What's Happening from Environmental Building News
July 1, 2007
GSA Moving Towards Ban on Underfloor Air
Underfloor air distribution (UFAD) is an approach to ventilation in commercial and institutional buildings in which conditioned air is distributed through a plenum created by raised (access) floors, which also typically carry electrical and communications cabling. It is a favorite strategy in green building circles because of claims that it can provide better occupant comfort with reduced energy costs and increased air quality, while allowing for easy maintenance and flexible use of space (see
EBN
Vol. 7, No. 1). However, UFAD is under scrutiny at the U.S. General Services Administration (GSA), said to be the nation’s biggest landlord and also a champion of green building.
GSA is now working through a biennial revision of its “P100” facilities standards, which establish design standards for its construction projects. P100 is a largely prescriptive document that notes areas in which GSA wants to surpass national building codes, according to GSA’s chief engineer,William Holley, P.E., as well as incorporating recommended best practices and lessons learned. The current version, issued in 2005, does not address UFAD specifically, but does specify raised floors in office spaces to promote flexibility. The document also references the U.S. Green Building Council’s LEED Rating System, which, in LEED for New Construction version 2.0/2.1, encouraged UFAD. That combination of factors has contributed to approximately 8 million ft
2 (750,000 m
2) of UFAD installations in GSA buildings, said Holley.
Results from many of those installations, however, have not been positive. “The leakage rates [from the plenum] are much higher than we anticipated,” said Holley, noting that GSA has seen leakage rates of 60%–100% in several installations, when 10% would be more in line with expectations. That amount of leakage can lead to numerous other problems—lack of thermal comfort, wasted energy, ventilation noise, and condensation in the plenum. Holley cited poorly defined trade responsibilities regarding the design, construction, and use of the plenum as the main source of GSA’s problems. “Everybody’s working in that plenum,” including the architect and engineer during design and various trades during construction, said Holley, “but industry-wide it’s not too clear who is responsible for making sure that it operates as a plenum.” According to Holley, GSA is likely to recommend ducted systems for UFAD, which should solve the air leakage problem but will increase fan energy use and reduce flexibility and user-controllability.
UFAD expert Blair McCarry, P.E., of engineering firm Stantec, acknowledged potential gaps in design and construction processes like those described by Holley, but said that procedures can be established to avoid problems. “In the construction process we’ve had initial zones constructed as a mockup and then done leakage tests right in the beginning,” McCarry said. “It highlights for tradespeople that this is something we want to pay attention to and also gives a reference point for construction details in other areas.”
McCarry speculated that GSA might be having problems that other project teams aren’t facing. “For GSA, each new building has a new team, and [with UFAD] you’re directed to do something differently, and there aren’t up-to-date guidelines,” he said. Holley resisted that notion: “I’m not convinced that we’re having more or fewer problems than anybody else,” he said. “I’m not sure if anybody else is examining their buildings to see how it’s working.”
Following an extensive internal commentary and review process, GSA plans to publish the new P100 document by the end of 2007. Holley noted that GSA is likely to wait for development of better design and load calculation guidance—now under development in an ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) committee—before recommending non-ducted UFAD.
– Tristan Roberts
For more information:
U.S. General Services Administration
Facilities Standards for the Public Buildings Service
www.gsa.gov (click on “Buildings” and then “Facilities Standards”)
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