positive determination regarding Standard 90.1-1999, each state is required to certify that it has reviewed and updated the provisions of its commercial building code regarding energy efficiency to meet or exceed Standard 90.1-1999.”

The DOE gave the states two years (until July 15, 2004) to submit certification of compliance. The graphic below shows the status of each state's commercial energy code adoption process as of mid-November 2004. These mandatory statewide codes include all state-owned buildings.

This graphic is courtesy of the Building Codes Assistance Project (BCAP). BCAP is a not-for-profit organization working to save energy in homes and buildings by promoting the adoption and implementation of building energy codes. BCAP provides free assistance to states and municipalities in their efforts to adopt and implement both commercial and residential energy codes. BCAP also functions as a clearinghouse of information about building energy codes and other measures for saving energy in homes.

Notes: 1) Code implementation depends upon voluntary adoption by local jurisdictions. 2) 90.1 is mandatory for state-owned buildings.

This map should be used in conjunction with BCAP's Status of the State newsletter. Go to www.bcap-energy.org to view the current issue.