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Energy Modeling for Leed Certification

Posted by: Robert L. Cave on Thursday, October 27, 2016

Any design engineer who has worked on a LEED certified building would tell you the most difficult phase of the process might be developing the energy model calculation. This calculation - which is required for LEED certification at any level - can help guide the architect and design engineers through the certification process, making everyone’s job easier. However, in order for that to happen, the energy model calculation has to be developed early in the process, and it has to be done carefully. Otherwise, the complexity of the energy modeling process makes cost overruns, rework and confusion much more likely.

The energy modeling process: Get it right the first time

The good news is that there are several tools that can be used to model the LEED building configuration. These tools compare the building’s energy efficiency to that of a base building design, as LEED certification requires a minimum of a 10% gain in energy efficiency over the standard spelled out in ASHRAE Standard 90.1. It is the size of this difference that determines the level of LEED certification awarded to any proposed facility: the greater the difference, the higher the level of certification.

If you are looking for a starting point, the DOE Energy Gage program is a user-friendly option that makes it easy to develop base buildings.

Building the energy model: Data requirements

Before embarking on the energy model process, the architect will need access to certain data, including:

  1. Building materials including size and shape, color and shading.
  2. Building location and orientation.
  3. Surface areas for walls, windows, roof and floors.
  4. Thermal insulation resistance (R-value) of the building walls and roof.
  5. Heat Transfer Coefficient (U-Factor) for windows and doors.

Additionally, engineers working on the project will need to know about:

  1. Weather data and outdoor air conditions.
  2. Indoor design conditions.
  3. Building occupancy periods and schedules for heat generating appliances, including lighting and equipment.

This information is included in the building HVAC cooling load calculation, and then summarized in the energy model itself. Now is the ideal point in the process for the design engineer to discuss energy requirements for the building envelope with the project architect.

Insulation and thermal resistance: How much is enough?

The thermal resistance (R-value) of the installed insulation should be defined on the architect’s drawings. However, ASHRAE 90.1 also includes specified minimums for both residential and non-residential standard building construction. You should use the R-values that are most appropriate for the climate zone where the building will be located. For our zone (2A), a commercial building should meet the minimum values in the code.

Residential requirements may vary. The design engineer should always consult the latest version of ASHRAE 90.1, to ensure that the required minimum R values are up to date.

Exceeding these thermal resistance minimums is a necessary aspect of designing a LEED building. While a well-designed building envelope can account for some energy efficiency gains, most derive from efficiencies in water use, waste water disposal, and devices like HVAC components and lighting fixtures. In particular, improvements to the building envelope, HVAC, and lighting efficiencies all affect the cooling load calculation, which is a required part of the energy model.

Investing adequate time and effort to develop the energy model early in the design stage is critical to the success of any LEED project. By using the input data described in this post, a design team can achieve a building cooling load and energy model calculation that meets all the requirements listed in the Minimum Program Requirements list, which spells out the project requirements for eligibility for certification under the USGBC LEED criteria.