By Christian Bergland, Building Energy Exchange
7/23/19 Update: In early July 2019, thanks in part to New York City legislators, New York State legislators tripled the property tax abatement for green roof systems from $5.23 per square foot of green space to $15, while also extending the abatement to 2024. The abatement applies to areas subject to sewer overflow during rainstorms and those lacking in green space.
Most components of the Climate Mobilization Act (CMA) take effect years in the future, giving building owners time to prepare. The sustainable roofs component of the legislation enshrined in Local Laws 92 and 94, however, will take effect as soon as November of this year. With the implementation horizon so near, it is crucial for building owners to understand what the legislation entails, why it is being implemented, and how installing solar photovoltaic (PV) panels and green (vegetated) roofs can benefit their portfolios.
What’s in the legislation?
Local Laws 92 and 94 mandate that any roof undergoing major construction be covered in either solar panels or a green roof system. Covered construction projects include new construction, vertical and horizontal extensions, and major modifications to the roof requiring a permit. Simple roof repairs and membrane replacements do not trigger the sustainable roof requirement. If triggered, the legislation requires that all available roof area be covered in either solar panels or a green roof system, excepting space required by New York City Fire Code and utilized by mechanical equipment.
Unlike other components of the CMA, the sustainable roofs legislation applies to all buildings in New York City, regardless of size. Within that, exemptions are available for buildings that cannot accommodate a minimum system size – 4kW – of solar or 200 square feet of green roof, and in the future the City may develop additional feasibility criteria beyond those included in the legislation. While these exemptions will not apply to most large commercial buildings, they represent a compliance pathway for smaller single-family homes with roofs too small for solar systems and where green roofs may not be possible, due to shading, City Code, or other factors.
Why is the legislation being implemented?
Solar panels have a significant role to play in achieving the central goal of the Climate Mobilization Act: moving New York City away from a carbon-focused energy regime towards one based on renewable energy sources. New York City’s rooftops represent a largely untapped renewable energy resource, providing New Yorkers with locally produced clean energy. Along with flexible ownership models, building owners and developers are able to reap the benefits of solar energy, often at little to no up-front cost.
While planted green roof systems do not produce power, they offer a suite of benefits to buildings, neighborhoods, and the city at large. The most immediately noticeable impact of green roof systems is their role in cooling and insulation (more on the latter below). Due to the urban heat island effect, dense urban agglomerations like New York City are often significantly warmer than surrounding areas, as much as 5°F on average. This heating effect is exacerbated in the evening, when temperature differentials between urban areas and their surroundings can top 20°F. The maps below do a good job of illustrating the relationship between density, green space (or lack thereof), and urban temperatures:
Thermal infrared satellite data measured by NASA on one of the hottest days in New York City’s summer, NASA Earth Observatory, August 14, 2002.
According to Columbia University research, as much as two-thirds of New York City’s localized warming over the last century can be attributed to the heat island effect. With warmer temperatures comes increased air conditioner usage, resulting in a feedback loop of increased cooling loads and enhanced heat island effect as air conditioners emit heat.
Conventional roofs – particularly asphalt roofs – absorb heat throughout the day, releasing it at night, representing heat sinks spread throughout the city. Thanks to evapotranspiration – the process of plants transpiring, or releasing water – green roof systems can result in roofs that are as much as 30–40°F cooler than conventional rooftops. When manifested across the city, green roofs can impact temperatures at the neighborhood and city-level, significantly mitigating the urban heat island effect by removing heat sinks.
Resulting benefits are manifold. Heat island mitigation allows for cooler streets, making for more pleasant outdoor spaces, something any New Yorker who has sought refuge outside of the city on a July weekend can appreciate. Indoor spaces are also impacted, as cooler temperatures result in more limited cooling loads, thus limiting emissions from air conditioners and heat resulting from their use.
Beyond heat island mitigation, green roof systems also assist in stormwater management. Impervious surfaces like asphalt rooftops and pavement predominate in urban areas like New York City, making up as much as 40% of ground surface and preventing water from penetrating the ground when it rains, resulting in flooding. One of the more prominent examples of urban flooding exacerbated by impervious surfaces was the disaster that ensued when Houston was hit by Hurricane Harvey in 2017, a situation that The Atlantic summarized succinctly in the headline, “Houston’s Flood is a Design Problem.”
In New York City, where stormwater runoff and sewage share a network of 150-year-old pipes, heavy rains result in untreated effluent flowing directly into our waterways, with devastating impacts on local water quality and aquatic life. Heavy rains also cause localized flooding in neighborhoods throughout the city, resulting in unpleasant conditions for local residents.
Unlike impervious conventional roof surfaces, green roof systems absorb rainwater. In practice, this allows for significant flood mitigation, resulting in health and quality of life benefits at both the neighborhood and city level. At scale, green roofs have the potential to meaningfully impact how water flows through New York City, slowing it down on its way to the sea, with resultant benefits for local residents and wildlife.
In addition to benefiting aquatic life by mitigating water pollution, green roofs also represent a form of habitat creation for insects and birds. Degree of impact varies depending on roof type. Extensive green roof systems, which consist of relatively shallow soil and ground cover, typically benefit a limited range of insects. Alternatively, intensive green roof systems, consisting of deeper soil and a diverse range of plant life, including bushes and trees, can support additional invertebrate and avian biodiversity.
How can solar panels and green roofs benefit owners?
At this point, solar PV systems’ ability to allow building owners to monetize their roofs and receive low-to-no-cost energy should be well understood. Less well understood are how solar can help building owners comply with other components of the Climate Mobilization Act, as well as the ways in which installing solar panels can benefit their buildings.
Local Law 97 imposes emissions limits on New York City’s buildings, with associated penalties for buildings exceeding those limits. Emissions calculations are based on energy source, with associated coefficients for oil, gas, grid energy, etc. Solar panels produce clean, renewable energy without any associated emissions, providing building owners with a source of energy that does not count towards their carbon allotment. On-site production of solar energy can result in a capacity credit for the host building, the details of which will be determined through the rulemaking process. Beyond their direct energy production benefits, solar panels can help limit building energy use by providing rooftop shade, limiting cooling load and resulting in minimized space cooling.
As noted above, green roof systems also result in cooler rooftops, similarly limiting cooling loads. This positive benefit is compounded by green roofs’ thermal and sonic insulation effects. Outside of the direct impact of green roofs on rooftop temperatures in the summer, they also provide thermal insulation in the winter, minimizing thermal transfer through the roof. The thicker soil substrates of intensive green roofs enhances this benefit. Both extensive and intensive green roof systems, meanwhile, offer sonic insulation benefits, with plants blocking high frequency sound and the soil substrate blocking low frequency sound. In concert, the above benefits result in occupant spaces that are more pleasant, comfortable, and affordable to operate than those of conventionally roofed buildings.
There are also long-term roof maintenance benefits to installing green roof systems. By eliminating the extreme temperature fluctuations typical of asphalt roofs, green roof systems also minimize thermal expansion and contraction stress. According to Columbia University researchers, minimizing these strains can result in roof systems that last twice as long as conventional asphalt rooftops, allowing building owners to minimize expensive, disruptive rooftop work.
What should you do to comply?
Every building is unique, and Local Laws 92 and 94 provide flexibility for building owners to determine which sustainable roof system makes the most sense for their building. Due to space constraints, zoning restrictions, and shade resulting from adjacent buildings, solar PV systems will make more sense for some buildings, while green roof systems will make more sense for others.
While there are advantages for building owners and occupants to simply installing solar or green roof systems and achieving compliance, more advanced, creative compliance pathways can offer additional advantages. Biosolar roofs, for example, seamlessly integrate green roof systems with solar, planting an entire rooftop and installing solar over the planted sections. Biosolar systems offer building owners the benefits of both solar and green roofs, providing renewable energy along with the insulating and cooling effects of green roofs. In fact, biosolar systems can even result in improved solar system performance: solar panels operate best at moderate temperatures, and green roof systems keep rooftops cool, allowing for optimal system performance.
Integrated battery storage can also improve solar system efficacy. Buildings do not always produce energy when they – and the grid – most need it. Residential buildings, for example, might produce energy in the afternoon, when residents are at work and there are relatively few building occupants. Batteries get around this issue, storing energy for when buildings need it most, or releasing it to the grid at times of peak demand. For a more detailed exploration of solar and battery integration, check out the Building Energy Exchange’s Solar Photovoltaics & Batteries Tech Primer.
Local Laws 92 and 94 represent groundbreaking legislation promising benefits to both building owners and everyday New Yorkers. Building owners benefit in the form of buildings with lower cooling loads, enhanced energy efficiency, and improved tenant spaces. New Yorkers at large, meanwhile, get to experience those enhanced tenant spaces, while also enjoying a city that is both cooler and less prone to flooding.
The Building Energy Exchange is here to provide reports, case studies, and educational events to help you identify optimal compliance pathways for your building in this evolving legislative landscape. For more information, consider reviewing our previous blog posts on the CMA:
- The Climate Mobilization Act
- The Climate Mobilization Act, Building Electrification, and the Grid of the Future
Still have questions? Visit our Climate Mobilization Act Series website, a series we’ve developed to demystify the components of the Climate Mobilization Act and connect our community with relevant solutions. If you’re still in need of guidance, Let us know!