Services & Maintenance: Suspended Ceiling Or An Open Plenum?
By Greg Taylor
Published in the May 2008 issue of Today’s Facility Manager
Inmany facilities, chances are the ceilings are comprised of anacoustical panel suspended in a metal grid system. That is because thistype of ceiling is still the most popular system for commercial andinstitutional spaces, ranging from offices to schools to hospitals.
However, an architectural design trend often referred toas the “exposed structure” or “open plenum” look is becomingincreasingly popular. It is exemplified in part by the warehouse lookin which an overhead HVAC system and roof deck are fully exposed andpresented as an integral part of the interior space, thereby providinga feeling of spaciousness and economy.
With a traditionaloverhead HVAC system, a suspended ceiling is used, in part, to coverthat equipment, but in this case, it is neither needed nor wanted. Inaddition, no provision is generally made for the absence of theperformance benefits that are taken for granted with a suspendedacoustical ceiling in place.
Inorder to gauge the impact of suspended ceilings on such performanceaspects as life cycle costs, energy savings, fire safety, andacoustics, a number of studies have recently been conducted. One ofthese was initiated by the Ceilings & Interior Systems ConstructionAssociation (CISCA) and conducted by the construction consulting firmof Barry Donaldson & Associates in Croton on Hudson, NY.
Thestudy evaluated life cycle costs by looking at initial expenses forsuspended ceilings versus open plenum designs. It also looked at annualoperating costs (including HVAC and lighting), periodic maintenance,and space reconfiguration.
Construction Costs.The Donaldson study found that, for its prototype office space, initialconstruction 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 suspendedceiling, flexible ducts, and cable tray would only partially offset thecost of a return fan, return air ductwork, and conduit for the openplenum design. Moreover, the cost of recess mounted light fixtures in asuspended ceiling was relatively close to that of pendant mounted lightfixtures in an open plenum design.
Thus, to justify asuspended ceiling design, the initial cost must be offset by enhancedperformance and reduced operational expenses, such as lower energy use,easier maintenance, and decreased cost of renovation andreconfiguration.
Energy.The Donaldson study found that energy use in its open office planexamples in various geographic locations was lower for suspendedceiling designs than for open plenums (see Table 1). One reason forthis was the use of a return air plenum with low static pressures andfan horsepower instead of a ducted air return with high staticpressures and fan horsepower. A suspended ceiling with a return airplenum is also more effective in removing the heat generated bylighting, thereby reducing the air conditioning load.
Basedupon the relatively short simple paybacks, the study concluded thatenergy savings alone could justify the use of a suspended ceiling.
Light Reflectance. The Donaldson study also noted that suspended ceilings typically havehigher, more uniform light reflectance than open plenums. With unevenand somewhat darker surfaces, open plenums often have a lightreflectance around 50%; suspended ceilings typically provide areflectance of at least 70%.
This is a key factor, becauseincreasing the reflectance of a ceiling can have a positive impact onlighting and HVAC energy consumption, especially when used inconjunction with an indirect lighting system.
To determine that impact, Brinjac Engineering, amultidiscipline consulting engineering firm based in Harrisburg, PA,studied two suspended acoustical ceilings. One had a light reflectanceof 75% and the other, 90%. It found that, compared to the 75%reflective ceiling, the 90% reflective ceiling achieved an averagereduction of nearly 24% in lighting energy costs when used withindirect lighting (see Table 2).
In addition, the 90%reflective ceiling allowed spacing between indirect luminaires to beincreased, thus reducing the total number of luminaires needed toachieve light levels similar to the 75% ceiling.
The studyalso found that reducing the number of lighting fixtures lowered theheat load on the cooling system. Compared to the 75% reflectiveceiling, the 90% reflective ceiling reduced the annual HVAC energycosts up to 9.1% for an indirect lighting system and up to 7.4% for a2′x 2′ recessed parabolic troffer system, depending on geographiclocation.
Maintenance.In regard to maintenance, the Donaldson study noted that, although itis difficult to define different requirements and costs for a suspendedceiling versus an open plenum design, there may be savings inunnecessary periodic cleaning of ducts, pipes, and raceways thatcollect dust; in not having to paint or finish exposed equipment andsystems; and less overhead maintenance activities in general.
Assuminga painted open plenum design is used, the study’s cost analysisestimates an increase of 10% in the maintenance cost of cleaning andrepainting the plenum. This finding is corroborated by a similar studyconducted by Project Time and Cost, a consulting firm in Atlanta, GA.Its research notes maintenance costs of suspended ceilings aregenerally known and budgeted. However, that is not currently the casewith open plenums due to a lack of real life data, although ProjectTime and Cost estimates it to be approximately 10% higher.
Thefirm states one reason for the higher costs is that all repair andmaintenance work is exposed to view and thus must meet a greater levelof aesthetically acceptable “finish.” It also notes that suspendedceilings prevent dust and small leaks from reaching occupied spacesbelow, where they can affect desks, computers, and other property.
Reconfiguration. The Donaldson study found that suspended ceilings could reduce the costof reconfiguration or “churn,” including moves to and from existingworkplaces and furniture relocation.
According to the study,suspended ceilings provide an adaptable and accessible interior finishthat allows for reconfiguration of building systems to accommodatechanging work and space needs. In office spaces, for example, the useof flexible ductwork, modular power and telecommunications cabling,light fixtures with modular “pigtail” connections, and return airtroffer light fixtures allows for easier and less costly changes andreconfigurations.
On the other hand, fixed components found inopen plenums, such as rigid metal ductwork, rigid metal conduit, hardwired power and telecommunications connections, and mounted lightfixtures are more difficult and costly to move.
Fire Safety. Another performance benefit that a suspended ceiling can provide is anextra margin of fire safety. That is because the ceiling represents asignificant percentage of the surfaces in a room and is critical tocontrolling the growth of a fire within a space.
Withouta suspended ceiling, there is no physical separation between theelements of building services (e.g. ductwork and piping) and the spacebelow. In contrast to an open plenum, an Underwriters’ Laboratories(UL) fire resistant rated ceiling system not only provides thatseparation but also creates a known, specified fire resistance period.
However,even a conventional acoustical ceiling can provide a limited degree offire resistance. That is because most fires start small and can becontrolled early by the sprinkler system. A conventional ceiling maythus remain intact and provide resistance to the movement of smoke,fire gases, and spread of flame into the space above.
Arecent study by Hughes Associates of Baltimore, MD on smoke detectorand sprinkler activation time demonstrates the difference ceilingheight makes. The fire protection engineering and consulting firm foundthat, in building spaces where a suspended ceiling is not in place, theheight of the space was greater, and the size of the fire could thus belarger at the time of smoke detector or sprinkler system activation.
Inaddition, sprinklers are generally designed to be installed under acontinuous ceiling without obstructions like those created by ductworkin open plenums. Sprinklers installed in a continuous ceiling thusimprove the chances of reliable performance.
Acoustic Environment. Over the years, studies measuring employees’ satisfaction with theirworkplaces have identified noise as a cause of dissatisfaction. Openplenum designs that reveal building service elements can cause acousticproblems, because sound reflecting off the deck above can result inexcessive reverberation.
Any large space of this type willusually need some sound absorption to control overall noise levels. Inaddition, if the exposed deck is less than 15′ high, reflectionsbetween open plan cubicles can cause distractions for occupants.
Manynoise issues related to exposed structure designs can be addressedthrough the use of acoustical canopies and clouds—two types of freefloating ceilings. Table 4 documents the difference in reverberationtime and the overall level of sound that canopies and clouds can makein an exposed structure space. (Reverberation time is a measure of thetime required for loud reflected sounds such as a handclap to becomeinaudible.) The results for a continuous or “wall to wall” ceiling arealso included for comparative purposes.
When planning aceiling strategy, facility managers should take performance factorsinto 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 protected].
Have you had experience with both suspended ceilings and open plenum? Share your thoughts by sending an e-mail to [email protected].
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