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Ceiling Systems: Open Plenum or Suspended?

Ceiling Systems: Open Plenum or Suspended?

A new study suggests the difference may be more than just style.

By By Sam Oches, Editorial Intern | August 11, 2010
This article first appeared in the 200809 issue of BD+C.

Though suspended ceilings (above, right) remain the most popular ceiling system option, facility managers report that 17% of their total building square footage still features an open plenum configuration (above, left).
A new study sponsored by the Ceilings & Interior Systems Construction Association (CISCA), the trade group representing the acoustic specialty ceilings and interior systems industry, suggests the difference between a suspended and open plenum ceiling design might be more than just a question of aesthetics. It might be thousands of dollars in energy savings.
The CISCA study is the first to compare the two types of systems based on initial construction costs, recurring energy costs, and the resulting payback period. “Ten years ago it was unusual for owners or developers to be doing detailed energy analyses on their buildings,” said Barry Donaldson. His firm, Barry Donaldson Associates, Croton-on-Hudson, N.Y., carried out the ceiling study for CISCA, whose members include Armstrong World Industries, Simplex Ceilings, and USG. “There's just so much more focus on green issues and sustainability today that people are digging down deeper, not only trying to understand the building itself as a whole, but understanding the fine tuning of individual building systems,” he says.

For much of the 20th century, of course, open plenum ceilings were the choice of designers and building owners, largely for reasons of initial cost. By the 1960s, as building systems started becoming more complex and high-performing, with electronics and communications wiring wiggling throughout the work space, suspended (or “dropped”) ceilings quickly became the more popular ceiling option.

The CISCA study examined construction and energy costs of prototype office and food retail buildings in five cities with widely varying climates: Chicago, Charlotte, Oklahoma City, Orlando, and Phoenix. Office buildings and food retail stores have two of the highest penetration rates of open plenum ceilings of any building type.

The prototype office building was a low- to mid-rise office structure with 120,000 sf and a typical nine-foot floor-to-ceiling height. The food retail store was set at 40,000 sf and a typical floor-to-ceiling height of 18 feet.

According to the study, suspended ceilings in the prototype office building cost 14.7% to 22% more to construct than open plenum ceilings, and 4.1% to 10.6% more in the prototype food store.

However, the study found, suspended ceilings saved as much as 10.3% a year in energy costs in the prototype office building, and as much as 17% a year in the prototype food store. This gave office buildings with suspended ceilings a payback period of as little as 5 years and food stores a payback period of as little as 0.9 years, or a little less than 11 months. Since the study was conducted in 2006 and 2007, the savings today could be even greater, given the huge leap in energy prices in the last year, according to Donaldson.
Where the savings come from
Most of the energy savings associated with suspended ceilings can be attributed to downsizing the HVAC system. Open plenums have much greater volumes of air to service than suspended ceiling systems, which requires higher static pressures and fan horsepower.

“The amount of fan energy to deliver air from the core of the building out to the workstation is expensive,” Donaldson said. “And when you're running air through both supply ducts and return air ducts, the sheer friction of the duct requires higher fan horsepower.” He added that energy use is the cube of the fan horsepower, meaning if you double the fan horsepower, you need eight times as much energy to provide that power.

Lighting in suspended ceilings can also help lower energy costs, said Donaldson. By diffusing light and distributing it more evenly to the work plane, suspended ceilings increase lighting performance and reduce the energy needed to offset heat created by the lighting.
Open plenums not fading away
While the CISCA study certainly suggests a suspended ceiling may be a wise call in future building plans (and may also help the project earn LEED credits), don't expect the open plenum design to disappear quite yet. For many building owners and developers, the ability to create a spacious, contemporary, or even utilitarian look justifies the use of open plenum ceilings.

According to Armstrong World Industries, facility managers reported that 17% of their total square footage is open plenum space, and 60% of architects reported having specified an open plenum space in the last two years. Aesthetic and cost considerations have kept open plenum ceilings popular in some quarters.

“The open ceiling certainly does work well for retailers,” said Kevin Dougherty, a principal in retail at Cubellis, Boston. “It reinforces the idea that this is going to save you money. It gives more of that warehouse feel.” This creates a sense in consumers' minds that the “low-cost look” must be saving them money on their purchases.

Dougherty said he would generally be cautious about specifying an open plenum design unless intentionally aiming to downplay the building's appearance to go along with a low-end retail brand.

Donaldson agrees that an open plenum design can do wonders for certain stores, especially big-box retailers. Transportation centers, such as bus and train stations, could also benefit from open plenum design because their elaborate mechanical systems would be more accessible to facilities staffs.

But he doesn't think the CISCA study will completely change designers' minds about whether to use a suspended ceiling or an open plenum. “There are a lot of reasons why people design something one way or another,” Donaldson said. “This study is the kind of information that helps them make better decisions.”

Download the complete CISCA study at http://www.cisca.org.

Construction cost (per floor)Energy (Btu/sf/yr)Energy cost ($/sf/yr)
Suspended Ceiling$164,63651,443$1.53
Open Plenum$141,23455,175$1.68
Suspended Ceiling$95,24746,225$1.90
Open Plenum$79,67050,664$2.11
Oklahoma City
Suspended Ceiling$99,52847,314$1.09
Open Plenum$86,76051,724$1.21
Suspended Ceiling$96,45346,609$1.75
Open Plenum$79,06251,668$1.95
Suspended Ceiling$108,15246,712$0.92
Open Plenum$92,84051,606$1.02
The CISCA study compared construction and energy costs for prototype buildings in the office and retail sectors, in five cities with different climates.

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