Alley 24

Case Study

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Project Overview

  • Owner: Vulcan Real Estate; PEMCO Mutual Insurance Co.
  • Location: Seattle, WA
  • Building Type: Mixed Use-Office, Retail
  • Size: six-story building, 180,000 sf office, 23,000 sf retail
  • Completion Date: February 2006
  • Utilities: Seattle City Light (Electricity); Puget Sound Energy (Natural Gas)

Located in Seattle, Washington’s burgeoning South Lake Union neighborhood, Alley24 is one of the nation’s first LEED-certified mixed-use projects, offering a new model for sustainable, urban development. The urban infill project is sited on a full city-block within a 60-acre urban redevelopment effort, one of the largest in the US and anchored by three historic brick laundry buildings. The two-acre development integrates 180,000 SF of office space (six floors), 28,000 SF of retail shops and 172 residential apartments within the context of a highly sustainable, pedestrian-oriented development.

The commercial portion of the project, containing five stories of office over ground floor retail, is known as Alley24 East. Roughly three floors of the office space are occupied by NBBJ, the project architect. (Additional space is taken by Skanska USA the office building general contractor and a PR/Advertising agency.) As anchor tenant, NBBJ was faced with a challenge—balance their interest in innovation and experimentation in the performance of their own space, with the need for a resource-efficient building that would also meet the requirements of a multi-tenant, speculative office space. By all accounts, they met these goals by delivering a high performance building that integrates daylighting (88 percent of the building is daylit); a hybrid ventilation system with the ability to use building mass for night-time cooling (90 percent of the building can be ventilated or cooled with operable windows); and an underfloor air distribution system. Building tenants benefit from exceptional local control of lighting, operable windows and air conditioning. It is one of the first mixed-use developments to report the measurable occupant benefits of a sustainable building, with one tenant reporting a 30 percent decrease in sick days and another reporting a 10.3 percent increase in productivity per person.

As reported in an April 2008 Building Performance Review developed by the New Buildings Institute, the integrated strategies have resulted in a building Energy Use Index of 53 kBTU/sf-yr, a 43 percent reduction in energy use, as calculated by the ENERGY STAR Target Finder tool.

Building Energy Use Index (EUI)









Alley24 East has earned LEED Core and Shell Silver certification and both NBBJ and Skanska offices have been Certified Gold under the LEED Commercial Interiors rating system. In addition, Alley24 anchors the neighborhood’s pilot status within the LEED Neighborhood Development program. The National Association of Industrial & Office Properties (NAIOP) recognized Alley24 as both the 2006 Sustainable Development of the Year and the 2006 Mixed-Use Development of the Year. In 2009, Alley24 and NBBJ’s offices were honored with CoreNet’s Sustainable Leadership Awards for both interiors and architecture. As this article is being written, NBBJ’s office is currently under consideration for CoreNet’s top honor, the Global Innovation Award.

Design Process

Large, collaborative goal setting charrettes, with participation from the owners, architects and project consultants, focused on urban design and sustainable strategies. With this foundation the project team was able to set sustainability goals early in schematic design. Collaboration was maintained through regular meetings allowing flexibility for the project team to determine the right solutions for the specific market type and client. For example, the design team originally wanted 100 percent natural ventilation, but prospective tenant concern regarding comfort levels would not support that approach. As a solution, the design team created an innovative hybrid HVAC system that meets both tenant concern for comfort and the client’s desire for high levels of energy efficiency.

Extensive 3-D computer modeling supported the high level of team collaboration, with the owner and mechanical, electrical and lighting engineers working together to assess performance of the building through daylighting studies, HVAC modeling, shading studies and computational fluid dynamic modeling of ventilation options.

In order to develop a regionally-relevant and community-sensitive design program, two design sessions were held with area residents and business owners to generate ideas for features that would add value to the neighborhood, with follow-up meetings that included community members, the neighborhood’s community council and city planners. Building tours and field trips to Portland and Stockholm were taken to provide additional insight into urban design, sustainability, daylighting and neighborhood building. Multiple pro-forma/programming options were studied to help determine the mix of retail, office, apartments and parking. These activities were supplemented by firm-wide brainstorming/critiques (a benefit from NBBJ being the anchor tenant).


Site restrictions, combined with Seattle’s north/south street grid pattern, resulted in a building with large areas of difficult-to-shade east and west facing façade. Analysis of the impact of Seattle’s mild climate upon the building suggested benefits from operable windows and nighttime cooling of building mass. Daylighting solutions were crafted with operable windows placed high and low, with a combination of fixed and operable exterior window shades. The building has a relatively narrow floor plate which allows for deeper daylight penetration. The building’s HVAC system turns off within an occupant-defined temperature range, with the tenants assuming responsibility for opening windows to maintain comfort and effective ventilation.


A typical office operating schedule of 6:00 AM–7:00 PM is used for building lighting and HVAC. Comfort settings are typically 68°F heating and 74°F cooling.

NBBJ and their clients decided that a multi-tenant occupancy would best be served by a building that could function in multiple modes. High performance capabilities could be realized by tenants who were willing to change their behavior—adopting extended comfort ranges, encouraging greater use of the operable windows, letting lighting levels vary as daylight fluctuates. At the same time, the building would meet the needs of other tenants who might be less interested in behavior change to achieve high performance and would maintain the flexibility to readily adjust for changing tenants over the building’s life.

In NBBJ’s offices the lighting designers decided that the daylight contribution, combined with the high reflectance interior surfaces, meant that a design criteria of 25 foot candles on desk tops would be sufficient. A modest number of task lights were purchased for those who felt a need for higher lighting levels.


The design team optimized the building envelope to take advantage of Seattle’s mild climate and natural daylighting while minimizing the negative effects of direct solar heat gain. Operable window placement areas were studied to optimize the effects of natural ventilation and several shading studies were performed to determine the best solutions to mitigate solar heat gain while ensuring effective daylighting.

The window head height is at 10 feet, to enhance both the daylighting and natural ventilation features of the building. Forty percent of the window area is operable and 25 percent of the operable units are mounted low and 75 percent near the ceiling, to enhance the passive ventilation performance (the operable windows provide passive ventilation to 90 percent of the building floor area). The windows, with a combination of fixed and operable exterior shades, are placed at different heights to optimize daylighting and views. In addition, the HVAC system turns off when outside air temperatures are mild with the tenants then responsible for opening windows to maintain comfort and ventilation.

Precast concrete mass was sized to temper thermal conditioning requirements and reduce cooling loads, when the air handler economizer cycle is employed to circulate large volumes of cool night air through the under floor air distribution system, to cool the concrete floor slabs. At the time this case study was written, that operating strategy had not yet been fully implemented.


Alley24 is the first North American building to incorporate exterior sunshades and automatic reflector blinds. The automated window shading system includes fixed sunscreens at operable windows and exterior motorized four inch blinds that are retractable when not needed.

A hybrid HVAC system, which could function in a variety of modes, was considered an essential feature for a speculative, multi-tenant office building, allowing individual tenants to determine the functionality and level of performance that meets their requirements.

The lighting power density in NBBJ’s offices is an efficient (and impressive) 0.73 watts/sf, using 100 percent indirect luminaires with single high output T5 lamps. The tenant improvement flexibility available to other tenants, has led to other offices with lighting power densities of roughly 0.97 watts/sf (midway between the LPD of the old code 1.0 /sf and the new code 0.95 w/sf).

Office building common space lighting (restrooms, conference rooms, etc) is on motion detectors and all office lighting is on timers for normal office hours, requiring active after-hour control. To meet the requirements of the Seattle Energy Code, perimeter luminaires incorporate daylight dimming controls.

Design Strategies & Features

Hybrid / Mixed-Mode HVAC System
There is substantial industry interest in the design and operation of hybrid HVAC systems. When asked for more detail about how the Alley24 system works, Joe Malaspino of CB Richard Ellis (CBRE), the Alley24 property management firm, explained the operation of the system on the floors occupied by NBBJ:

“At an outside air (OSA) temperature less than 63°F the main air handlers operate in economizer mode and electric reheat is performed on the perimeter zones via fan terminal units. At OSA temperatures greater than 63°F and less than 80°F supply fans are turned off, relief fans modulate to maintain a static pressure inside the space, and tenants open windows as needed. As the OSA temperature rises above 80°F the system enters mechanical cooling, supply fans are restarted and the windows are closed by the occupants. On the fourth, fifth and sixth floors operation is essentially the same, but with slightly different temperatures for operational changeover, as determined by the occupants of those floors.
The air handlers are not equipped with internal heat; reheat is accomplished at the terminal units. The discharge air temperature setpoint is on a reset schedule referencing the OSA temperature. The dampers modulate to maintain a discharge air temperature of 64°F when the OSA is >= 58°F. As OSA temperature decreases below 58°F the discharge air temperature is reset upwards to a maximum of 69°F when OSA is <= 38°F.
During occupied modes there is always a mechanical portion of the fan system that is on, thus assisting the natural ventilation and air flow for code required minimums. Mechanical cooling is locked out below 64°F OSA temperature.”

According to Malaspino, building management has “tweaked the settings to find the limits of the occupants” and concluded that the building could be effectively cooled with “pure natural ventilation” at the settings used by NBBJ. Some areas with southern exposure experience some comfort issues during the cooling season and CBRE is determining the steps necessary to subzone that area of the building or to explore the benefits available from implementing the night ventilation of thermal mass strategy.

Free Cooling
The under floor air distribution (UFAD) system greatly extends the hours per year that the building can operate with “free cooling” (circulating cool outside air rather than using air conditioning), because the UFAD operates at a higher supply air temperature than in buildings that mix air from above. To further maximize “free cooling” hours from the use of operable windows, a variety of systems are used to alert Alley24 occupants to open and close windows—red and green indicator lights tied to the building’s HVAC system, for some, while others are alerted through an email sent to the floor’s window monitor. Following the anchor tenant’s lead, tenants who initially insisted on air conditioning, converted to the hybrid model within a few months of occupancy, resulting in the use of natural ventilation by all building tenants the majority of the time.

POE / Measurement & Verification

Early in 2008, a Building Performance Review was conducted by the New Buildings Institute, comparing 12 months of building energy use to the national building stock, using the ENERGY STAR rating system and to a set of other completed LEED buildings. The results confirmed that Alley24 East demonstrates excellent energy performance. The project achieved an ENERGY STAR Rating System score of 97, placing it in the top three percent of the national building stock. The project’s energy use also ranks it in the upper half of the energy performance of a sample of LEED buildings. Alley24 does not quite achieve the Architecture 2030 energy performance criteria of 50 percent less energy use than comparable buildings, but programs are under way to improve performance.

Efforts to improve energy performance are reinforced by Alley24’s lease structure. All leases are triple net, with tenant spaces separately metered, by the half-floor. For example, NBBJ is billed with an itemized listing of five distinct meters, corresponding to the 2½ floors that they occupy and providing an opportunity to track area by area, month by month.

The performance review also surveyed building occupants to determine their satisfaction with building characteristics and performance related to acoustics, air quality, lighting, temperature, whole building performance, productivity and health. In all categories, the building achieved high degrees of occupant satisfaction relative to data from comparable buildings used for benchmarking,

Next Steps: Identifying Tenant-by-Tenant Opportunities

There has not yet been any whole building coordination among tenants regarding energy use. According to CBRE’s Joe Malaspino, “it is something we have discussed but have not yet pushed that button. We are currently finishing up the whole building ENERGY STAR process. The next step would be to get the tenants to do an ENERGY STAR profile of their individual spaces. This would allow us to identify the areas for improvement.”

Lessons Learned

Establish a Meaningful Energy Performance Baseline
NBI’s performance review indicated that the Alley24 energy model was not a useful predictor of the building’s energy use. This finding is consistent with NBI’s decision to benchmark building energy performance against similar buildings, rather than against a modeled code-based baseline. While energy simulation models offer a useful methodology to evaluate design strategies against one another, actual building performance often diverges, significantly and with good resaons, from what has been modeled. NBI’s approach parallels the 2030 Challenge, measuring energy performance goals and actual performance against a stock of existing buildings.

Design for Flexibility and Innovation
According to NBBJ’s Margaret Montgomery, principal and sustainable design leader, “In this building, I think the biggest win was that we, as tenants, were willing to entertain ideas involving both alternate systems and ways of living.” While Alley24 was designed to offer the flexibility of a speculative office project, the design team benefited from anticipation that they and Skanska would be anchor tenants. Says Montgomery, “there were decisions made about how flexible the design intent could be; what the sliding scale is from conventional occupancy to ‘innovation-ready’ tenants and how far the building should be able to accommodate in the future.”

NBBJ has benefited from the ability to take prospective tenants on tours of Alley24, perhaps at the rate of one per month. Montgomery further explains, “The building has been a very good tool that helps to explain what we mean by mixed-mode ventilation and allows clients to experience what it’s like in an open office with daylighting and natural ventilation.”

Find a Balance Among Simple and Complex Systems
The operable windows and fan assist hybrid ventilation have led all office tenants to embrace the hybrid ventilation system and to forego mechanical cooling during much of the time (even those tenants who initially required its availability). For occupants, the inherent simplicity of operable windows, with almost immediate comfort feedback when they are opened and closed, delivers operating flexibility and direct control to tenants. These combined elements have led tenants to trust their ability to remain comfortable over an extended set of comfort conditions, without having to depend on air conditioning and sophisticated building controls.

On the other hand, complex automated systems do not always work as well as hoped. For example, Alley24’s automatic shades were designed with a single zone for each façade. In retrospect, there should have been additional zones within each orientation to take into account shading that occurs from surrounding buildings. In addition, internal shades have been added to mitigate glare from the upper windows. (The external blinds were for the vision glazing portion only, with the daylight section—the higher lights—left unshielded.)

Living in Our Own Experiment – The Importance of Post-Occupancy Evaluation
The final words are from Margaret Montgomery, “The post occupancy evaluation was an excellent start to learning how the building performs and living in our own experiment has been invaluable. Ongoing collaboration and communication with the building manager helps us to stay abreast of the issues he’s finding, and how he’s solving them. That, in turn, helps us to be smarter on the next project. The new building and its sustainability features have begun to change our workplace culture. For example, one year after moving in, our employees were integrating the opening and closing of windows into their everyday habits. We were able to experience the implications of these design solutions and saw how people could expand their comfort levels to include an extra five degrees cooler and warmer in exchange for a healthy and inspiring work environment. That kind of experience feeds back into our practice and has allowed us to more confidently make recommendations to our clients and to increase their understanding of important ramifications of design decisions they will be making about building performance.”


Architect, Lighting Design:

Structural Engineer:
Magnusson Klemencic Associates

Mechanical Systems Designer:
Flack & Kurtz

Mechanical Design Build: 

Holmes Electric

Commissioning and LEED certification services: 
Egis Real Estate Services

General Contractor: 
Skanska USA

Property Management: 
CB Richard Ellis 

Contacts & Resources

Seattle City Light

Puget Sound Energy




Integrated Design Lab | Puget Sound

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