Article #120 - Why are There So Many Approaches for Energy Code Compliance?
Over the years, the International Energy Conservation Code (IECC)/Model Energy Code (MEC) has grown to provide different approaches of varying simplicity and flexibility to meet user needs. The simpler approaches are less flexible but are generally easier to use. Some of the approaches have considerable overlap.
A prescriptive approach lists the minimum R-value or maximum U-factor requirements for each building component such as windows, walls, and roofs. For lighting systems in commercial buildings, 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.
A trade-off approach involves trading enhanced energy efficiency in one component against decreased energy efficiency in another component. These trade-offs typically occur within major building systems (e.g. envelope, mechanical) or in commercial lighting.
A performance approach (also known as a systems performance approach) compares a proposed design with a baseline or reference design and demonstrates that the proposed design is at least as efficient as the baseline in terms of annual energy use. This approach allows the greatest flexibility but may require considerably more effort. A performance approach is often necessary to obtain credit for special features such as a passive solar design, photovoltaic cells, thermal energy storage, fuel cells, and other nontraditional building components. This approach requires an annual energy use value. There are several commercially available software tools that perform this analysis.