Raised Access Floors: The Foundation of Flexibility and Efficiency  

Many architects practicing over the past 20-30 years or so have seen the appearance of raised flooring systems that allow for flexible access to the space below. One of the first applications of this type of flooring was for large, complex, mainframe computer systems that usually required separate rooms with specific climate control and wiring requirements. Today access floor systems are becoming increasingly common for a variety of reasons.

Access floors are defined as a system of panels and supports that create a raised floor above the actual structural floor. By raising the floor up, a space is created in between the raised floor and the building structural floor where functional components like wiring for power, voice, and data can be routed and plumbing lines located. This space in between has also become increasingly valuable for heating, ventilation, and air conditioning (HVAC) distribution either as a plenum space or with defined ductwork. The United States Green Building Council (USGBC) has identified this type of HVAC system as a way to improve indoor air quality through their Leadership in Energy and Environmental Design (LEED) program.

General overview shot of office space with access floor. Photo Courtesy of Steelcase

Access Floor Construction

Access floors include two different types made up of several components with various options as follows.

Access Floor Types. There are two types of access floors: full-height and low-profile.

Full-height access floors are ideal for new construction. This type needs to be used if underfloor HVAC routing is desired. Also, this type needs to be coordinated early in the development or schematic design phase of the process to allow for drops in the floor and smooth transitions to eliminate excessive ramping.

Low-profile access floors can be used in both new buildings and renovation work, but they are optimal for renovations due to their minimal impact on the floor-to-ceiling cavity.

Access Floor Components: Access floors consist of two primary components: the floor panel itself, which comes in various types and surfaces, and the pedestal supports, which also come in various types and adjustment options. Both are described further below:

Typical Access Floor Components. Photo Courtesy of Steelcase

• Floor Panels: Panels are fairly standardized, regardless of manufacturer, and have the following attributes:

Sizes: Generally two feet by two feet for full-height access floors.

Materials: Most panels have a steel exterior with either a cement fill or wood core. A typical cement-filled panel has a top and a bottom steel pan. The bottom pan is comprised of a series of domes for structural efficiency. The panel works like a miniature waffle-slab, which creates the most efficient strength to material ratio.

Surface options: The most common finish option is to use painted panels, particularly if the floor will be carpeted. High pressure laminate (HPL) panels are also available and dissipate static, preventing damage to sensitive electronic equipment when used in computer environments.

HVAC floor diffusers: When the access floor includes an underfloor HVAC system, adjustable air diffusers can be placed in the floor panels to allow supply air from the underfloor plenum to enter the workspace. In this manner, occupants may readily control airflow volume and direction through the adjustable portions of the diffusers. More diffusers allow for more individual control within space.

• Pedestal Supports: Pedestal supports are the pieces that hold the floor panels and raise them up above the structural floor of the building. They are typically made of galvanized steel and include a threaded head to allow for specific height adjustment and floor leveling.

Standard height pedestals: Standard sizes allow for finished floor heights (FFH) between six inches and 24 inches in one-inch increments. The addition of a threaded rod allows for final adjustments in one-eighth-inch increments to ensure a level floor. Typical applications are up to 24 inches, but some applications will go even higher to accommodate additional utility routing.

Low-profile pedestals: They are typically used in applications where there are building structure floor-to-ceiling height restrictions or where a facility is looking to more easily upgrade its wiring and technology infrastructure. Typically, these pedestals allow for finished floor heights (FFH) between two and one-half inches and five inches. However, it should be noted that, since typical panels are one and one-half inches thick, the actual clearance can be less than one inch on a two and one-half inch FFH due to uneven floors. This can add significant technical challenges and coordination issues in routing utilities. In these cases, a true fixed-height low-profile floor may be a better solution.

Other Options: Additional types of pedestal supports are available to address specific building needs. For example, ramp type pedestals use a swivel head to allow for an adjustable slope or a 1:12 ramp slope for ADA compliance. Similarly, seismic pedestals have larger bases and thicker steel walls to withstand lateral forces. Usually adhesive attachment will be adequate to comply with many seismic code requirements. Occasionally, in higher seismic zones, fasteners and stringers may need to be applied.

Specification and Design Considerations: Based on the information described above, some relevant details are important when designing or specifying access floor systems:

Finished Floor Height (FFH) Requirements: With the various types and adjustment options of access floors, different finished floor height requirements can be achieved.

Five-inch minimum FFH is needed for adequate clearance to route wires.

Eight-inch minimum FFH is needed for pressurized plenums to distribute underfloor HVAC systems in small floor plans.

Twelve-inch minimum FFH is needed for most underfloor air distribution (UFAD) systems and horizontal wire management.

Load Ratings and Surface Type: Access floors are categorized or referenced by their load ratings and/or surface type. Floors are referred to in terms of concentrated load rating, with 1,000, 1,250, or 1,500 pounds; and finished surface type, for bare or HPL.

For example, a high-pressure laminate floor panel with a 1,000 pound load capacity is referred to as a "1,000-pound HPL." Similarly, a bare panel with a 1,250 pound load rating is referred to as a "1,250-pound bare."

The various types of load testing used by the Ceilings and Interiors Systems Construction Association to determine panel load capacity (Figure 1 − Access Floor Load Testing) are based on "Recommended Test Procedures for Access Floors" published by the Ceiling & Interiors Systems Construction Association (CISCA).

Advantages of Using an Access Floor

There are six main advantages to using access floors.

1. Higher quality underfloor air distribution compared to overhead air distribution.
In order to understand the differences, an examination of both traditional overhead air distribution and underfloor air distribution is in order.

Overhead air distribution: Traditionally, in commercial and institutional environments, conditioned supply air is delivered into an occupied space through ductwork and diffusers spaced evenly in the ceiling overhead. Prior to reaching individuals within the space, the supply air is mechanically mixed, making it uniform in both temperature and pollutant distribution. Just as the supply air is delivered at the ceiling level, so too, the return air is often collected at the ceiling and exhausted through the plenum that is created in the space between the suspended ceiling and the structure above. This common method of conditioned air distribution has resulted in a few well-known issues, however (Figure 2-Traditional Overhead Air Distribution Issues):

Figure 2: Traditional Overhead Air Distribution Issues

• Air quality: When heated air rises, it also carries dust particles (pollutants) up toward the supply air ceiling diffusers. Then, in turn, the pollutants are pushed back into user occupied space by the supply air. This can contribute to a build-up of poor air quality in the space.
• Temperature control: Because the mixed air is uniform in temperature, there is usually no opportunity for user adjustment or control. This results in the single most common occupant complaint that the air temperature is either too hot or too cold.
• Energy use: Because warmed air naturally rises, more energy is required to push cooler, conditioned air down into the user zone. Thus, air needs to be cooler than otherwise desired to overcome the hot air barrier so that it reaches users at the necessary comfort level.

Underfloor Air Distribution (UFAD): Underfloor Air Distribution (UFAD) allows for a preferred method of air distribution to take place. Referred to as "displacement ventilation," this process locates air supply vents at the bottom of the occupied space and the return air vents at the top of the space. Hence, a UFAD system allows ducted supply air from the HVAC equipment to enter into the space between the access floor and the structural slab creating a continuous volume low-pressure plenum. From there, the conditioned air rises easily into the user's zone through diffusers located strategically in the floor. As the air moves through the room, it gains heat from users, computers, equipment, and lighting. It continues to move upward until it is exhausted out of the space through the return air plenum in the ceiling. This type of conditioned air distribution has been shown to provide the following benefits and advantages (Figure 3-Underfloor Air Distribution System):

Figure 3: Underfloor Air Distrinution System

• Improved air quality: With underfloor air distribution, there is an increase in the total amount of air flow and ventilation since room air and supply air are more thoroughly mixed and more complete air changes are possible. Thus, as the conditioned air rises up throughout a room, it collects more particles and pollutants pulling them away from the user directly into the return air system where pollutants can be removed or reduced. This reverses the tendency of overhead systems to push pollutants down towards the users in the space and creates lower levels of contaminants in the occupied spaces and rooms. Independent studies have documented these results. (Figure 4-Air Quality Comparison):

Figure4: Air Quality Comparison

• Improved thermal comfort: In addition to improved air quality, underfloor air distribution can offer a more comfortable range of air temperatures. Because the air does not need to be pushed down to reach users in the space, as is the case in overhead distribution, the air temperature does not have to be heated or cooled as much. Hence, the plenum temperature can be adjusted to simply allow for the normal variations within occupied rooms and be set closer to typical desired levels. Therefore, the air temperature ranges are much smaller, allowing for greater thermal comfort. Further, by properly locating floor and ceiling diffusers, drafts and cold spots can be virtually eliminated.
• Temperature control: Providing adjustable floor diffusers that allow users to control the volume of air entering their space gives them the ability to determine their own individual comfort level. It has been generally acknowledged that greater worker productivity results when a user has the ability to control the air in their work space. This makes sense in light of the fact that the number one and number two complaints at work are "I am too cold" and "I am too hot." Occupants with no control are believed to be twice as sensitive to temperature changes while those with more control have fewer complaints. While more diffusers allow for more occupant control, they do not add to the total overall air flowing into the space; that is determined by the pressure within the plenum beneath the floor.
• Energy use:Underfloor air distribution has been tested and shown to reduce the amount of energy used for heating and cooling typical commercial and institutional spaces. First, less energy is needed to deliver the air through the underfloor plenum compared to overhead systems. Typically, HVAC system fans and motors can be reduced in size, since only .05 inches of static pressure is necessary to deliver air through an underfloor plenum. Additionally, overhead distribution systems require cooler temperatures, meaning bigger chillers and more energy use. (Figure 5- Less energy is used with underfloor air):

Air diffusers located in floor panels for greater air flow and temperature control. Photo Courtesy of Steelcase

2. Flexible power and data management
Another advantage of access floors is the easier management of power and data, both during construction and afterwards during occupancy. Access floors create an open cavity for fast and easy distribution, and management of power and data cables.

At installation, wires and cables can be run on the building's structural subfloor with virtually no obstructions. When moves, changes, or additions are needed, power and cable can be easily accessed through the floor panels. By contrast, traditional methods require removing suspended ceiling panels, pulling wires through conduit, disrupting the workplace all while working on ladders versus picking up an access floor panel, disconnecting the modular wiring and moving it.

Modular wiring systems allow for improved management of power, voice, data and other wiring needs. Photo Courtesy of Steelcase

3. Increased space flexibility
Overall, access floors provide a clear increase in space flexibility over traditional flooring methods. When relocating, panel locations and types are easy to change and exchange. When expanding spaces, more floor boxes can be added for power, voice, and data systems. In either case, air quality and temperature control can be maintained by simply adding adjustable floor diffusers. And when it comes time for a facility to upgrade itself, easy underfloor access to building services means easy updates to technology with minimal disruption.

Workspace flexibility is enhanced through the use of raised access floors. Photo Courtesy of Steelcase

4. Reduced construction time
Access floors are installed after the wiring, HVAC, and plumbing are installed. That means each of these trades has easy and open access to the structural floor, saving coordination time. Significant time reductions are possible by eliminating the need to hang things in the air, meaning no need to "fish" wires through a suspended ceiling, and not having to use ladders or lifts. And, because less ductwork is required for underfloor HVAC systems, less time is needed for ductwork fabrication and installation. Finally, because access floors have the capability for nearly perfect level floors, the remaining construction above them can go smoother and faster.

Typical access floor construction and installation. Photo Courtesy of Steelcase

5. Reduced building height
By requiring less building materials, e.g., building ductwork, the overall floor-to-floor height can be reduced, contributing to overall cost savings. In traditional overhead HVAC distribution, a large plenum space is required for supply air and return air ductwork, wiring, and sprinklers. In access floors, underfloor plenum heights are determined by the largest HVAC components (note that minimal ductwork is used in the underfloor plenum), requirements for underfloor cabling, and clear space for underfloor airflow. Similarly, a smaller ceiling space is required for return air and wire distribution. All of these factors make a five to 10 percent reduction in floor-to-floor heights possible when using access floors with underfloor air distribution compared to overhead distribution. (Figure 6-Building height comparison):

6. Reduced costs
Access floors can generate a positive impact on first costs of building construction, and on life cycle building costs.

Construction Costs: Numerous cost comparisons from development and construction companies have shown that access floors can be highly cost competitive with traditional construction methods for the shell, fit-out, HVAC, plumbing, lighting, electrical, and communications utilities.

Figure 8: Cost of change Trends

Life cycle costs: Beyond first costs, access floors can also help reduce on-going costs. (Figure 7) Cost of change benchmarks and (Figure 8) Cost of change Trends illustrate the rising cost and rate of change in typical office layouts. The cost per change has gone up 22 percent and the rate of churn has increased three percent. Access floors make the changes easier, require less time, and hence, less cost to perform.

LEED Certification Contribution

Access floors can also benefit building owners and managers through their potential to contribute to United States Green Building Council's (USGBC) Leadership in Energy and Environmental Design (LEED) certification. As most architects are already aware, there is a growing interest and concern in the building industry to take a more pro-active role in caring for the environment. The reasons for this concern are varied but include several research findings.

LEED rated building. Photo Courtesy of Steelcase

Reports from the Worldwatch Institute indicate that 40 percent of raw stone, gravel, and sand, and 25 percent of virgin wood is used in commercial buildings. Commercial buildings were found to use 40 percent of the energy, and 16 percent of the water supplies in most communities. Construction and demolition waste are about equal to municipal garbage.

In response, the USGBC has been recognized for promoting a transformation in the design and construction of buildings through the LEED program. This program is an independent rating system that defines green or sustainable buildings by commonly accepted standards of measurement. It is based on a point rating system that awards recognition at various levels. Achieving the base minimum number of points earns the distinction of a LEED certified building. Achieve increasingly higher point levels, and the distinctions become LEED Silver, Gold, or the ultimate level of Platinum.

More important than the distinction, however, achieving LEED certification for a building can promote energy savings, environmental preservation, healthier work spaces for people, and greater value to the building. Over time, the associated utility and incremental construction cost savings can result in the complete payback of the LEED related measures within a relatively short period of time.

Depending on the focus of the design objective, access floors can contribute to a variety of possible LEED points. They can do so, typically in three main categories:

Materials and Resources (MR), where the access floor components contain 25 to 50 percent or more recycled material content, and are manufactured within 500 miles of the project location.

Indoor Environmental Quality (EQ), where they are part of an overall underfloor air distribution design that increases ventilation effectiveness, reduces the use of volatile organic content (VOC) materials, allows for individual temperature control, and thermal comfort.

Innovation and Design Process (ID) where the access floor is part of an overall innovative design approach to sustainable building design.

Access floors have come a long way since they first arrived to address the needs of mainframe computers. With all of the attributes and advantages described above, plus the ability to create a healthier, more flexible, and sustainable environment, it's easy to see why they have been dubbed, "the foundation of flexibility and efficiency."

LEED Details, Versions, and Variations

As described on their web site, the USGBC has three different variations of the well-known rating system where raised access floors and underfloor air distribution systems can make a significant impact.

1. LEED − NC:LEED for New Construction and Major Renovations (LEED-NC) is a green building rating system that was designed to guide and distinguish high-performance commercial and institutional projects, with a focus on office buildings. Practitioners have also applied the system to K-12 schools, multi-unit residential buildings, manufacturing plants, laboratories and many other building types. It is important to note that USGBC requires a formal registration of a project in order to start an ongoing review file during the design and construction of a project. Certification at either the base, silver, gold, or platinum levels only occurs after the building construction is complete and all relevant documentation has been verified for certification to be awarded. LEED-NC is currently being used in two different versions:

LEED-NC Version 2.1. This rating system has been in use for the past few years as the basis for all LEED-NC project registrations and certifications. It includes specific intent, requirements, submittals, technologies, and strategies for each credit and the associated points.

LEED-NC Version 2.2. This is the latest version of this most widely used rating system. USGBC has announced that LEED® for New Construction (LEED-NC) v2.2 has been approved by its membership, and launched on November 11, 2005 at the Greenbuild Conference and Expo in Atlanta, Ga. This means that this new version 2.2 becomes the basis for registering and applying for LEED-NC certification instead of prior versions. USGBC has extended the deadline for LEED-NC version 2.1 Project Registrations until December 31, 2005. As of January 1, 2006, LEED-NC version 2.2 will be the only LEED-NC version available for registration. Projects will NOT be able to register for LEED-NC version 2.1 after December 31, 2005. Note that all previously registered LEED-NC version 2.0/v2.1 projects will still be able to apply for certification using the standard in place at the time of project registration. And, unlike the transition from v2.0 to v2.1, in this revision, projects will not be able to mix different versions of credits within one submittal. If projects are registered for version 2.0 or version 2.1, and a determination is made that version 2.2 would work better, a switch can be made entirely to version 2.2 with no additional registration cost.

LEED-NC is based on a total of69 points with award levels as follows:

Figure 9 - Improved Thermal Comfort

Figure 9 - Improved Thermal Comfort

Within that framework, access floors have the potential to contribute to the following LEED-NC points:

Materials and Resources (MR)
13 total possible points; up to four points applicable to access floors:

• MR 4.1 ~ 4.2 25 or 50 percent Recycled Content of Materials 1-2 points
  −Total weighted recycled content versus costs.
• MR 5.1 Manufactured Locally within 500 miles 1 point
• MR 5.2 Mfg. locally with Regional Materials (500 miles) 1 point

Indoor Environmental Quality (EQ)
15 total possible points; up to nine points applicable to access floors

• EQ 2 ~ Increased Ventilation Effectiveness (air-changes) 1 point
• EQ 3.1 ~ Indoor Air Quality Plan during construction 1 point
• EQ 3.2 - Indoor Air Quality Plan before occupancy 1point
• EQ 4.1 to 4.4 ~ Low Voss paints/adhesives/carpet/wood 1-4 points
• EQ 6.2 ~ Individual Controls temperature/lighting 1 point
• EQ 7.1 and 7.2 − Thermal Comfort per ASHRAE 55 1 point

Innovation in Design (ID)
Five total possible points.

• ID 1.1 to 1.4 ~ Innovation in Design 1-4 points

2. LEED-EB: The LEED Green Building Rating System for Existing Buildings (LEED-EB) is a set of performance standards for the sustainable operation of existing buildings. The LEED-EB criteria cover building operations and systems upgrades in existing buildings where the majority of interior or exterior surfaces remain unchanged. LEED-EB maximizes operational efficiency while minimizing environmental impacts. It provides a recognized, performance-based benchmark for building owners and operators to measure operations, improvements, and maintenance on a consistent scale. It is a road map for delivering economically profitable, environmentally responsible, healthy, and productive places to live and work, by addressing the following:

• Whole-building cleaning and maintenance issues including chemical use
• Ongoing indoor air quality (IAQ)
• Energy efficiency
• Water efficiency
• Recycling programs and facilities
• Exterior maintenance programs, and
• Systems upgrades to meet green building energy, water, IAQ, and lighting performance standards

LEED-EB is based on a total of85 points with award levels as follows:

Within this framework for existing buildings, access floors have the potential to contribute to the following LEED-EB points: Figure 10-Personal Control

Figure 10: Personal Control

3. LEED-CI: LEED for Commercial Interiors (LEED-CI) is the much anticipated green benchmark for the tenant improvement market. LEED-CI gives the power to make sustainable choices to tenants and designers, who do not always have control over whole building operations. LEED-CI is the recognized standard for certifying high-performance green interiors that are healthy, productive places to work, are less costly to operate and maintain, and reduce their environmental footprint.

LEED-CI addresses the specifics of tenant spaces primarily in office, retail and institutional buildings. It is part of a comprehensive suite of LEED assessment tools under development by the USGBC to promote green design, construction, and operations practices in buildings nationwide. A companion rating system for Core & Shell developments (LEED-CS) is currently under development. Together, LEED-CI and LEED-CS will establish green building criteria for commercial office real estate for use by both developers and tenants. Some projects may have only one applicable rating system while others may have more. USGBC encourages the project team to tally a potential point total using the rating system checklists for all possibilities. The project is a viable candidate for LEED certification if it can meet all prerequisites and achieve the minimum points required in a given rating system. If more than one rating system applies, then it is up to the project team to decide which one to pursue.

LEED-CI is based on a total of57 points with award levels as follows:

Within this framework for existing buildings, access floors have the potential to contribute to the following LEED-CI points:

All successful LEED projects begin with a fully integrated design team, in which all professional disciplines work together toward the project goals. While each needs to be aware of the other's contributions and participate in the decision-making, none can or will have the knowledge and experience to complete a project unassisted. Hence, the use of a LEED Accredited Professional on the design team is recommended and most likely to garner the greatest success. For more information contact USGBC atwww.usgbc.org

Information About Case Studies

• A growing body of research has been emerging to study and assess raised access floors for commercial and institutional installations. The most referenced body of evidence includes several articles and reports prepared by public and private sources. These include an article appearing in
• Architectural Record in 1984, authored by Gary Hall, that analyzed the characteristics of using raised access floors based on a 10 percent churn rate. In 1997 AT&T prepared a similar report on Intelligent Building Analysis using a 30 percent churn rate. The most relevant article, perhaps, appeared in 1992 when the General Services Administration (GSA) prepared an analysis and report specifically investigating the advantages and disadvantages of the use of raised access floors. Details of this report are summarized below.

GSA Report #7211-911C

• Subject building size 251,000 total square feet, 150 square feet per work station
• Churn (change) rate: 20 percent
• Specific items addressed: Access floor system and modular power system

  Advantages determined in the report include:

  • Lowest NPV
  • Lowest initial cost of access floor option
  • Lowest life cycle cost
  • Easiest and fastest to reconfigure
  • Lowest design coordination
  • Lowest labor costs
  • Highest residual property value
  • Best solution for modular overhead lighting

  Disadvantages identified include:

  • Proprietary systems
  • Greater floor-to-floor heights (potential reduction with underfloor supply air not evaluated)

• Regarding first costs, a private building cited by a raised access floor supplier has agreed with the results of the GSA study that first costs can be lower using a raised access floor with underfloor air distribution compared to a traditional structural floor system with overhead air distribution. First costs of tenant fit-out can also be analyzed. (Figure 11):

Peter J. Arsenault, AIA, NCARB, LEED-AP is an architect and consultant focused on green building and sustainable community planning, based near Syracuse, NY.