Adventures in LEED Energy Modelling

I’ve been super busy over the last month or more – between our 7-month-old and work, it’s been crazy! Work for me lately has been all about LEED Energy Modelling and ASHRAE 90.1 Appendix G. For those of you who may not be aware, a significant number of LEED points are up for grabs related to optimizing energy performance in a building. The basic procedure is to model a strictly defined reference building, your proposed building and then compare the two. The percent difference defines how many points are awarded.

This was only my second project like this (I also did a project to judge energy performance against MNECB) and I have found it very enlightening! The vast majority of energy models I’ve done in the past were for the purpose of judging the impact of energy retrofits, so these models were all of existing buildings, calibrated to a base year. At first glance, an energy model is an energy model, but when you get into the details, I was surprised at how fundamental a difference there could be between the two concepts.

Two Purposes, Two Concepts

The main goal of a LEED model is to judge how well your proposed building (or renovation) will compare to the reference baseline building. This is a way to just how “green” your building is in terms of energy performance. While the main goal of a baseline-calibrated energy model is typically to judge the performance of a specific energy conservation measure. From these two core goals, a huge amount can be different in your energy modelling work-flow – from assumptions, required documentation, modelling tools, etc. Below I will try to compare and contrast these two model types.

  • Sometimes many more assumptions, sometimes fewer. Depending on the flow of information from your design team, a LEED model may be more of less information than an existing building. The opposite to this problem though is that the floorplans of an existing building never change half way through your modelling process!
  • Poor documentation of existing buildings is very common. Rarely does the operator of an existing building have all the documents you need unless it’s a new building or owned by an organization which has strict documentation and record-keeping guidelines.
  • Significant experience is required to judge the difference between what an existing building is supposed to be doing compared to what its actually doing. You may have only older, out-of-date drawings, the control system may be lying or the building operator may have misunderstood your questions. When you are looking at energy use patterns that simply don’t make sense or are completely different than the base year energy data, you need to put on your detective hat to track down what is actually going on in the building.
  • Only one model is typically required for an existing building. LEED models can (and should) be constantly evolving throughout the design phase. ASHRAE 90.1 also requires the model to be simulated at various azimuth angles to take into account the effects of site orientation. Count the baseline building as well and you end up with a minimum of five models! There are tools that can help with the baseline model generation, but it still requires extra work.
  • Due to the use of standardized space type categories for various purposes from lighting power density to scheduling, a compliance-type model like those created for LEED will often require much more detailed (and careful!) zoning to ensure representation of the building as the referenced standard requires. One could argue that this is a good thing and should be done for any building type because proper zoning is very important.
  • If, during a LEED project, the modeller is unsure about the purpose of a piece of equipment or the intent behind the ventilation system design, he or she can simply call the mechanical engineer and ask! This is rarely an option for an existing building!
  • LEED and other programs have prescribed ways to deal with a lack of information. Usually it’s simply a matter of following the defaults from ASHRAE 90.1 or the MNECB. In an existing building, the “best practice” values may not be accurate due to the building not operating as intended.

As you can see, there are some big difference between these types of energy models! There are other differences of course, and several other types of energy modelling as well. I plan to cover a few more in a post at another time.

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About Matt

Matt is a mechanical/building engineer who specializes in whole-building energy modelling, energy efficiency and solar buildings. He's worked for about 9 years analyzing the energy use patterns of buildings. He studied the energy performance of a low-energy, solar house for his Masters thesis. Matt has experience with EnergyPlus and eQuest.

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