Access Floors
Design flexibility, energy efficiency and healthy environments
by Bill Reynolds
A sustainable approach to building design brings with it demands for flexibility, energy efficiency and healthy work environments. Interior spaces need to respond rapidly and cost effectively to organizational and technological changes. Meanwhile, steadily increasing energy costs require building systems that operate with improved efficiencies while ensuring a healthy, comfortable environment for building occupants.
Raised access floors respond to these demands by offering an integrated, cost-effective approach to the distribution of power, voice, data and heating, ventilation and air-conditioning services while providing interior space flexibility and improving indoor air quality. However, to reap all the benefits of a raised access floor, architects, engineers, builders and building owners need to share responsibility for the proper design, construction, and maintenance of the plenum beneath the floor.
An access floor is a raised floor system comprising an understructure and welded steel floor panels filled with lightweight cement. The understructure provides positive positioning and lateral retention, ensuring that the floor is soundly supported on all contact points. The 24in.square floor panels use typical finished floor heights from 3in. up to 36in. on stable understructure that can also offer a height adjustment levelling device to ensure the floor is level even when the slab is not.
Raised access floors provide pathways for any type of service distribution system, modular wiring, passive or active zone cabling and HVAC service. Power-voice-data [PVD] termination boxes can be installed directly in the modular floor panels to provide convenient, flexible access to all of these services.
Services, UFAD and LEED
The resulting under-floor pathway created by the raised floor panels provides housing for any type of service distribution system, modular wiring, passive or active zone cabling and HVAC service. Power-voice-data [PVD] termination boxes can be installed directly in the modular floor panels to provide convenient, flexible access to all of these services.
Under-floor service distribution responds to equipment layout and technology change quickly and easily, and with almost no wasted materials. Modular plug-and-play wires and cables can be reused continually no matter how often reconfiguration occurs. On-site facility managers can access and/or move wiring and cabling services by simply removing the panel that contains the PVD termination boxes and swapping it with a panel in another desired location.
An access floor provides similar flexibility with respect to under floor air distribution [UFAD]. Air diffusers used in access floors are not attached to ducts and allow building occupants to adjust the direction and volume of air flowing from the under-floor plenum into the occupied space. PVD boxes and air diffusers can be relocated anywhere services are desired as spatial layout or technology and occupant needs change.
Using the space under the access floor for air distribution requires minimal ducting because the entire space is used as the service plenum to provide fresh air directly to the 6ft. high occupied zone closest to the floor. As fresh air enters the zone from the floor, it replaces existing “contaminated” air rather than diluting it. Pollutants and stale air in the zone are carried to the ceiling by natural convection and removed through return outlets. Because the air pathway is situated under the access floor, users can easily adjust air direction and volume for their individual comfort through the modular floor-mounted diffusers.
UFAD systems save money at initial installation and throughout operation compared to overhead systems by speeding installation time, eliminating costly, rigid overhead ductwork, and reducing HVAC energy through reduced fan energy. The static pressure required for an under-floor air distribution system is .05- to .10-inch wg [water guage], which represents approximately one-third of the pressure required for an overhead system.
Not only is under-floor air distribution a cost-effective alternative to traditional overhead systems, but it can also help contribute toward the accumulation of points for LEED certification in the areas of Energy and Atmosphere, Materials & Resources, Indoor Environmental Quality, and Innovation in Design. Significant contributions can be realized for optimizing energy performance, material reuse, recycled content, increased ventilation, controllability of systems, thermal comfort, and daylighting - just to name a few.
Access floors offer additional benefits to building owners. For example, conventional HVAC construction requires a large ceiling void for fixed service pathways. Using under-floor service distribution eliminates the need for this additional overhead space, which can reduce initial building material costs. Reducing the ductwork overhead enables new buildings to achieve a 5 to 10% reduction in slab-to-slab heights, resulting in materials savings, construction cost reductions and faster construction rates.
Access floors also contribute to better work environments. In addition to improving indoor air quality and enhancing occupant comfort, the floors increase design choices and day-lighting opportunities through the elimination of traditional overhead HVAC ducts for systems and nonstructural columns used to contain wiring. Such subtle benefits can pay off with increased employee productivity and decreased absenteeism.
Under-floor service distribution eliminates the need for additional overhead space for fixed service pathways, and reduces slab-to-slab heights by 5 to 10% resulting in savings in materials and construction costs
Sealing the plenum
However, to achieve these advantages and realize the significant energy savings described above, the under-floor plenum must be properly sealed. In fact, plenum integrity is one of the most important aspects of designing, constructing and maintaining an under-floor air distribution system.
Numerous examples of best practices demonstrate that the process of ensuring plenum integrity begins with the architect. An architect designing a building with under-floor air distribution must provide specifications and details for constructing a plenum with proper sealing and integrity.
Architects must consistently place specification and performance requirements in all divisions of the CSI specifications and provide general construction specifications for pre-bid and preconstruction meetings, the assembly of a preconstruction mock-up, quality inspections, and air leakage testing procedures with expected results.
Educating the trades is key to success here, and the general contractor is best positioned to ensure that all subcontractors are informed of and perform the sealing requirements as outlined in the specifications and details provided by the architect. Ongoing communications with the trades regarding the prevention of air leakage is critical, from pre-bid meetings through testing and remediation.
Commissioning
For the commissioning agent, the job begins with pre-bid and preconstruction meetings and the opportunities they provide to reaffirm sealing requirements and assist with providing information to subcontractors. In addition to inspecting and testing the preconstruction mockup, the commissioning agent should regularly perform quality inspections throughout the construction process, test the completed plenum and report any plenum integrity sealing issues.
Commissioning agents, as well as other members of the project team, need to have an understanding of the definitions for various types of air leakage and testing procedures. A critical aspect of success prior to testing the plenum is the inspection and reporting form. The commissioning agent should use the form to report plenum integrity to the general contractor during construction. The reporting form, in turn, acts as a checklist against which the general contractor can review the sub-contractors work and initiate remediation where necessary.
Essentially, access floors look and feel similar to traditional floors, but what lies beneath offers far more sustainable advantages. By taking a holistic approach that engages everyone involved in the building design and construction process and that emphasizes the significance of plenum integrity, building owners can take full advantage of a green-driven solution that increases building flexibility, energy efficiency and control, enhances indoor environmental quality, and reduces construction and operating costs.