My first energy modeling assignment, about six years ago, was for a building that was nearing the end of construction. The owner wanted a second opinion, or third-party energy model, to see how it would compare to the energy modeling results given by the design team for LEED certification. What they really wanted to know was how much they should actually expect to pay for utilities during their first year or two. They probably didn’t trust the LEED energy model, thinking that it was skewed to show better performance than actual. Since this introduction to the powerful tool of energy modeling, my mind has been focused on proving to myself and others that the most useful investment for energy modeling is to influence the building envelope and HVAC system at the beginning of design and conceptualization.
So, the question becomes: How do we, as energy modelers, help shape the design and specifications of a building to maximize energy efficiency? I think the answer comes with experience and a determination to analyze as many building envelope and system design options as possible. Since engineers and energy modelers work alongside architects and owners in designing a building, the key is to focus on a conversation that helps all parties understand what is at stake with their design choices and how powerful an energy simulation can be in finalizing those choices. Finding out how a particular design choice changes the energy performance of a building is a very useful skill to have.
The challenge with this skill is that the answers are almost never available immediately. For example, in a face to face meeting with an architect I will usually not be able to answer this question before starting an energy model:
“What is the R-value and assembly type that will make our wall most efficient while costing the least?”
Maybe the best answer is something like:
“Let’s decide on the number of assembly types that you are considering using for this project, and I will complete some analysis to narrow us down to an optimal solution.”
One of my previous managers gave me some great insight and a story of how this conversation can evolve. We were working on the design of a new small commercial building to be used as a community educational center. My boss handled much of the consulting before I came on board to finalize the HVAC and plumbing design. The owner and architect were very interested in this being an energy efficient and sustainable building, but it wasn’t in their budget to get LEED certification. My boss agreed to do some level of energy modeling to help optimize the building envelope design, including windows, shading, insulation, and thermal mass. The architect hadn’t been exposed to a lot of energy modeling on past projects, and they were delightfully interested in the process of analyzing their options. For example, they were really interested in a rammed earth wall for thermal mass and the energy model gave them some insight into how helpful this could be if it were located with the optimal orientation. When the design process was about two thirds complete, the earth wall was replaced with traditional framing because the project was getting to be over budget.
They also decided on a roof insulation value of R-30 based on the recommendations of my manager through the energy model and industry standards. During construction, the roof cavity turned out to be deeper than expected, so the architect and contractor decided to fill it up with insulation. When construction was finally complete, the architect wanted to show the owner how they got additional insulation for free and how much money on energy it would save. They convinced my boss to run the energy model again with the roof increased from R-30 to the actual installed value of R-55. He reported back to them with the results of an annual energy cost savings of $7.
With all of their enthusiasm about energy modeling, I don’t think the architect had a good overall concept of which parts of the building envelope were responsible for most of the energy consumption. They were disappointed when they realized that the roof improvement was pretty much insignificant. The way I look at the building envelope is that its energy performance is only as strong as the weakest link. If 70% of the heat loss or gain is through windows, and the size and specifications of the windows are already decided, then it is probably a losing battle to try to improve the walls or roof.
I am sure that we all learned a lot from this experience. This type of experience, along with running the numbers early on in design, is a large part of what it takes to really contribute to an energy efficient building envelope design.