Conquering Carbon

Facade designs are key to meeting stringent New York City carbon reduction requirements
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Sponsored by the Ornamental Metal Institute of New York
By Barbara Horwitz-Bennett

Learning Objectives:

  1. Explain New York City Local Law 97 in large buildings and its framework of carbon reduction replacing energy efficiency as the key sustainability metric.
  2. Identify key issues and shortcomings with older building facades, and review design tools and strategies for facade and building envelope retrofits.
  3. Recognize how enhanced thermal performance and airtightness can significantly boost energy efficiencies in facades.
  4. Describe the concept of carbon trading where densely populated buildings can purchase emission credits from buildings that are below the carbon cap.


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This course can be self-reported to the AIBC, as per their CE Guidelines.
This course is approved as a Structured Course
This course can be self-reported to the AANB, as per their CE Guidelines
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Approved for Core Learning
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Course may qualify for Learning Hours with NWTAA
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In what is being described as the most ambitious climate legislation for buildings enacted by any city in the world, New York City is requiring large buildings to progressively ratchet down their carbon emissions starting in 2024. By 2030, approximately 50,000 buildings, 25,000 square feet or larger, must reduce their emissions by an aggressive 40 percent, from a 2005 baseline, or face steep fines.

Image courtesy of NYU Center for Urban Science and Progress

This map depicts large buildings in Lower Manhattan and west Brooklyn required to comply with Local Law 97 carbon emissions reductions.

“There are plenty of cities crafting impactful legislation around the built environment’s role in climate change, but no piece of legislation is as far-reaching as the emissions limits in Local Law 97,” reports Adam Roberts, director of policy, American Institute of Architects AIA New York.

While the road to 2030 compliance will be challenging and costly, the city will ultimately be looking at a savings of more than 5.3 million metric tons, the carbon equivalent of the entire City of San Francisco’s annual emissions.

“This is a very comprehensive approach. We do not know of any other city that has adopted that scale of carbon savings,” explains John Mandyck, chief executive, Urban Green Council, New York whose organization worked hand in hand with the city in drafting the new legislation.

Though Local Law 97 (LL97) from the Climate Mobilization Act impacts less than 4 percent of the city’s buildings, they represent nearly 60 percent of the city’s building area and account for more than two thirds of greenhouse gas emissions.

“We have to take aggressive action and we have to focus on the buildings that are the largest polluters,” explains Gina Bocra, AIA, LEED Fellow, Department of Building’s Chief Sustainability Officer, New York.

The legislation is also unique in that it addresses carbon reduction, as opposed to energy efficiency. Whereas architects are used to measuring their sustainable designs by energy use, they will need to broaden their perspective to account for the source of this energy.

“Energy sources have varying carbon intensities depending on grid region and fuel source,” explains Michael Woods, senior project manager, Perkins and Will, New York. “Carbon, on the other hand, is a measure that casts a wider net over the thermodynamic processes at play and also speaks to the way cities everywhere will be measuring the impact of buildings within the broader context of climate action plans.”

Erik Olsen, managing partner, Transsolar KlimaEngineering, New York, further qualifies the significance of this climate legislation by the fact that it impacts the largest city in the United States, targets existing buildings, and is outcome-based.

Putting things in perspective, the legislation completely changes the cost-benefit analysis for envelope retrofits for existing buildings. Whereas previously, it was hard to justify costly envelope retrofits based on energy savings alone, now, with fines factored in, it is much easier to make the case for investing in the building facade.

To better understand the difference between an outcome-based code and traditional codes that are linked to a simulated energy modeling comparison to a baseline building, Olsen explains that the former approach is not well correlated to actual building performance as a lot can happen between design simulation and actual operational performance. This can be caused by a number of factors, including:

  • Not accounting for all energy losses in the building such as thermal bridging
  • Inaccurate assumptions/inputs
  • Simulation tools not calibrated to real building performance
  • Value engineering
  • Poor installation
  • Building operation modeling does not take into account variables like human behavior and building operator training.

“So, with our typical simulation-based code compliance system (IECC/ASHRAE 90.1 etc.), we do not know, nor have to justify to a code official, that what we built performs how we expected it to perform,” he explains.

On the contrary, with LL97’s ongoing regulation of carbon emissions, the design team’s scope extends beyond the certificate of occupancy to building operation and training occupants.

“This means that design teams are forced to deal with issues like thermal bridging, which are not adequately addressed by conventional codes or performance evaluation standards, but can cause significant energy gaps in real performance. It also means that commissioning of building envelopes will be widespread to ensure the quality of installation is achieved,” says Olson.


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Originally published in Architectural Record
Originally published in November 2020