Article #156 - Three Aproaches to Commercial Compliance

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 are typically based on worst-case assumptions, and all requirements must be met exactly as specified.

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 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.

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.

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 a 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.

The three approaches can produce different results. Performance approaches require a higher degree of detail so that an individual building can be designed to meet the IECC requirements exactly. 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 does not generally account for many of the features that affect energy use, such as the effect window orientation and external shading have on solar heat gain. Trade-off approaches fall somewhere between the prescriptive and performance approaches in flexibility and complexity.

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