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In new construction, the best option is to locate the duct system within the conditioned space. Doing so can reduce your heating and cooling costs and improve your indoor air quality. When all the ducts are inside the building envelope, even if return leaks do occur, the air infiltrating the system is already conditioned. Supply leaks can still be a problem in that you won't get even distribution of conditioned air throughout the home. Therefore, proper sealing of ductwork is still very important—even when the duct system is located within the conditioned space. Note that the Florida Building Code, among other things, requires all duct distribution systems be sized and designed in accordance with recognized engineering standards such as ACCA Manual D or other standards.

ACCA stands for Air Conditioning Contractors of America http://www.acca.org

Source: Wendell A. Porter, Hyun-Jeong Lee, and Kathleen C. Ruppert, The Duct System, UF/IFAS FCS3263, 2008.

Additional Resources

U.S. Department of Energy – Better Duct Systems for Home Heating and Cooling - http://www.nrel.gov/docs/fy05osti/30506.pdf 

U.S. Department of Energy – Improving the Efficiency of Your Duct System - http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/27630.pdf 

U.S. Department of Energy – Minimizing Energy Losses in Ducts – http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12730