Accurately computing the solar irradiance in building performance software (BSP) is necessary to reliably predict the thermal behavior of many building related processes as well as the performance of renewable energy generators. While most of the simulations are performed at higher resolutions, up to 1-minute, the solar radiation data is often provided with weather data files at an hourly time scale. This work investigates the way popular building simulation programs interpolate the hourly solar radiation data and proposes a new algorithm that fixes deficiencies of other common methods. Program algorithms are identified and described and compared against high resolution measured irradiation data for three stations in the United States.