Energy Modeling, Building Size, and BIM—What's Cost-Effective?

Energy modeling Q&A: first some answers on cost, and then it's your turn to ask (or answer) some questions.

Chris Schaffner

There is so much confusion about energy modeling--what it should cost, what benefits it offers, how to approach it--that clear statements addressing these questions are like a breath of fresh air.

When I was privy to a private email exchange that included a short treatise on this topic from Chris Schaffner, principal of The Green Engineer in Concord, Massachusetts, I got his permission to share it.

First, the question:

I've often heard that energy modeling generally becomes cost-effective on projects that exceed 50,000 square feet. Do you agree, or is there a better threshold?

And Chris's reply:

Thoughts:

1. There are two kinds of models--documentation models, performed after the design decisions have been made, and design-phase models, used to make decisions. Documentation models are never cost-effective. (This is why the current LEED 2012 draft has requirements for early design models.)
2. It can be cost-effective on any size project depending on the questions you need answers to.
3. It gets less expensive as the building gets bigger--all other things being equal.
4. Systems complexity has more of an impact on modeling costs than size.
5. In Massachusetts, we have something called the "Stretch Energy Code," which can be adopted by cities and towns as an optional, more stringent code. The Stretch Code requires energy models for projects 100,000 ft2 or greater, except that labs and healthcare must model at 40,000 ft2 or greater. So that is one potential idea of when it becomes cost-effective.
6. Looking at how much I might charge for generic office buildings, assuming modeling results in a modest 10% energy cost savings:

• At 20,000 ft2, my modeling costs are recovered in savings in 4.2 years
• 50,000 ft2 – 1.9 years
• 100,000 ft2 – 1.1 years
• 200,000 ft2 – 0.6 years

(All those look pretty good to me – I should raise my prices!)

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A screen capture from Hevacomp energy modeling software

There was also a corollary question: Do you see this number decreasing as BIM usage increases?

Here's what Chris had to say about that:

Currently, at least in my practice, BIM is not having an impact on modeling costs. Whatever theoretical savings are there are usually overwhelmed by all the deficiencies in the BIM model. Most of our energy models are still done in eQuest, which doesn't play well with BIM.

What will bring down modeling costs will be COMNET-compliant software that can self-generate robust baseline case models.

For a much more detailed discussion of design-phase energy modeling, and what non-engineers should know, see Marc Rosenbaum's paper from Greenbuild 2003 in Pittsburgh.

For background on how Building Information Modeling (BIM) and works with energy and other sustainability analysis tools, see the EBN feature "Building Information Modeling and Green Design."

What's your experience with the costs and benefits of energy models? Do you agree that documentation models are never cost-effective? Have you figured out how to make BIM support energy modeling? Let us know!