Feature Article

Access Floors: A Step Up for Commercial Buildings

Access flooring has long been used for cables. To get the most out of the investment, though, it should also be used for underfloor air distribution.

Photo: InterfaceAR
Simply put, access flooring is a winner.

Long used in computer rooms, access floors are now finding their way into office buildings and other commercial space, where they can dramatically reduce renovation costs while saving energy and improving indoor air quality—especially when providing an underfloor plenum for conditioned air distribution. This article takes a look at access floors, their many benefits, and some of the developments under way in this rapidly growing field.

Understanding Access Floors

The basic idea with access, or raised, floors is to elevate the floor, providing a plenum for electrical wires, communications cables, and air distribution. The floor can be raised by as little as 212 inches (60 mm) to more than three feet (900 mm). When used for air distribution, the floor is typically raised at least 14” (360 mm) above the slab. The access floor is usually a modular system of bolt-together steel or aluminum pedestals and concrete or metal floor panels. A few low-profile access flooring systems are made from molded plastic or aluminum. The floor surface can be carpet tile, vinyl composition tile, high-pressure melamine laminate on wood composite, decorative finished concrete, finished metal (steel or aluminum), or a natural material, such as linoleum or cork. The floor cavity is accessed by lifting up panels.

These passive air diffusers from Krantz swirl air, causing it to mix quickly with surrounding air.

Source: Krantz
Access floors have been around for more than 20 years, but until very recently they were only used in computer facilities, research laboratories, cleanrooms, specialized manufacturing buildings, and other spaces where a readily accessible subfloor space was absolutely necessary. Within the past few years, access floors have begun to find widespread use in standard office buildings. William Odell, AIA, a group vice-president of Hellmuth, Obata & Kassabaum’s (HOK’s) St. Louis office, believes that access floors will soon become the default for flooring in commercial office buildings.

Access floors make so much sense not only because they make it extremely easy to reconfigure a commercial space, but also because they can often provide very cost-effective air distribution in large, multifloor buildings. A conventional commercial building today has a network of supply ducts in the ceiling cavity above a dropped ceiling, with the entire cavity also serving as the return-air plenum. Conditioned air is blown down through ceiling-mounted diffusers where it mixes with the room air, and return air exits the occupied space through ceiling-mounted registers. A significant problem with this arrangement, according to Harry Gordon, FAIA, of Burt Hill Kosar Rittelmann Associates of Washington, D.C., is poor air circulation, which can be especially problematic in an office with workstation cubicles that interfere with the air circulation.

With an access floor air distribution system, the entire floor plenum is pressurized, and air is delivered to the occupied space through specialized floor-mounted diffusers.

These diffusers “swirl” or “twist” the air, causing it to mix very quickly with room air. This conditioned supply air is delivered at very low pressure, and directly to the occupant’s level. From there it gently mixes with the room air and rises to return grills mounted at ceiling level.

Benefits of Access Floors

Access floors used for conditioned air delivery offer numerous economic and environmental benefits:

Ease of reconfiguring the space

Photo: InterfaceAR

From a building owner (or occupant) standpoint, the most obvious benefit of access floors is ease of reconfiguring the workspace. According to E Source, a Boulder, Colorado-based technical information service, one-third of all office space in the U.S. undergoes a floor plan change every year! With access flooring, the cost and complexity of modifying the floor plan can be dramatically reduced, offering significant economic savings.

For example, consider the Owens Corning World Headquarters in Toledo, Ohio. Completed in September 1996, the three-story building was built with access floors throughout—some 275,000 sq. ft. (25,550 m2). In the first year of occupancy, the building experienced a remarkable 130% churn, according to Jim Eckert, the Director of Real Estate and Facility Management. (In other words, every employee moved an average of 1.3 times that year.) The access flooring system enabled those moves to be made very inexpensively—just $140 per workstation move, Eckert told

EBN. “My estimate is that we saved at least a half-million dollars in the first year in relocation costs,” he said. That comes out to about $1.80 per square foot ($19.40/m2) in savings in a single year! Note that potential savings with churn is not always present; it is only a benefit in a building where significant movement of employees and workstations occurs.

To reconfigure office space with access flooring, the floor panels are unscrewed from the support grid (first removing carpet tile if the floor is carpeted) and lifted up to expose wiring and cabling systems, which are modified as needed. In the case of the Owens Corning project, special Titus diffusers, with wider-than-normal flanges, were used to facilitate reuse of carpet tiles when the space was reconfigured. Even if a workspace is rarely reconfigured, evolving communications and networking technologies demand periodic updating of cable and wire systems. These upgrades are greatly simplified with access flooring.

Along with saving money, increasing the ease of remodeling can have significant environmental advantages.

Many modifications, such as changing the computer network cabling system, can be done simply by removing a section of carpet tile and lifting up floor panels, without modifying walls—drywalling and painting (and associated IAQ impacts) are not required. And unlike conventional flooring, with a modular access flooring system, most if not all of the carpet tile can be reused, eliminating both the solid waste impacts from disposing of old carpeting and the energy and other environmental impacts resulting from manufacture of new carpet or carpet tile.

Eliminating ducts also reduces resource consumption. Richard Craig, the U.S. representative for industry-leading Krantz diffusers (and the man who introduced this air distribution strategy to the U.S. in the 1980s), claims a 40% to 70% reduction in sheet metal ducting in a typical project. In evaluating potential resource benefits of eliminating supply ducts, the additional metal and other materials required for the access flooring system should be taken into account—there may not be a net reduction in material use.

EBN is not aware of any such analysis to date.

Energy savings

Use of an access floor plenum for conditioned air distribution can dramatically reduce energy use in a number of important ways. Less fan energy is required to distribute air, because the air is not being forced at high velocities through ducts, and because the air mixing in the room is more natural. Clark Bisel, P.E., a principal at Flack & Kurtz Consulting Engineers in San Francisco, is designing systems to deliver air at a pressure of less than 110 of an inch water column (25 Pa), compared with a ducted air supply system that typically delivers air at more than 12” water column pressure (124 Pa). “The floor plenum is like a huge duct,” he notes. To make this system work properly, the envelope has to be tightly sealed, but that should be a part of quality construction anyway.

Richard Craig, the U.S. distributor for Krantz diffusers, argues that the differences in

total static pressure for these two HVAC systems are just as great. A conventional system with overhead air distribution, according to Krantz, requires a total static pressure of 4” to 5” w.c. (995-1,244 Pa), compared with just 1.0” to 1.25” w.c. (249-311 Pa) for an underfloor air distribution system. Other engineers estimate a more modest reduction in required pressure. Krantz suggests that with a 15,000 square foot (1,400 m2) floorplate, the air handler needed for each floor can often be downsized from 25 to 10 horsepower (19,000-7,500 watts) with underfloor air distribution. In addition to the first-cost savings for the smaller motors, he claims that yearly savings in electricity will be $4,000 to $6,000 per floor, at 8¢/kWh.

Delivery of conditioned air at the floor level also means that the air doesn’t need to be as cold. Conventional HVAC systems in commercial buildings deliver air at about 55°F (13°C), while the supply air can be as warm as 63°F (17°C) in access-floor-delivery systems. This is possible because the conditioned air is delivered in the occupied zone of the room and because the delivered air does not pick up heat from the lights near the ceiling, as occurs with ceiling air supply. The upward air flow carries away unwanted heat before it reaches the occupied space. The actual cooling load may be no different, but, as Harry Gordon explains, “your entire delta-T [difference in temperature between supply and return air] moves up a few degrees.” This means that the water doesn’t have to be chilled as much and the chiller efficiency increases—in an example presented by E Source, from 0.60 kW/ton to 0.37 kW/ton (COP 5.9 to COP 9.5).

The higher-temperature supply air also means that outside air can be used for cooling more of the time (in engineering lingo, an “extended economizer range”). Because there are so many hours with the outside air below 63°F (17°C), Bisel calls this HVAC strategy “a cousin to natural cooling in many climates.” Even in more severe climates, chilled air can usually be delivered at least 3°F (1.7°C) warmer with this approach, according to Rich Kroko, president of Interface Architectural Resources (InterfaceAR™), which manufactures access flooring systems. Note that in humid climates the higher chiller temperature will result in less moisture removal, so the dehumidification requirement cancels out much of the energy savings from this shift in temperature range.

Additional energy savings can sometimes be realized by virtue of the thermal mass in the floor. Concrete floor panels, for instance, can store “coolth” and thus reduce temperature swings and peak cooling requirements.

In San Francisco’s moderate climate, Bisel claims energy savings of up to 50%, or $0.15 to $0.25/square foot per year ($1.60 to $2.70/m2yr.), compared with a conventional HVAC system. Richard Craig, who has been involved with the HVAC design of 60 buildings in the U.S. with access floor air distribution systems, claims that 30% to 50% savings in energy use is typical.

Improved indoor air quality

The “displacement air flow” that results when an access floor is used for conditioned air delivery should result in better air quality, according to David Bearg, P.E., of Life Energy Associates in Concord, Massachusetts. “Displacement air flow is significantly more efficient at removing air pollutants,” notes Bearg, who wrote

Indoor Air Quality and HVAC Systems (1993, Lewis Publishers). There should be less lateral mixing of air than with conventional air delivery, and those pollution sources that are also heat sources (for example, copiers and laser printers) should rise upward and out through the return grills, where filtering should remove pollutants.

Delivery of fresh outside air is also better. Richard Craig told

EBN that with conventional air delivery systems, more than 50% of the supplied outside air never even reaches the occupied zone. Therefore, to achieve a true 20 cfm (9.5 l/s) of outside air per person, one would actually have to provide 40 cfm (19 l/s). With a floor delivery system, on the other hand, 100% of the supplied outside air gets into the occupied space. According to David Houghton, P.E., in an E Source paper “Turning Air Conditioning on its Head,” designers of a building in Toronto used this principle to justify a 50% reduction in outside air flow while maintaining the same fresh air delivery to occupants.

There is also a reduced likelihood of mold growth in the supply ducts with the warmer supply air, because the relative humidity of the air is lower. Dan Nall, P.E., with Flack & Kurtz in New York, explains that in a conventional system, the 55°F supply air is at 93% RH, while with a displacement air system 60°F to 62°F is more typical and much less hospitable to biological agents.

Indoor air quality expert Hal Levin, however, cautioned in a 1995 issue of

IAQ Update that under certain circumstances, the floor plenum could become an IAQ problem—such as if an office worker accidentally spills a drink into the floor diffuser and mold growth ensues in the plenum. When using the floor plenum for air distribution, it is very important to clean it thoroughly during construction—before the access flooring system is installed. Another IAQ concern with some access flooring systems is formaldehyde emissions from wood composite floor panels. This offers an opportunity for use of MDI-bonded straw particleboard as the structural panel (see


Vol. 6, No. 3, page 5).

Increased ceiling height

Access floor plenums are usually significantly shallower than ceiling plenums. As a result, it is often possible either to increase the ceiling heights in a building or to reduce the slab-to-slab distance between floors. Conventional practice today typically provides an 18” (260 mm) to 24” (600 mm) plenum above a dropped ceiling. By eliminating the supply ducts, a much shallower ceiling plenum can be designed (say 12” or 300 mm), or the dropped ceiling can be eliminated altogether—with exposed ducts used for return air and pendant light fixtures used in place of recessed fixtures. This extra ceiling height can be used to improve natural daylighting by having windows extend higher than normal on outside walls and using light shelves to reflect daylight deep into the workspaces. In this way, the need for electric lighting can be significantly reduced in many buildings. Note that if the dropped ceiling is eliminated, extra expense (and potential IAQ impacts) will be incurred in painting the ceiling, exposed sprinkler pipes, etc., and other strategies may be required to deal with acoustics.

Alternately, the reduced loss of vertical space can be translated into reduced height between floor slabs. By reducing the slab-to-slab height by 8” (200 mm) on every floor, it is possible to provide an additional floor in an 18-story building. This can increase the profitability of a project as well as result in significant resource savings.

Enhanced comfort and control

Properly installed and operated, an access flooring system with air distribution should provide enhanced comfort and controllability compared with conventional HVAC systems. By pressurizing the floor to a uniform pressure (say 0.15” water column, or 37 Pa), balancing is extremely easy. Floor diffusers provide some level of individual control—both through their placement and how they are operated. The German diffuser made by Krantz is highly efficient at mixing supply air with room air. According to Craig, within a few inches of the diffuser vertically and horizontally, the air will be within 1⁄2°F (0.28°C) of the surrounding air temperature in the room. This is referred to as a “high-induction” diffuser because it very effectively induces circulation of a large volume of surrounding air. Like most other floor diffusers on the market, this is a passive system that works without a supplemental fan.

It is possible to integrate an access floor air distribution system with a comfort system that provides individualized climate conditions to each workstation, such as Johnson Controls’ Personal Environmental Module (PEM). PEMs are more commonly used with ducted air supply, but HOK recently used them in the new 250,000 square-foot (23,200 m2) headquarters building for Johnson Wax in Racine, Wisconsin, which has an access floor air distribution system. Other such products are under development (see “New Developments” below).

With access floor air distribution, extra effort may be required to ensure that perimeter spaces will achieve comparable comfort to interior spaces. With a well-insulated envelope and high-performance glazings, pockets that are too warm or too cold should not be a problem. Without adequate attention to envelope design, however, it is sometimes necessary to incorporate special radiant heating elements or additional floor diffusers.

Wiring and cabling savings

An access floor alows quick and easy modification of wiring.

Source: Owens Corning
With access flooring, the construction schedule for electrical rough-ins can often be more flexible because the wires are readily accessible.

Also, because floor modules with outlets and cable jacks can be located anywhere in the floor, it may be possible to get by without electrified furniture (furniture with integral cable trays and wiring). This can reduce furniture costs by $1,000 per workstation, according to Dan Nall. Finally, modifications to the wiring and cabling can be made very easily—often with in-house staff, avoiding the expense of union electricians.

More forgiving design

Despite the newness of access floor air distribution systems and their sophistication in terms of energy performance, the HVAC design is actually quite straightforward, according to Trox USA, Inc., another German manufacturer of floor diffusers. According to a technical bulletin the company provides, “underfloor air distribution systems are both easier to design and more forgiving of slight oversights and miscalculations than are overhead systems, largely because they do not require final runout duct.” The inherent adaptability of these systems allows for easy modification if, for some reason, performance is not meeting the needs.

The floor level requirements are also more forgiving, notes Harry Gordon. With access floors, floor leveling costs can usually be avoided, because the support posts provide for precise height adjustment.

Overall cost savings

First Cost Savings with Access Floors

Because air distribution access floor systems are still relatively new in the United States, there is little data on costs and measured savings. Flack & Kurtz Consulting Engineers compiled cost information for both a conventional HVAC system and an access floor system for a 50,000 square-foot (4,650 m2) building in California (see table).

The extra cost of the access flooring was $8 per square foot ($86/m2), but when other costs relating to finishes, conditioned air distribution, and wiring systems are included in the comparison, the access flooring system actually saves $2.70 per square foot ($29/m2) in construction cost. Most designers familiar with access floor air distribution systems who were interviewed by

EBN claimed significant first-cost savings, but some indicated that first costs would be somewhat higher. Craig claims that an access floor system used for air supply will save an average of $1.50 to $3 per square foot ($16-$32/m2) in construction cost. Engineer Paul Scanlon, P.E., of Burt Hill Kosar Rittelmann Associates, believes typical savings to be about $2 per square foot ($21.50/m2), while Dan Nall of Flack & Kurtz’s New York office estimates that an access floor system will

increase first costs by $1 to $1.50 per square foot ($10.75–$16/m2).

Along with the savings in installed cost for an access floor system that is used for air distribution, there are very significant savings in operating cost, as outlined in the discussion on benefits above. Overall HVAC costs can be reduced by as much as $0.25 per square foot ($2.70/m2) per year in California’s moderate climate, according to Bisel of Flack & Kurtz. Researchers at Carnegie Mellon University suggest a 20% to 30% reduction in energy costs with this air distribution system, according to material provided to

EBN by Krantz. Reduced churn costs can be even more significant, saving more than $1 per square foot ($10.75/m2) per year, according to Eckert at Owens Corning.

New Developments in Access Flooring

The access flooring industry is highly dynamic today, quickly expanding as more designers come to appreciate its advantages, and evolving to meet new needs. There are three principal manufacturers of access floor systems in the U.S.—Tate Access Flooring (originally a division of U.S. Gypsum), Interface Architectural Resources (InterfaceAR™), and Maxcess Technologies (a subsidiary of Hitachi)—along with several regional producers or packagers of access flooring systems.

InterfaceAR, with strong environmental directives from parent company Interface, Inc. (see page 15), is probably the most active in pursuing new technologies, materials, and strategic alliances. The company was formed in the early 1990s and in February 1996 acquired C-TEC, which had been producing access flooring since 1982. (C-TEC was originally a Westinghouse company.)

InterfaceAR recently entered into a joint venture with the office furniture manufacturer Herman Miller to pursue integrated comfort systems for the office environment based on a raised-floor platform. This joint venture, which does not yet have a formal name, will likely develop systems to address the fact that different individuals have different requirements when it comes to comfort. According to John Doris, InterfaceAR Vice President for Technical Services and Advanced Technologies, productivity-boosting systems will provide task conditioning that, in effect, “tops off” the climatic conditions at each workstation. Their approach will differ from what Johnson Controls does with their Personal Environmental Module (PEM), which controls

zone conditions as well as task conditions.

The floor diffuser industry is also seeing a great deal of activity. There are currently four manufacturers of floor diffusers with a strong presence in the U.S.: Krantz (distributed by Eurotech Products), Trox USA, Titus, and Tate. Krantz and Trox are German companies, with Krantz having been the leading provider of floor diffusers since the late 1980s and Trox only recently having entered the U.S. market. Titus, a U.S. company that is the world leader in

ceiling diffusers, now offers floor diffusers and is likely to become a major presence in this market (their products were recently used in the Owens Corning World Headquarters access floor system). Tate Access Floors makes its own line of floor diffusers, including fan-assisted models. The fan provides a greater degree of controllability, but comes with the penalty of increased complexity, electricity consumption, and motor heat that some say should be factored into the cooling load calculations. Most other floor diffusers on the market are passive.

InterfaceAR is actively pursuing low-toxic and natural materials for use in their access flooring systems. “We’re getting more and more demand for natural finishes,” said Doris, who is overseeing research on such materials as straw particleboard, bamboo, cork, natural rubber, and natural linoleum. “[Interface Chairman] Ray Anderson has encouraged us to stretch ourselves and look at options,” he told

EBN. Recycled content is an important part of that directive. The company’s new Tri-Tec™ access flooring system for cleanroom applications is made from 100% recycled aluminum, including both the support structure and the panels, according to company president Rich Kroko. The aluminum is made from a mix of post-consumer and post-industrial waste.

Perhaps the most environmentally innovative new development is a residential access flooring product made from 100% recycled material that InterfaceAR expects to introduce in the third or fourth quarter of 1998. According to Doris, this will be a very low-profile raised floor designed for the do-it-yourselfer for home offices and home entertainment rooms. It will be manufactured in 4-foot by 8-foot sheets (1.2 m x 2.4 m) and suitable for retrofit applications. This product should make finished basements in homes more comfortable and less prone to mildew by raising the carpeted floor above the concrete slab.

A small Canadian company, Camino Modular Systems in Etobicoke, Ontario, deals with recycling at the other end. The company salvages and refurbishes access floor systems from older buildings that are undergoing major remodeling or being torn down. These systems are sold at prices 10% to 20% below those of new systems (see


Vol. 5, No. 5, page 14). Because access floors are relatively new for all but computer facilities, however, and because they are inherently adaptable, there should not be a significant supply of salvaged floor systems for a long time. Project Manager Matt Roberts of the company told

EBN that they refurbished 10,000 to 25,000 square feet (930-2,300 m2) of access flooring last year. This is just a sideline to their primary business of producing their Woodcore line of new access flooring, which the company sells throughout Canada and in some foreign countries (but not the U.S., since the company is also a distributor for Tate Access Flooring). In the U.S., Irvine Access Floors in Eldersburg, Maryland salvages and refurbishes access flooring systems.

As access flooring systems become better understood over the next few years—particularly as the economic advantages of air distribution through access floors are appreciated—we can expect to see a rapid increase in their use in the U.S.

According to diffuser manufacturer Trox USA, less than 2% of commercial office space in the U.S. is currently being built with access flooring systems that are used for air distribution, compared with as much as half of commercial space in Japan and Europe. Not only is the U.S. likely to catch up with the rest of the developed world, but with companies like InterfaceAR, it may well move ahead with a new generation of environmentally friendly access flooring systems.

For more information:

Access Flooring Systems:

Interface AR

3700 32nd Street

Grand Rapids, MI 49508

616/977-8600, 616/977-8620 (fax)


Maxcess Technologies, Inc.

A Subsidiary of Hitachi Maxco, Ltd.

P.O. Box 2450

Summerville, SC 29484

803/821-1200, 803/821-0405 (fax)

www.mtiaccessfloor.comTate Access Floors (also diffusers)

P.O. Box 278

Jessup, MD 20794

410/799-4200, 410/799-4207

www.TateAccessFloors.comAccess Flooring Salvage & Refurbishing:

Camino Modular Systems, Inc.

89 Carlingview Drive

Etobicoke, ON M9W 5E4


800/370-0226, 416/675-2424 (fax)

Irvine Access Floors, Inc.

1311 Londontown Blvd., Suite 135

Eldersburg, MD 21784

410/766-8870, 410/766-2644 (fax)


Krantz (Swirl diffusers)

Eurotech Products, Inc.

3835 Deer Run

Denver, NC 28037

704/483-2050 (phone & fax)

Titus (Architectural Products Group)

990 Security Row

Richardson, TX 75081

972/699-1030, 972/918-8880 (fax)

www.titus-hvac.comTrox USA, Inc. (Swirl diffusers)

1005 Alderman Drive, Suite 103

Alpharetta, GA 30202

770/569-1433, 770/569-1435 (fax)

Johnson Controls, Inc. (Personal environmental module)

P.O. Box 423

Milwaukee, WI 53201


Published January 1, 1998

(1998, January 1). Access Floors: A Step Up for Commercial Buildings. Retrieved from https://www.buildinggreen.com/feature/access-floors-step-commercial-buildings