A building is very similar to the human body; it is a network of separate, but interrelated and interconnected systems transferring fluids, electricity and data. The circulatory system, the respiratory system, the nervous system, even the digestive system clearly correlate to the various building systems that distribute energy, provide ventilation, manage the building and remove waste.

A building energy simulation characterizes these relationships between the systems to help fully understand a project’s needs. Simply put, building energy simulation is a tool used by designers to illustrate the implications of their design decisions before a building is constructed.

An energy model is a representation of a building for the purposes of the building energy simulation. It consists of all the design and operating parameters associated with the energy consumptions of that building. Design parameters include wall and roof constructions, window performance values, installed electricity for lighting and user equipment, occupant numbers, ventilation requirements, air and water distribution system descriptions, and cooling and heating equipment details.

Operating parameters include operational schedules for the lights, user equipment, occupants, thermostats and fans as well as HVAC controls and utility data. The simulation takes in the energy model data, combines it with historic weather data, and estimates the performance of the entire building and system components for every hour of the year.

Energy modeling tools, such as eQUEST (a front end to DOE-2), EnergyPlus, IES Virtual Environment, Trane TRACE, and Carrier HAP provide three distinct functions: a design tool throughout all phases of design, an accounting tool for the end of design, and a financial justification tool after occupancy and calibration.

As a design tool, energy models provide a comparative analysis between options. This can occur during concept stages to determine the energy impacts of an architectural form or can occur later in the design to evaluate the impact of a specific glazing choice.

As an accounting tool, energy models are most often used to compare a proposed design against a benchmark. The Leadership in Energy and Environmental Design (LEED) rating system, state energy codes, and federal tax incentives are all examples of using energy models in this capacity.

As a financial justification tool, energy models must be used very cautiously. Once an energy model has been justified and calibrated to real utility and operations data, energy service companies and retro-commissioning agents can utilize these models as the basis for performance contracts and other financial configurations. It is critical that the energy models are properly validated and calibrated prior to utilizing them in this fashion.

It is also critical that energy models are regarded merely as tools — the true path to improving building performance is an approach founded on a consistent and integrated design. This approach is an evolution of the design process from the “lost opportunity approach” that provides the minimum legal performance and the “upgrade approach” that maintains the standard design, but incrementally improves the performance of individual components.

The integrated-design approach targets energy and other performance issues at the most fundamental conception of design and relies on an ongoing holistic design philosophy and interdisciplinary communications to insure optimum performance. It is in the integrated design approach where building energy simulations are essential. A truly integrated design cannot proceed without a better understanding of the building at hand.

That essential knowledge is learned through multiple simulations of the energy model to test concepts and technologies, pinpoint performance of individual components, and characterize the interdependent relationships between building systems. Furthermore, now that the annual performance can be better ascertained, engineers, architects and owners are not limited to relying solely on the peak design conditions. Simply put, with better understanding, there is greater confidence in potential customized and integrated solutions.

As a cautionary note, the biggest misconception for building energy simulations resides in properly understanding the results. Energy models do not predict the future — there are simply too many unknown variables. Only in the special cases where the energy model has been validated and justified with real data can this comparison be properly used. The general mantra to always remember, “If it is good for the model, it is good for our design.”

Building energy simulation will never replace good design judgment but it will always calibrate and inspire it.


Kristopher Baker, PE, LEED AP, is a senior associate and Rob Bolin, PE, LEED AP is senior vice president with New York-based engineering firm Syska Hennessy Group.