Figure 1: Image of the building from the west side. Rendering by: D-Box Inc.

Designing a Green Apartment Building in New York City:

A Case Study

Rafael Pelli, Partner

Cesar Pelli & Associates, LLP, 322 8th Avenue, 18

th Floor, New York, NY 10001, 212-417-

9496, 212-414-9497 (Fax), rpelli@cesar-pelli.com

1. INTRODUCTION

The design of Twenty River Terrace is presented here as a case study of the first green

residential high-rise building in New York. The project is a public-private partnership

growing out a state government initiative to advance the environmental standards for

buildings. This project has been an exciting, often frustrating, and always illuminating

opportunity to explore our deep interest in how buildings are conceived, and how changes in

building practice lead to new expressions in building design. While Twenty River Terrace has

not led to a complete re-conception of what a green high-rise residential building should be, it

has offered us a glimpse of the possibilities and the challenges that we can look forward to as

a design firm, and more broadly, as a profession.

2. CASE STUDY

2.1 Project Description

Twenty River Terrace is a twenty-seven-story glass and brick residential tower that is

currently under construction in Battery Park City, a planned residential and commercial

neighborhood bordering the west side of New York City’s financial district, directly adjacent

to the site of the former World Trade Center. Although the building was developed privately

the lease terms stipulated that this be the first building to be designed under new

environmental guidelines instituted by the Battery Park City Authority, the government

agency that has overseen Battery Park City’s development. Our challenge was to meet these

new guidelines in addition to the Authority’s pre-existing design guidelines, which govern

everything from the building materials and window proportions to the building’s orientation

and height setbacks.

2.2 Overview

Our overall goal at Twenty River Terrace was to reduce the building’s environmental impact

during construction and over its lifetime use, and increase its energy efficiency in comparison

to a traditionally constructed New York City high-rise residential building of similar size.

More specifically, we faced a complex list of sometimes conflicting criteria as we set out to

meet three separate goals: meeting the Battery Park City Authority environmental design

guidelines (a requirement), qualifying for the recently enacted New York State Green

Building Tax Credit (a financial incentive pilot program), and qualifying for a LEED Gold

rating (a voluntary national green building rating system established by the US Green

Building Council.) Considering that there were no other comparable projects to refer to, there

was a large research component to the design team’s efforts.

In meeting the environmental and energy efficiency goals, we incorporated a broad range of

strategies. Natural gas absorption chillers increase energy efficiency and reduce peak

electrical loads, and the design re-captures waste heat to provide hot water to the apartments.

Building-integrated photovoltaic cells provide five per cent of the building’s peak electrical

load. A blackwater recycling plant provides treated water for use in the toilets and cooling

tower as well as roof and park irrigation. The roofs are extensively planted using a continuous

membrane. All interior materials were selected to reduce or eliminate off-gassing, and to

maximize recycled content. Accommodations were also made for future adaptations,

including setting aside a dedicated room for a fuel cell near the electrical room. While many

of the design solutions taught us a great deal, I will focus on three specific examples: the

exterior wall, the photovoltaic cells, and materials sourcing. These three elements perhaps

best illustrate the range of issues we faced in designing this project. Some of the issues we

encountered along the way were specific to building in Battery Park City, but many are

relevant to all of us who work within the field of environmental design.

3. EXAMPLES

3.1 Exterior wall

What was most interesting for us as we developed a more thermally efficient design for the

exterior wall was the way it forced us to adapt the design process itself. The design goals

required a much larger participation by many of the technical disciplines in the early

conceptual stages of design than is typical. Alternative wall designs were analyzed not only

by the architects, but by the energy consultants for energy performance and condensation

issues, by the contractors for schedule and constructability implications, by the mechanical

engineers for systems compatibility, and by the owner for cost and payback.

Our goal with the exterior wall was to optimize its performance. We were required to be

thirty-five percent more efficient than New York State energy code. Given the pre-existing

design guidelines, as well as scheduling and budgetary considerations, we had to use a

conventional brick and concrete block wall. The challenge then was to optimize the

combined thermal performance of the masonry wall and the windows, while avoiding

condensation problems arising from the humidification of the indoor air.

Before we could begin subjecting our ideas to computer modeling, the developer

commissioned a mockup of a typical New York City masonry residential wall and tested it to

establish a baseline, especially for air infiltration. Once we had that baseline, we then began

computer simulations of many of our ideas, including different layering methods, sealant

lines, window frames, and glass to come up with the most effective and cost efficient

solutions. For example, we tested various windows and glass and found that the type of glass

was not as critical a component in the overall performance as we originally imagined. The

type of window and the frame mattered more.

Casement windows were significantly more effective than typical sliding windows in

reducing air infiltration (a major factor on a riverfront setting), while cutting solar heat gain

was not as clear a benefit as in a commercial building because in a residential building heat

gain is advantageous in winter. The insulating value of the glass was an important

consideration, but the window frames quickly emerged as greater conduits for heat loss than

any change in glass could compensate for. Many of the solutions in the final design are

relatively simple and low-cost methods of increasing efficiency that can easily be

incorporated into the way new conventional residential

high-rise masonry walls are constructed in New York City.

3.2 Photovoltaic Cells

The installation of photovoltaic cells at Twenty River Terrace served a dual purpose. These

cells, which are laminated into a piece of glass, were required to provide at peak efficiency

five percent of the base building electricity. We also saw the photovoltaics as the clearest

public expression that there were energy and environmental goals incorporated into the

building. This strategy also interested the Authority who was trying to establish precedence

for the design and building professions, to familiarize people with this new technology, and to

encourage its use in new construction.

The interesting challenge for us as design architects was how best to integrate this relatively

new building material into the composition of the exterior. We committed ourselves early on

to using the photovoltaics as an important element symbolically in the façade, but the final

design evolved considerably as were learned of their technical and cost limitations.

Because the building exposure to the south was small and often in shadow, we wanted to

locate all the photovoltaic cells on the building’s west façade where we had uninterrupted

exposure and extra reflection off the water. First, we proposed deploying them in horizontal

bands across the length of the western side. But we subsequently learned that such a design

would increase the cost of the wiring, and reduce the overall efficiency of the photovoltaics.

We finally chose to place the photovoltaics in two primary clusters. One cluster is vertically

organized on the west façade above the entrance. The second cluster is installed on the

mechanical bulkhead at the top of the building. In both cases, the photovoltaics are treated as

a special, but integral, material in the building fabric, rather than an additive component. They

serve to highlight the entry and express the technology of the overall building systems. Initial

Figure 2: Diagram showing location of photovoltaic cells with two initial alternatives

estimates suggested the photovoltaic panels would be more economical to install on the

mechanical bulkhead walls, but the final bidding resulted in the panels within the main façade

being less expensive.

Once we solved the design challenges, we discovered that it was almost impossible to bid out

this system as a standardized product. Each of the suppliers has panels that look different and

have different electrical outputs. We had no choice but to adapt our design to what we were

able to buy. Although our initial choice was to use a charcoal colored cell, we ended up with a

more efficient blue mono-crystalline cell array.

Figure 3: Regional materials used in exterior wall construction

3.3 Sourcing

Many of the decisions in this larger field of sustainable design involve the weighing of

different priorities. This requires a level of subjective preference because it is difficult to

come up with an objective measure of why one solution is better than another.

Materials sourcing was a clear example of the difficulty of coming up with an ideal solution.

We were required to purchase brick from a manufacturer within five hundred miles of the

project. In this case, the intention of sourcing materials within a radius of five hundred miles

clearly had to do with minimizing the need for transportation. Not included in the assessment

of this requirement was the relative costs of different modes of transportation and the relative

environmental impact of producing the bricks at different manufacturing plants.

We found two companies that manufactured bricks that met both the Battery Park City

Authority design guidelines and our aesthetic preferences. One company met the distance

requirement, but used an older, less energy-efficient kiln for baking the bricks and had no

access to rail transport. The other company was well beyond the five hundred mile distance

requirement, but the plant conforms to the highest international environmental standards, and

they can ship by train, which is more energy efficient than shipping by truck.

So the question becomes: Is local sourcing more energy efficient and better for the

environment? In this case we met the sourcing guidelines, but lacked the full data to measure

overall environmental impact.

Figure 4: Terrace garden on the 17th floor. Design & Rendering: Balmori Associates

4. CONCLUSION

Each of the three elements of the design of Twenty River Terrace that I have chosen to

highlight offers a glimpse of both the opportunities and challenges we face as architects in

designing sustainable architecture, particularly at a time when this industry is still in its

infancy and undergoing rapid change. The policy initiatives behind this project, and the

interest and effort demonstrated by the private building developer, suggest the possibility for a

rapid evolution of these environmental design strategies into more broadly applicable design

standards. For this to take place, though, there will need to be access to better data as a basis

for comparison, further development in some of the technologies themselves and an

adaptation by the building trades to ease the cost of implementing these solutions. Finally, the

creative challenge remains to make architecture more than the solving of an accumulation of

technical challenges. The goal of architecture must be to enrich the human experience, and the

experiential qualities of architecture rely on not only the preservation of our environment, but

on our participation in it.