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VAC systems using underfloor air distribution (UFAD) promise multiple benefits. These include: improving occupant satisfaction by pro
viding control over personal air supply through use of manually adjustable diffusers; making building systems more flexible and easily reconfigurable, with the effect that "churn"* can be less expensive; improving room ventilation by delivering fresh air closer to occupants; and reducing building energy use due to decreased central fan energy, extended economizer cooling, and improved cooling-cycle efficiency.
These benefits come with a price. The initial cost of construction using UFAD systems is typically higher than in comparable overhead systems. In recent projects, there has been a $3 to $5 per ft2 ($33 to $55 per in 2) premium depending on specific building design, including the cost of the access floor. The challenge to a team designing a building with UFAD is to capture as many of the benefits as possible while keeping the initial cost increase to a minimum.
With that goal in mind, here are three design and construction topics to consider when using UFAD. These topics come from experiences on seven recent projects, and reflect feedback from owners, designers, contractors, occupants, and building operators.
Minimize Ductwork in the Plenum
A major benefit of UFAD is system flexibility. The raised-floor plenum provides a convenient and accessible route for all building services from the building core out to the point of use. Services include air distribution, of course, but also can include heating-hot-water piping for perimeter systems, electrical and telephone/data cabling (typically run in cable trays), sprinkler mains, domestic water piping, and drains.
In the best case, these services share the plenum with a "loose fit" that provides room for each to be modified easily. In the worst case, these co-existing and co-located services make coordination a complex and difficult task. Where conflicts occur, ductwork or "air-highways"** are typically involved because they take up more room than any of the other services. Detailing air-distribution crossovers invariably ends up with notched or penetrated ducts, making for a difficult or inefficient installation.
Furthermore, while cable can be pulled through the plenum easily, and there is usually enough room to work on piping, hard ductwork or plenum dividers are much more difficult to reconfigure and doing so requires access...