Energy in Building Design + Construction: Prescriptive vs. Performance
In my 17 years of professional experience, I have often been surprised by how much disconnect there is between the design + construction team when it comes to building energy analysis and code compliance. I will call out my fellow architects here and say I have been especially dismayed at times by first, the lack of understanding of some basic building energy performance concepts, and second, by the acquiescence by architects of all of the energy discussion to the engineering team.
It is important for architects, owners, contractors and really everyone on a design / construction project to have a feel for the basic concepts and language of energy analysis. Components such as glazing, HVAC, and insulation can make up not only a huge portion of the construction budget, but also have significant impact on the operational cost, thermal comfort, and indoor air quality issues down the road.
This is the first in a series of articles on the most common areas where I see a lack of knowledge from design and construction teams. These are drawn from my personal experience over the years where I have found myself spending time explaining these items to various team members.
Prescriptive vs. Performance Energy Code Compliance
In just about every energy code or green building program, there are two options for demonstrating energy compliance: Prescriptive and Performance. Understanding clearly which method is being used as well as how to analyze the results are paramount because this process affects soft, hard, and future operational costs.
Prescriptive is pretty basic to understand. There are a series of energy thresholds for each different component of the building. A project needs to comply with ALL of these to demonstrate compliance with the relevant Code. There is usually a checklist and one just simply checks off the boxes to make sure each material or system individually is adhering to the isolated requirement for that material. Below is a simple prescriptive checklist:
Performance on the other hand allows the design team to use a whole building calculation, which is a calculation of projected energy use based on the energy characteristics of the designed building. A similar calculation is also done for a baseline / code building. The total energy used for the design building is compared to the total energy used for the baseline building. Thus, if the baseline value for roof insulation is R-40, but in the design case you use an R-20, but the overall building still uses less overall energy than the baseline, it doesn’t matter. You still pass. This method allows the ability to horse trade material performance for costs. For example, you can use less energy efficient glass but use better wall insulation because the wall insulation is less expensive than the up charge for the glazing.
The Performance calculation can be complicated and requires someone well versed in software inputs. Thus, I often see smaller / residential projects using the prescriptive method. It is often assumed to be easier and less expensive in engineering costs to run down a simple checklist and check off the boxes, than it is to hire an engineer to do a complicated whole building energy analysis.
Conversely, on large commercial projects the budget is much larger. But more importantly, the cost impact of certain prescriptive requirements can be enormous and well outweigh the added cost of the energy calculations. For example, quite often the code glazing performance requirements are very restrictive and the resulting glass can be very expensive. On a 50-story glass tower, adhering strictly to prescriptive requirements for glazing can add hundreds of thousands of dollars, if not millions. In other words, a more efficient HVAC system would more than likely be less expensive than more expensive glass. Below is a snapshot from a typical performance calculation report, with the design case numbers in the middle column and the baseline in the far right.
In general then it is important for everyone to understand first which path / method is being used for code and / or green building energy compliance. Second, it is important to understand the cost implications of both doing the calculations and the impacts of different materials. For example, even on a small building, the added glazing expenditure may still outweigh the cost of doing the calculations.
—
Joe Snider is an architect, speaker, and author and owner of Joe Snider Consulting, a consulting firm that provides a full suite of sustainability consulting services to guide organizations to incorporate their sustainability missions into policies and daily practices. Joe Snider has been awarded the prestigious LEED Fellow status by the U.S. Green Building Council.
