Today, the NYC Mayor’s Office of Long-Term Planning and Sustainability released the first annual New York City Benchmarking Report. This landmark effort is required by Local Law 84, the first law enacted by the Green, Greater Buildings Plan (GGBP), and covers 12,565 private properties (municipal buildings began benchmarking a year earlier and their first report was issued in November 2011).
The report is the result of a foresighted and heroic effort. According to the report, of all the buildings benchmarked by law in the US, Local Law 84 alone accounts for about 61% of the impacted square footage! The data will eventually be available publicly (although the first year of data collection for each building type – municipal, non-residential, and residential – is not), which will be a treasure trove of information for the analysis of the built environment. Overall, the report is a fantastic 30,000-foot view of the city’s large buildings, boiling down complexity into clear, easily digestible chunks.
Buildings’ energy use ranges widely – even within similar building types
Among the more fascinating analyses in the report is a consideration of the factors that contribute to energy consumption. Multifamily building energy use intensity does not vary much, perhaps because they have more similar use patterns to each other. On the other hand, building use intensity for office buildings varies widely.
Several factors appear to correlate with office energy use intensity. For example, higher density of occupants and longer operating hours correspond to higher energy usage. Larger office buildings tend to use more energy per square foot than smaller ones, but it’s not immediately clear if this is due to the intrinsic nature of large building construction (inability to use natural ventilation, simultaneous heating and cooling, and complex systems) or other factors.
Another correlation is with construction date. The report splits building ages by twenty-year increments, and finds that energy intensity has steadily increased over each 20-year period. In fact, offices built since 1990 use almost 40% more energy per square foot than offices built before 1930!
There are a few possible reasons why this might be the case:
1. Tenant Profiles
Tenants with high computer use or other advanced technology needs might select newer buildings. If so, actual end uses would have to made more efficient to see improvements. Building infrastructure changes wouldn’t be the answer. In commercial buildings, since 70% of energy use is typically from tenant spaces, this warrants serious future study.
2. Increase in building system capacity
Newer building systems tend to have more lighting and HVAC than older ones do. A 1920s-era office building may struggle to keep up with HVAC needs on a hot day, whereas a brand new office building may have overcapacity all the time, causing AC units to run less efficiently. This can be improved in new construction through better building load projection during the design process, ensuring the building is geared to actual loads and not “rule of thumb” overestimates, as addressed by the Green Codes Task Force recommendation EE-2 Improve Analysis of Heating & Cooling Needs During Design).
3. Building system complexity
New buildings are more complex than older ones, with computer management systems controlling advanced, interrelated technologies. Sometimes these advancements save energy, but paradoxically, these complex buildings sometimes perform worse than their simpler brethren. Programs like GPRO, Building Operator Certification, Building Performance Institute Multifamily, and others address this maintenance and operations issue. In this case, we do have a hope of improving these buildings over time, as more emphasis is put on energy-efficient maintenance and operations. Benchmarking can help in this regard, since as tenants become more informed and look for the energy score of prospective spaces before leasing, owners will hopefully respond and attempt to improve their buildings’ performance.
4. Building envelope
Building envelope styles change over the decades. Newer buildings with high vision glass curtainwall construction may exhibit worse energy performance than older, masonry buildings with less window area. And though window energy performance has improved over time, we’ve given back those gains through greater increases in vision glazing area. This is one of the reasons the Green Codes Task Force is hoping to address new building envelope u-values in recommendation EF-3 Limit Heat Loss Through Exterior Walls).
More on this topic may become clear when benchmarking data is released in September. Nevertheless, the fact remains that there is a wide variability in energy use intensity, due to age and many factors, even among similar building types. This demonstrates the improvement that is possible in the City’s building stock – although as the report’s authors state, the City as a whole is not currently moving fast enough to meet the 2030 goal of 30% citywide greenhouse gas reduction.
It’s impressive how much work went into the report. The benchmarking data will be a great source for analyses that will ultimately help New York City and the nation create and implement strategies to cut energy use and greenhouse gas emissions. Congratulations are due to all who contributed to this effort.
