A detailed building energy simulation, FSEC 2.1, has been used to determine the relative significance of duct leakage and heat transfer on space-cooling energy use in Florida houses. A comprehensive calculation procedure has been developed to predict duct air leakage based on duct leakage areas and associated operating pressures. The effect of the leakage on building air infiltration and air-conditioning electrical demand is estimated based on the mass transport and the sources of the various airflows. Heat transfer to the duct system is estimated using calculations based on previous experimental research on duct conductances. Results show that the impacts of duct systems on air-conditioning loads are strongly time-dependent, exacerbating electrical demand during utility summer peak periods and increasing air-conditioner run-time. The impact of duct leakage was found to be of the largest magnitude, followed by heat transfer to the duct system itself. Air handler return-side air leak source temperature and enthalpy were also found to be significant in terms of air-conditioner loads. Detailed measurements of air-conditioner electrical demand taken in a house before and after duct leak repair is provided for comparison with simulation results.