The thermal and hygrothermal performance of a ventilated attic with and without an insulated ventilated nail base (IVNB)system over the roof deck was studied in a hot, humid climate. Seven test attics were constructed and instrumented for monitoringtheir field performance. Attics with the same configuration but having shingles with increased solar reflectance are also includedfor comparison. These test attics are instrumented to provide field data of temperature, heat flux, relative humidity, moisturecontent, and pressure differential across the roofing assemblies. Field data collected over a year suggests that the IVNB systemover the roof deck can drastically reduce heat influx during summer and prevent heat loss in winter, which may provide a costeffectivesolution for roof retrofitting to achieve high energy savings. During the winter, the IVNB system was found to preventmoisture buildup and, hence, may lower the condensation potential. Hygrothermal analysis of moisture content in roof deck andHVAC energy consumption calculations in various climate zones will be discussed to shed light on the total benefits of IVNB inthe scenario of an above-sheathing roof retrofit solution to achieve higher energy savings.