Creating an Air-Tight Enclosure for Under-Floor Air Distribution
Designing a raised floor system to include air distribution is not especially challenging. Installing the floor so that the air distribution functions properly, on the other hand, requires obsessive attention to detail. The objective in any such system is to get the air from the air-handling unit to the occupant-controlled floor air outlet with as little leakage as possible. But air, like water, has a great talent for finding and exploiting leaks. A raised floor has an extraordinary number of opportunities for air to escape.
Typically, there are 2’x2′ panels installed on 12- to 18-inch-high steel posts, leaving a void between the sub-floor and walking surface to serve as a plenum to distribute conditioned air to the floor air outlets and a pathway for conduit and flexible cable. Either self-adhering carpet tile or hard-surface flooring is then installed on top of the panels. So far, so good. But now we cut holes through the panels and carpet to enable the installation of flush-mounted power and tele/data outlets and air distribution devices, fed by conduit and cabling. Every place we penetrate the raised floor, and everywhere the floor meets a vertical element, air has an opportunity to escape.
The solution? Seal it. Seal everything. Seal every gap, no matter how small,[image 1] as though you were protecting astronauts from the airless void of outer space.
The first step is hiring an installer and sub-contractors (for HVAC, electrical, and tele/data), as well as a construction manager or general contractor, who are experienced with raised floors. Have the CM or GC assign one person who is responsible for all sealing and who works closely with the design team to ensure thoroughness and quality. It is also critically important to get everything right the first time. Raised floors have the great advantage of enabling space reconfiguration and technology upgrades quickly and easily, but identifying and remedying air leakage after the fact is no picnic.
Then seal everything that abuts, enters, or leaves the void between sub-floor and raised floor.[image 2] If the installer tells you that you don’t really need to seal this or that, don’t believe it. It is easier and less costly to seal everything than to try to determine after the fact where air is leaking. So seal the conduit, pipe and duct/air highway edges, and all penetrations. With conduit, pay particular attention to sealing the perimeter of its penetration and the end where any wiring leaves the conduit. [image 3] Check the joints of the air highways that direct air-flow into the plenum by sliding a piece of paper along the sub-floor joint under the duct partition. [image 4] If you see the leading edge of the paper emerge on the other side of the duct, have the joint sealed. Ensure that all interior and exterior building edges are tight, [image 5] ideally by detailing an edge strip everywhere the raised floor meets a wall and then sealing the joint; you may need a larger edge strip if the building has a curtain wall system.
For more detailed information and guidance, the US General Services Administration offers the PBS Guidelines for “Raised Floor Systems – with and without Underfloor Air Distribution”, a free downloadable reference. Several of the raised floor manufacturers have drawing details and specification sections that can be included in the project documents. They will also offer their expertise during design and installation.
Attention to detail is the hallmark of any successful project. But with an air distribution system under a raised floor, make that attention obsessive, compulsive, early, and often. So…
Just seal it!
Dennis Dyer , P.E. LEED AP is a partner at AKF Group’s Boston office. AKF Group LLC is headquartered in New York.