This paper describes a web-based integration of energy, daylight, and airflow engines that enable users to access a comprehensive retrofit tool via a web browser. Integration of the selected engines in an online tool requires simplification of the energy, daylight, and airflow simulations, as well as the building model, which are described in the paper. This process relies on OpenStudio as a middleware to perform energy and airflow simulations via EnergyPlus and CONTAM, respectively. Daylight simulations utilize the Daysim engine to develop lighting power schedules for a typical room. This tool ultimately merges the developed infiltration rates and lighting schedules into the energy model.