Green Building Law Blog
"Reasonably Relied Upon..." - The Growing Importance of Energy Modeling
Today's guest post was contributed by E. Mitchell Swann, P.E., LEED AP, a partner at MDC Systems
As a strong component of the sustainability initiative in buildings, energy use is rightfully taking its place as a leading metric in evaluating a building’s performance. Further emphasizing the importance of performance measurement is the expected roll out of an industry wide “Building Energy Performance” label which is intended to provide an objective comparison of energy use between buildings. Rating systems like Energy Star along with model energy codes look at both predictive energy use models and actual usage as crucial to determining a building’s true performance and rating. The USGBC’s newly issued LEED v3.0 rating system requires the initial certification, recertification and by extension the possibility of decertification of LEED buildings to be tied closely to comparisons of modeled and measured energy use over time.
In those rating systems where a project’s “end game” performance is evaluated against their ‘promise’ as presented in a predictive model developed during design, the quality of the prediction greatly influences the quality perception of the results.
On many projects energy modeling work is performed by a subconsultant to the design team or possibly as an independent member of the project team. The results of the modeling effort are extremely influential in making design decisions from site orientation to building envelope options to HVAC systems to control strategies. Clearly an error during the modeling stage can lead to major problems downstream especially with respect to energy use comparisons and possibly maintaining certification downstream.
But if an error is made, who do you turn to for redress? Obviously the first stop is at the door of the energy modeler. This might work in the independent team member scenario but not so well in the subconsultant situation. Dealing with a subconsultant to a member of the design team may require a ‘two stop’ stop. But what happens if the modeling consultant doesn’t have the liability insurance that design firms typically have? What if the error is not ‘caught’ until Year Two of operation and we discover that the system that was installed will never perform as modeled and the actual energy costs are expected to 15% higher than initially thought for the life of the building? What are the ‘damages’ incurred by the Owner? The cost of a building or system retrofit? The energy cost penalty for the life of the system? What about the engineer who reasonably relied upon the analysis provided by the energy modeler? If the initial model output is used to guide engineering and/or architectural designs on the project, is a ‘third party modeler’ providing design services? Do they need to be licensed as an architect or engineer?
These are just a few of the issues that can arise and while energy modeling has a fairly long history, it is common in the industry to consider models as a comparative tool used to evaluate design options, not to ascertain the precise amount of energy a building would consume in a year. The newly ‘codified’ need to compare prediction to reality would seem to introduce a new level of expected accuracy and with it potential exposure. Is this effectively a prediction (and promise?) of performance which traditional E&O insurance does not cover? Will insurance products need to be revised to accommodate this new potential risk?
Design professionals would do well to clearly define performance expectations and potential limitations on their design as well as key parameters and assumptions used in modeling facility operations. On evaluating performance downstream, there may need to be an ‘audit clause’ to allow the designers a chance to evaluate how the facility was maintained or operated and the impact on energy if there is a divergence between predicted and actual usage.
The importance of energy modeling and the importance of accuracy in modeling is growing especially if building ratings or certifications are linked to correlations between predicted an actual performance. Is this a good thing? Well, at one level it seems reasonable to require actual performance to be at or near what was “promised”. But it is also important to remember that construction is a complicated and multi-variable process with many inputs, pieces and actors. So a lot can happen that can impact final performance and it may be difficult to determine exactly all of the “whys”, “hows” and who if something doesn’t perform exactly as expected. And of course we all know how fickle the weather can be.
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This is an interesting and timely subject for discussion. Owners, both public and private, will increasingly be looking for designers to be accountable for their design modeling calculations. Designers will continue to push back by reciting factors beyond their control that may impair building performance and reduce energy efficiency (e.g., construction in accordance with plans and specs, operation and maintenance of mechanical systems post completion, and occupant thermal comfort settings).
Victor Schinnerer, the largest underwriter of professional liability policies in the US, advocates holding design professionals responsible for failure to achieve predicted energy optimization only if the design professional is solely at fault. Well represented owners are unlikely to agree to this language since it will be very rare for the facts to show that no other party (e.g., owner or contractor) bears any responsibility for failure to achieve targeted levels of energy performance. More reasonable and balanced language would call for the design professional to be responsible to the extent of its relative fault for failure of the building to perform as predicted.
"So a lot can happen that can impact final performance and it may be difficult to determine exactly all of the “whys�, “hows� and who if something doesn’t perform exactly as expected".
Your right.
You can't model occupant behavior and this is a very big variable.
Stepping back, is it better to not model than to model? Could a firm be blamed for designing a building without modeling? This sounds like a standard of care issue, rather than an absolute performance issue. We would rather model with uncertainty than not model at all.
Michael, Shari,
Thanks for this article - it raises some very good points, not least the widespread misunderstanding on appropriate uses of energy simulation.
Energy simulations done in the course of design are not typically intended to be predictions of actual energy consumption, demand, costs or GHG emissions. The actual energy performance of a building is a function not only of building design, but also of building construction, operation and occupant behaviour.
Energy simulation is best used as a tool to
- inform decisions on design alternatives with their relative energy/GHG merits; or
- document that the design meets requirements of applicable Codes or Standards.
Energy modellers must necessarily make assumptions on construction, operations and behaviour. Typically these assumptions are based on industry codes or standards (ASHRAE 90.1, etc.) or on modeller judgement, based on research and experience.
Energy modellers should not be held responsible for actual performance that differs from simulation results if their reasonable assumptions on construction, operations and occupant behaviour are not subsequently carried through - and modellers are not typically in a position to control these.
Of course, simulation assumptions *should* be reasonable, and modellers should be responsible for good research, documentation - and raising flags for unusual situations. However,this is not the same as being responsible for performance that doesn't match a compliance simulation.
Mike Sealander is right, this is a standard of care issue - and it cuts both ways. Designers should use the best available tools and information to make good decisions, and often this includes energy simulation.
Glad to see I touched a nerve (or a least tickled a hair or two on the back of a neck).
Mr. Gibbons offered the idea of it being difficult to find a party other than the design professional responsible for suboptimum energy performance but operations and maintenance are crucial (right Mary?) and much harder to pin down. A real challenge is that energy performance - or more broadly sustainability - is only measureable over time and a lot can happen over time. Most contracts don't address the idea of performance over time with "multi-player" inputs.
Energy modeling was used (late 70s thru 90s) as a tool to create relative comparisons between options not to definitively predict the absolute energy consumption of a building. Remember, a lot can happen on the way to your annual energy bill.
Energy modeling is valuable and necessary as a design tool and made all the more valuable and mandated as necessary by the adoption of standards that demand such, so it should be done. And it is a 'standard of care' issue.
The idea of [design professional] responsibility for performance being off the table "...if their reasonable assumptions on construction, operations and occupant behaviour are not subsequently carried through..." creates a foggy playing field. What is a reasonable assumption about operations and occupant behaviour? What is unreasonable?
Measuring performance is a tough matter and I don't think we've nailed down just yet how all of that will play out.
Glad to see I touched a nerve (or a least tickled a hair or two on the back of a neck).
Mr. Gibbons offered the idea of it being difficult to find a party other than the design professional responsible for suboptimum energy performance but operations and maintenance are crucial (right Mary?) and much harder to pin down. A real challenge is that energy performance - or more broadly sustainability - is only measureable over time and a lot can happen over time. Most contracts don't address the idea of performance over time with "multi-player" inputs.
Energy modeling was used (late 70s thru 90s) as a tool to create relative comparisons between options not to definitively predict the absolute energy consumption of a building. Remember, a lot can happen on the way to your annual energy bill.
Energy modeling is valuable and necessary as a design tool and made all the more valuable and mandated as necessary by the adoption of standards that demand such, so it should be done. And it is a 'standard of care' issue.
The idea of [design professional] responsibility for performance being off the table "...if their reasonable assumptions on construction, operations and occupant behaviour are not subsequently carried through..." creates a foggy playing field. What is a reasonable assumption about operations and occupant behaviour? What is unreasonable?
Measuring performance is a tough matter and I don't think we've nailed down just yet how all of that will play out.
The requirements in LEED v3 re: energy performance are for reporting the data. There is no penalty for not meeting expectations. You must simply agree to provide the information. Energy Modeling for LEED is as per Appendix G of ASHRAE 90.1-2007. It states in that standard that Energy Modeling is not to be used as a predictor of future energy use. Too many people who don't know about LEED are fostering misconceptions such as the one where LEED reuires you to meet the energy use predictions. It is just not true.