An under-floor plenum system (not access floor) for air distribution is an expensive approach to flooring, but it saves on air handling fan sizes, energy costs, and presumably fan life; and reduces HVAC noise due to lower pressure drop with this approach. For HVAC flexibility, don’t neck down your air ducts. Instead, run the same size duct work throughout the facility. It cost a bit more in sheet metal, but yields a big HVAC flexibility benefit.

- From Tradeline's Academic Science Buildings 2006 Conference