By now the slogan of the National Trust for Historic Preservation that "the greenest building is the one already built" is widely known. In an era of increased environmental awareness and rising fuel prices, however, the question is how can historic building stock be made more energy efficient in a manner respectful of its historic integrity and character. The other challenge is to find those improvements that, in the quest to save energy (and, by extension, money), do not in the long run cost more than they save. There are an increasing number of "sustainable solutions" in the marketplace today, but not all are good investments, provide tangible benefits, or are appropriate approaches for historic buildings. Often common sense, trained historic and/or aesthetic judgment, and the studies and assurances of those marketing the
solutions are used to determine what interventions are appropriate. In addition, practical and objective analysis tools are needed in the process, and that is the benefit of including energy modeling and life-cycle costing in assessing potential changes. These calculation tools can help all of those involved in a project to understand which solutions truly offer energy and operating-cost savings.
The use of computers to simulate annual energy consumption began as a result of the energy crisis in the 1970s. After the United States Department of Energy (DOE) was created by President Jimmy Carter, algorithms were developed to simulate the annual energy consumption of a building. These calculations were refined and further developed over the years, with the DOE-2 simulation algorithms gaining wide acceptance in the industry throughout the 1990s. Currently, use of these energy-modeling tools has become standard for any project that is pursuing Leadership in Energy and Environmental Design (LEED) certification from the United States Green Building Council. There are many energy-modeling software programs in use today, including Energy Plus, developed jointly by the University of Illinois and the Lawrence Berkeley National Laboratory. (1)
The basic concept of the energy model is to virtually create (or, in the case of preservation, recreate) a building, delineating not only its physical form but also other performance and usage variables. The simulation process includes a virtual model of the building geometry, the building materials and their characteristics, and the types of mechanical systems and lighting, along with other systems that may consume energy. The patterns of the occupants and their activity levels are added to the virtual model, and finally the weather-data files that reflect the particular locale are referenced for a complete hour-by-hour simulation of a typical meteorological year. (2)
Depending on the size of the building, creating this baseline model can be a process that takes 40 hours for a small, straightforward building, such as a suburban office building, to hundreds of hours for a large, complex edifice, such as a monumental campus building. Regardless of project size, the process is typically the same, although larger buildings tend to leverage the effort and cost of the model to greater effect since the improvements can produce larger energy savings. Once the baseline information has been entered and an existing-conditions model created, it is then possible to calculate the building's current energy-use footprint and to track what percentage of that consumption can be attributed to each of the building's components.
Before performing any analysis, however, this initial calculation should first be cross-checked against any energy bills or other records that may exist to help normalize the model to the actual operation of the building and to allow for more accurate predictions in the future comparisons. (3) For example, energy loss through air infiltration can constitute a sizable percentage of the overall total, ranging anywhere from less than five percent in a newly constructed building with careful air-barrier detailing to 40 percent or more in an old, poorly maintained building that has numerous gaps and openings at such locations as foundations, sill plates, windows, doors, sheathing, flues, and eaves. …