Commercial Compliance FAQs

For commercial buildings, you can show compliance using a prescriptive approach, a trade-off approach, or a performance approach. Only one approach is necessary to show compliance.

Q: What is a prescriptive approach?

For the building envelope, a prescriptive approach would list the minimum R-value or maximum U-factor requirements for each building component, such as windows, walls, and roofs. For lighting systems, a prescriptive approach would simply list the allowable watts per square foot for various building types. For mechanical systems and equipment, a prescriptive approach would list the minimum required equipment efficiencies. This approach is quick and easy to use, but you may find the approach somewhat restrictive because the requirements typically are based on worst-case assumptions and all requirements must be met exactly as specified.

See the COMcheck™ prescriptive packages for this approach.

Q: What is a trade-off approach?

A trade-off approach allows you to trade enhanced energy efficiency in one component against decreased energy efficiency in another component. These trade-offs typically occur within major building systems-envelope, lighting, or mechanical. You can, for example, trade decreased wall efficiency (lower R-value) for increased window efficiency (lower U-factor), or increase the roof insulation and reduce or eliminate slab-edge insulation. For lighting systems, the trade-off typically would occur between proposed lighting fixture wattages in various spaces within a building. The only trade-off allowed for mechanical systems and equipment is found in Chapter 8 of the IECC. You may trade off higher cooling equipment efficiency against a requirement for an economizer. The trade-off approach is less restrictive than the prescriptive approach because you describe the actual building design in the trade-off approach and may adjust individual component requirements.

The COMcheck software automates this approach.

Q: What is a performance approach?

A performance approach (also known as a systems performance approach) allows you to compare your proposed design with a baseline or reference design and demonstrate that the proposed design is at least as energy efficient as the baseline in terms of annual energy use. This approach allows great flexibility but requires considerably more effort. A performance approach is often necessary to obtain credit for special features, such as passive solar, photovoltaic cells, thermal energy storage, fuel cells, and other nontraditional building components. This approach requires an annual energy analysis for the proposed and the reference buildings.

Q: Which approach is best for a specific building?

The choice of approach depends on the complexity or uniqueness of the building, and the amount of time and money available for demonstrating compliance. The prescriptive approach allows quick review of the requirements. If these requirements are too restrictive, try a trade-off approach. For example, if the window area of a building exceeds that allowed by the prescriptive approach, a trade-off approach might work. If nontraditional components are involved or if energy use trade-off between building systems (e.g., envelope, mechanical) is desired, try the performance approach.

Q: Do the three approaches produce different results?

Yes, they can. Performance approaches require a higher degree of detail so that an individual building can be designed to exactly meet the IECC requirements. Prescriptive approaches tend to be somewhat conservative and use worst-case default assumptions so the prescriptive packages will apply to all buildings. Although the prescriptive approach may result in a more energy-efficient building because of its conservative assumptions, this situation is not always the case. The prescriptive approach generally does not account for many of the features that affect energy use, such as the effect window orientation and external shading may have on solar heat gain. Trade-off approaches fall somewhere between the prescriptive and performance approaches in flexibility and complexity.

Q: Why are there so many compliance methods?

The different methods reflect the differing influence of engineers and code officials. For example, Chapter 7 of the IECC reflects the engineering viewpoint by covering all possible situations and systems in a building. Chapter 8, originally developed for simple buildings and later enhanced for more complicated ones, reflects the need of code officials for a simple, easy-to-enforce set of requirements.

Q: Is it possible to use all these approaches in my state?

The 1998, 2000, 2001, 2003, 2004, and 2006 IECC, and the ASHRAE/IESNA Standard 90.1-1989/1999/2001/2004 contain requirements for all three approaches. If your state has adopted any of these codes directly, you may be allowed to use all three compliance approaches. Always consult your local code office for specific requirements in your area.

Q: What compliance tools and materials are available for these approaches?

DOE provides a set of free COMcheck products that includes paper-based prescriptive requirements and software-based trade-off requirements for the 1998, 2000, 2001, 2003, 2004 and 2006 IECC and ASHRAE/IESNA Standard 90.1-1989/1999/2001/2004. DOE also provides COMcheck-Plus (discontinued) that implements the performance approach for codes based on ASHRAE/IES Standard 90.1-1989. The ASHRAE Standard is used directly to demonstrate compliance with the prescriptive approach and includes the ENVSTD software to demonstrate the trade-off approach.

Q: What is the relationship between ASHRAE/IESNA Standard 90.1-1989/1999, the Model Energy Code, and the IECC?

Since the early 1970s, ASHRAE has developed standards for commercial building energy use. Model code organizations have incorporated these standards into the Model Energy Code (MEC) and the IECC. The MEC codes were developed and maintained by the Council of American Building Officials (CABO). With the advent of the International Code Council (ICC), the task of maintaining the MEC codes was passed to the ICC and resulted in the release of the two IECC codes. The first edition was released in 1998 and updated in 2000, 2001, 2003, and 2006.

Q: Where can I get the compliance materials?

You can download COMcheck compliance materials from this site.

Q: Why does COMcheck 3.1 include all below-grade walls with above-grade walls when calculating window-to-wall ratio (WWR)?

ASHRAE approved an interpretation in June 2005 for including basement walls in the WWR calculation based on the definitions of "building envelope", "wall", and "gross wall area" in Section 3.2 of Standard 90.1-2004. The interpretation states that in buildings with conditioned space below-grade, the gross wall area extends from the top of the surface of the floor of the lowest conditioned space to the bottom of the roof of the highest conditioned space. This interpretation was deemed to also affect 90.1-1999 and 90.1-2001. Therefore, with the release of COMcheck 3.1, below-grade walls are included with above-grade walls when calculating WWR when 90.1-1999, 90.1-2001, or 90.1-2004 is the selected code.