` Information & Insight, Year 27, No. 1, March 2022 | WDP & Associates

Information & Insight, Year 27, No. 1, March 2022

LL97: NYC’s Landmark Climate Legislation from the Perspective of a Building Scientist

by John M. Civitillo Jr., PE, CPHC®, LEED AP BD+C, Senior Engineer

Figure 1. New York City Midtown Skyline, 2020


As part of the Climate Mobilization Act, Local Law 97 was passed into law by NYC legislation in April 20191 and is being heralded as one of the most ambitious climate change legislations in the world.  Most energy codes and sustainable rating systems focus on energy conservation in new construction, but it is estimated that two-thirds of the building area in existence today will remain in 20502.  Currently, this regulation is the only legislation that sets quantifiable limitations on the carbon emissions of an existing building stock, and its impact on the City is expected to be broad-reaching and significant.  

Figure 2. New York City Carbon Emissions (millions of metric tons of CO2e) (Source - NYC Greenhouse Gas Inventory, 2016)

The purpose of the new legislation is to progressively reduce energy consumption of buildings that account for roughly 60% of the City’s entire energy consumption3 (Figure 2) ultimately resulting in an 80% reduction in carbon emissions in 2050 relative to a 2005 benchmark level.  Buildings that do not comply with the requirements will be subject to a steep fine. Building owners will be forced to take inventory of their existing energy usage to understand the physical and behavioral changes required to meet the requirements of the law.  
There are inherent complications in making sweeping energy performance improvements within the constraints of an existing building, and it is important that a building professional be engaged to assist in developing deep energy retrofits to mitigate the risk of introducing long-term detriments to the integrity of the building.  The involvement of a building enclosure consultant with expertise in building science and the hygrothermal behavior of exterior wall assemblies is critical for energy performance upgrades of an existing building enclosure. The most diligent building owners have already begun planning and consulting with experts to minimize waste and maximize efficiency.


Figure 3. Building Energy Efficiency Ratings Currently Issued under Local Law 33 give a Basic Understanding of a Building's Current Energy Performance

Who Do These Limitations Apply To?

Every existing building within the City limits that is 25,000 SF or greater will be required to demonstrate compliance, including future construction that will be substantially completed at the time a new limitation is instituted. The Urban Green Council estimates that as much as 60% of NYC’s building area will be subject to these requirements4. However, some of the buildings meeting the floor area criteria will be permitted to claim exemptions and follow alternate compliance methods, such as government buildings, religious buildings, and buildings with rent-controlled units. Buildings that will be required to comply with this legislation are already assigned an energy efficiency grade under Local Law 33 (Figure 3).

Where Does Carbon Come from and How is My Building Emitting It?

Carbon dioxide is the byproduct of burning fossil fuels to create energy, which is then used to power various systems and loads in a building, such as lights, heating and cooling systems, appliances, and plug load to name a few.  In some cases, fuels, such as natural gas, are delivered to the building and consumed on-site to power building systems or produce heat. Most of the fuel associated with buildings is consumed off-site at a power plant where the energy from the combustion process is eventually delivered through a grid as electricity or steam.  Both site and source fossil fuel consumption are considered in this equation. While building HVAC systems physically consume energy to provide comfort, the efficiency is directly impacted by the effectiveness of the building enclosure (Figure 4).

Figure 4. Infrared Imaging (left) of Building Exterior (right) Reveals Energy Loss Through Air Leakage

How Does the Local Law Limit Emissions?

The legislation is primarily concerned with limiting operational energy usage, which is the energy required to facilitate the ongoing habitation of buildings and the needs of its occupants (heating and cooling, appliances, miscellaneous loads) and can be quantified by utilities and fuel consumption over the course of a calendar year and normalized for carbon equivalency.  Greenhouse Gas (GHG) emissions are computed on a per-square foot basis of floor area by converting various energy sources and fuel supplies of a building to the equivalent metric tons of carbon dioxide (tCO2e) output.  The conversion factors provided by the bill depend on occupancy category as defined by Building Code (Residential, Mercantile, Institutional, etc.). 

Some fuel sources are ‘cleaner’ than others in that they emit less carbon per unit of energy output, but these metrics can vary depending on several factors and can change depending on your geographic region.  For example, electrical energy is one of the most adaptable consumable energy metrics as it can be generated from any number of fuels at the power plant. As areas like the Northeast have migrated towards the use of more renewable energy, such as solar, wind, and hydroelectric power, electrical energy in this region is significantly cleaner than in the Midwest where electric is still primarily produced from burning fossil fuels.  Accordingly, the City of New York has developed conversion factors for various energy sources that they feel are representative of the region and consider things like energy source and delivery distance, and may be adjusted for subsequent reporting periods at the recommendation of an appointed advisory board.

When Do the Limitations Go into Effect?

The initial energy limitation benchmark goes into effect in January 2024 and is intended to target the worst-performing 20% of buildings in the City4.  Compliance reporting for the calendar year 2024 must be completed by May 2025, and annual reporting will be required each subsequent May.  The next benchmark will go into effect in 2030, and it is anticipated that 75% of buildings covered by the legislation will not meet those limitations without instituting changes4.  Two more future benchmarks are anticipated to be implemented to reach the 2050 goals; however, these limitations have not yet been set.

What are the Paths to Compliance?

Building owners can take the following steps to ensure compliance:

  1. Implement changes as required to reduce building energy usage within Local Law 97 limitations.  Changes may include building system modifications and upgrades, policy and administrative changes to target occupant behavior, or deep energy retrofits.
  2. Carbon credits can be purchased to offset the actual measured energy consumption of a building.  However, these credits are only allowed to be purchased from a specific source, and a finite number of credits exist to be purchased. 
  3. Submit for an adjustment to the limitation requirements based on restricted ability to retrofit.  Examples include historic preservation, space limitations, and financial hardship.

Buildings that remain non-compliant will pay a fine of $268 per each tCO2e in excess of the respective building’s allowable limit. To put this into perspective, a 50,000 square foot office building that is 10% over the 2024 limitation would be subject to a fine of over $11,000 for each year that it does not reduce its energy consumption.  If the same building did not implement any reductions in energy use by 2030, the annual fee would increase to $64,000. 

The Benefits of Developing a Plan Early

As the first benchmark is set to address the worst performing buildings in the City, high-performing buildings and even average buildings will not need to implement changes for 2024.  For buildings that are not projected to comply, the necessary changes may be minor.  However, the first step is to understand how your building currently performs to determine how soon you will be affected by Local Law 97.  A building owner should look to their current utility usage or review their Local Law 84 reporting which is public information. A straightforward calculation will indicate if a building’s existing energy performance would meet the limitations of 2024 or 2030.  

Despite the relatively small number of buildings that will be impacted in 2024, all building owners should start planning for the multi-year process of identifying and implementing energy savings upgrades.  The sooner this process is started, the more the costs of energy improvements can be distributed over time and coordinated with regularly scheduled maintenance and upgrades.  This will effectively save money and limit the need for rework.  All building systems and components have a finite anticipated life span, and components that have reached the end of their useful life should be repaired or replaced with consideration of the opportunity for energy enhancements.  Having a plan in place can make upgrading a component easy, even during an emergency repair where an owner might otherwise be tempted to replace in-kind to get things back online quickly.

Figure 5. Thermal Modeling Simulation Programs can be Used to Identify and Quantify Thermal Bridges and Assess the Risk of Interior Condensation

From the Perspective of a Building Scientist

The planning process should be both long term and wholistic.  Long term means that changes and upgrades in individual components and systems should be made with consideration given to future energy performance goals and not simply meeting the next benchmark.  A wholistic approach means that building systems that consume energy should be considered as interrelated and not as isolated components of the equation.  This is especially important for a building’s HVAC systems and the building enclosure, which are inseparable. Coordination between the HVAC engineer and the building enclosure consultant is essential to limit rework and maximize efficiency.  For HVAC upgrades to be appropriately sized, they must consider the enclosure performance and vice-versa.  As smaller mechanical systems are typically more efficient, forward-looking building owners may find it beneficial to frontload building enclosure retrofits to be able to size new mechanical equipment once to meet the ultimate required energy performance metrics. 

Building enclosure retrofitting is a complicated undertaking and on the higher end of the cost spectrum of changes to be made, thus many owners will elect to postpone this work until there is no alternative.  It will still be important for these owners to be able to quantify the performance of the existing enclosure.  Unlike mechanical systems that typically offer plaques with model numbers and relevant performance data, often little information is known about the materials and properties of products that make up the building enclosure.  Without field measured performance data, energy modelers are forced to make conservative assumptions about the performance of the enclosure.  However, unlike energy modeling required by LEED or modern energy codes, model results can no longer be relative as compared to a ‘baseline’ building.  Instead, models must attempt to represent the real-life, quantifiable energy usage of a building since there are real consequences associated with exceeding the limitations set by LL97.  Building owners may choose to engage an enclosure consultant to determine the in-situ performance of the various existing enclosure assemblies to assist in developing a calibrated energy model that will produce more accurate results. This calibrated energy model also serves to identify energy conservation measures that would be most beneficial to the building. 

Figure 6. Risks of Improper Enclosure Retrofit Strategies and Execution Include Concealed Structural Deterioration (left) and Mold Growth (right)

Whether enclosure retrofitting is pursued early or postponed until absolutely necessary, most buildings will eventually find a need to seek out building enclosure improvements, and some may require a complete overhaul, such as a deep energy retrofit project.  Either way, modifying the building enclosure within the constraints of an existing building is a very complex undertaking; and building owners should engage qualified enclosure consultants with experience in building science analysis (Figure 7) and design of enclosure retrofits.  Most enclosure retrofits involve dozens of unique conditions, primarily at interfaces between cladding systems and: other cladding systems, the structure, and adjacent structures.  These specific conditions must be carefully detailed since consequences with the enclosure can be severe.  For this reason, the building enclosure consultant should be experienced in considering the building science and hygrothermal impacts of enclosure retrofit work.  The addition of insulation changes the behavior of a wall assembly and the way in which it separates the interior and exterior climate. If not designed and executed properly, modifications to increase insulation and air tightness can lead to accelerated deterioration of the exterior walls, the structure, or problems with mold (Figures 5 and 6).  Worse still, these potential problems often occur within the wall assembly where they are hidden from view until they have advanced beyond repair.  

Figure 7. Steady-State Hygrothermal Analysis Confirms Importance of Interior Air Tightness with Interior Insulation, Given Change in Dewpoint Location at Inner Masonry Wythes

Even though it may reduce useable floor area, in most cases, building owners will prefer to add insulation to the interior to preserve the architectural features and overall aesthetic of the building’s exterior.  However, the addition of interior insulation is the most dangerous retrofit in NYC’s climate zone in terms of long-term durability and requires careful planning and meticulous execution of air and vapor control layers. This is particularly true for older buildings that do not incorporate any dedicated water control layers or membranes but instead rely on a mass of masonry to store moisture and allow it dry out slowly over time.


The carbon limitations set by Local Law 97 are unprecedented, far-reaching, and anticipated to impact most existing buildings and future construction in New York City.  The most responsible building owners have begun the process of planning for changes, upgrades, and retrofits.  The demand for professionals and contractors to implement the necessary energy efficiency upgrade work is anticipated to be a very high, and there is likely to be a shortage in capable firms to handle the workload.  The sooner a plan is put in place the less likely that a building will be subject to potential surge pricing or miss a deadline due to lack of availability. 

Planning is the key to implementing efficient and cost-effective upgrades to meet the progressive requirements of the local law.  Building owners should engage design professionals in the planning process with other stakeholders to guide the plan towards the most streamlined and cost-effective solution by correctly prioritizing upgrades and retrofits and limiting rework.  Owners must recognize that HVAC systems and the enclosure must be examined simultaneously and that design consultants with experience in building enclosure retrofits, specifically considering building science and the hygrothermal behavior of exterior wall assemblies, can help to prevent the risk of serious future problems.


  1. Local Laws of the City of New York for the Year 2019 No. 97, New York City Council, April 18, 2019
  2. Architecture 2030, obtained from https://architecture2030.org/buildings_problem_why/
  3. Inventory of New York City Greenhouse Gas Emissions in 2016, Issued by The City of New York, December 2017
  4. NYC Building Emissions Law Summary - Local Law 97, Urban Green Council, 2020

Please note WDP's publication is not intended to provide specific engineering advice or legal advice.  Every project has unique characteristics that should be evaluated in context of the work to be performed and, where prudent, should include review and input by legal counsel.

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