Traditionally, attics have been insulated at the flat ceiling with deliberate venting to manage heat and moisture. Having all the duct air distribution system in the conditioned space is the best building science approach, however this is not always the preferable solution for builders that like to use the roof space for air distribution. Ducts in attics are exposed to very intense heating and cooling conditions. Not only do we have thermal conduction, convection and radiation exchanges occurring but important duct leakage losses that create significant energy balance losses. In the recent IECC 2018 code, deeply buried attics have been credited for reducing the energy losses. Due to the complex thermal performance in each zone adjacent to the buried ductwork these credits seem to be not consistent with the benefits achieve by buring the ductwork in insulation. The thermal performance of buried ducts was examined in both 2-D and 3-D to determine the actual improvement as a function of duct size, duct insulation and attic insulation. In this paper we will describe the field/laboratory and modeling effort undertaken by to quantify the performances of duct systems. Climate zone 2A is included in the field testing, that encompass both hot/dry climates and hot and humid climates. Attics with deeply buried ducts and low air duct leakage had excellent thermal performance and exhibited the best overall thermal performances. Indeed, based on the thermal simulation analysis of deep buried ducts the thermal performance is better than the credit given to them by IRC 2018 code.