This paper describes a transient heat and mass transfer model of residential attics. The model is used to predict hourly ceiling heat gain/loss in residences with the purpose of estimating reductions in cooling and heating loads produced by radiant barriers. The model accounts for transient conduction, convection, and radiation and incorporates moisture and air transport across the attic, Environmental variables, such as solar loads on enter attic surfaces and sky temperatures, are also estimated. The model is driven by hourly weather data which include: outdoor dry bulb air temperature, horizontal solar and sky radiation, Ll,ind speed and direction, relative humidity (or dew point), and cloud cover data. The output of the model includes ceiling heat fluxes, inner and outer hear fluxes front all surfaces, inner and enter surface temperatures, and attic dry bulb air temperatures, The calculated fluxes have been compared to experimental data of side-by-side testing of attics retrofit with radiant barriers. The model predicts ceiling heat flows with an error of less than ten percent for most cases.