Services & Maintenance: Suspended Ceiling Or An Open Plenum?
By Greg Taylor
Published in the May 2008 issue of Today's Facility Manager
In many facilities, chances are the ceilings are comprised of an acoustical panel suspended in a metal grid system. That is because this type of ceiling is still the most popular system for commercial and institutional spaces, ranging from offices to schools to hospitals.
However, an architectural design trend often referred to as the “exposed structure” or “open plenum” look is becoming increasingly popular. It is exemplified in part by the warehouse look in which an overhead HVAC system and roof deck are fully exposed and presented as an integral part of the interior space, thereby providing a feeling of spaciousness and economy.
With a traditional overhead HVAC system, a suspended ceiling is used, in part, to cover that equipment, but in this case, it is neither needed nor wanted. In addition, no provision is generally made for the absence of the performance benefits that are taken for granted with a suspended acoustical ceiling in place.
In order to gauge the impact of suspended ceilings on such performance aspects as life cycle costs, energy savings, fire safety, and acoustics, a number of studies have recently been conducted. One of these was initiated by the Ceilings & Interior Systems Construction Association (CISCA) and conducted by the construction consulting firm of Barry Donaldson & Associates in Croton on Hudson, NY.
The study evaluated life cycle costs by looking at initial expenses for suspended ceilings versus open plenum designs. It also looked at annual operating costs (including HVAC and lighting), periodic maintenance,and space reconfiguration.
Construction Costs.The Donaldson study found that, for its prototype office space, initial construction costs for a suspended ceiling could range from almost 15%to 22% more than for an open plenum, depending on geographic location.It also found that, in general, the additional cost for the suspended ceiling, flexible ducts, and cable tray would only partially offset the cost of a return fan, return air ductwork, and conduit for the open plenum design. Moreover, the cost of recess mounted light fixtures in a suspended ceiling was relatively close to that of pendant mounted light fixtures in an open plenum design.
Thus, to justify a suspended ceiling design, the initial cost must be offset by enhanced performance and reduced operational expenses, such as lower energy use,easier maintenance, and decreased cost of renovation and reconfiguration.
Energy.The Donaldson study found that energy use in its open office plan examples in various geographic locations was lower for suspended ceiling designs than for open plenums (see Table 1). One reason for this was the use of a return air plenum with low static pressures and fan horsepower instead of a ducted air return with high static pressures and fan horsepower. A suspended ceiling with a return air plenum is also more effective in removing the heat generated by lighting, thereby reducing the air conditioning load.
Based upon the relatively short simple paybacks, the study concluded that energy savings alone could justify the use of a suspended ceiling.
Light Reflectance. The Donaldson study also noted that suspended ceilings typically have higher, more uniform light reflectance than open plenums. With uneven and somewhat darker surfaces, open plenums often have a light reflectance around 50%; suspended ceilings typically provide a reflectance of at least 70%.
This is a key factor, because increasing the reflectance of a ceiling can have a positive impact on lighting and HVAC energy consumption, especially when used in conjunction with an indirect lighting system.
To determine that impact, Brinjac Engineering, a multidiscipline consulting engineering firm based in Harrisburg, PA,studied two suspended acoustical ceilings. One had a light reflectance of 75% and the other, 90%. It found that, compared to the 75%reflective ceiling, the 90% reflective ceiling achieved an average reduction of nearly 24% in lighting energy costs when used with indirect lighting (see Table 2).
In addition, the 90%reflective ceiling allowed spacing between indirect luminaires to be increased, thus reducing the total number of luminaires needed to achieve light levels similar to the 75% ceiling.
The study also found that reducing the number of lighting fixtures lowered the heat load on the cooling system. Compared to the 75% reflective ceiling, the 90% reflective ceiling reduced the annual HVAC energy costs up to 9.1% for an indirect lighting system and up to 7.4% for a2'x 2' recessed parabolic troffer system, depending on geographic location.
Maintenance.In regard to maintenance, the Donaldson study noted that, although it is difficult to define different requirements and costs for a suspended ceiling versus an open plenum design, there may be savings in unnecessary periodic cleaning of ducts, pipes, and raceways that collect dust; in not having to paint or finish exposed equipment and systems; and less overhead maintenance activities in general.
Assuming a painted open plenum design is used, the study’s cost analysis estimates an increase of 10% in the maintenance cost of cleaning and repainting the plenum. This finding is corroborated by a similar study conducted by Project Time and Cost, a consulting firm in Atlanta, GA.Its research notes maintenance costs of suspended ceilings are generally known and budgeted. However, that is not currently the case with open plenums due to a lack of real life data, although Project Time and Cost estimates it to be approximately 10% higher.
The firm states one reason for the higher costs is that all repair and maintenance work is exposed to view and thus must meet a greater level of aesthetically acceptable “finish.” It also notes that suspended ceilings prevent dust and small leaks from reaching occupied spaces below, where they can affect desks, computers, and other property.
Reconfiguration. The Donaldson study found that suspended ceilings could reduce the cost of reconfiguration or “churn,” including moves to and from existing workplaces and furniture relocation.
According to the study,suspended ceilings provide an adaptable and accessible interior finish hat allows for reconfiguration of building systems to accommodate changing work and space needs. In office spaces, for example, the use of flexible ductwork, modular power and telecommunications cabling,light fixtures with modular “pigtail” connections, and return air troffer light fixtures allows for easier and less costly changes and reconfigurations.
On the other hand, fixed components found in open plenums, such as rigid metal ductwork, rigid metal conduit, hardwired power and telecommunications connections, and mounted light fixtures are more difficult and costly to move.
Fire Safety. Another performance benefit that a suspended ceiling can provide is an extra margin of fire safety. That is because the ceiling represents a significant percentage of the surfaces in a room and is critical to controlling the growth of a fire within a space.
Without a suspended ceiling, there is no physical separation between the elements of building services (e.g. ductwork and piping) and the space below. In contrast to an open plenum, an Underwriters’ Laboratories(UL) fire resistant rated ceiling system not only provides that separation but also creates a known, specified fire resistance period.
However,even a conventional acoustical ceiling can provide a limited degree of fire resistance. That is because most fires start small and can be controlled early by the sprinkler system. A conventional ceiling may thus remain intact and provide resistance to the movement of smoke,fire gases, and spread of flame into the space above.
A recent study by Hughes Associates of Baltimore, MD on smoke detector and sprinkler activation time demonstrates the difference ceiling height makes. The fire protection engineering and consulting firm found that, in building spaces where a suspended ceiling is not in place, the height of the space was greater, and the size of the fire could thus be larger at the time of smoke detector or sprinkler system activation.
In addition, sprinklers are generally designed to be installed under a continuous ceiling without obstructions like those created by ductwork in open plenums. Sprinklers installed in a continuous ceiling thus improve the chances of reliable performance.
Acoustic Environment. Over the years, studies measuring employees’ satisfaction with their workplaces have identified noise as a cause of dissatisfaction. Open plenum designs that reveal building service elements can cause acoustic problems, because sound reflecting off the deck above can result in excessive reverberation.
Any large space of this type will usually need some sound absorption to control overall noise levels. In addition, if the exposed deck is less than 15' high, reflections between open plan cubicles can cause distractions for occupants.
Many noise issues related to exposed structure designs can be addressed through the use of acoustical canopies and clouds—two types of freefloating ceilings. Table 4 documents the difference in reverberation time and the overall level of sound that canopies and clouds can make in an exposed structure space. (Reverberation time is a measure of the time required for loud reflected sounds such as a hand clap to become inaudible.) The results for a continuous or “wall to wall” ceiling are also included for comparative purposes.
When planning a ceiling strategy, facility managers should take performance factors into consideration to choose the most fitting design for their spaces.
Taylor is senior analyst of marketing research for Armstrong Ceiling Systems in Lancaster, PA. He can be reached at email@example.com.
Have you had experience with both suspended ceilings and open plenum? Share your thoughts by sending an e-mail to firstname.lastname@example.org.
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