DescriptionFreestanding high tunnels are cost-effective, plastic film-covered growing structures that use very little to no modern environmental control technology. Natural ventilation is used to control temperature and humidity. This dissertation investigates design and management decisions that impact the high tunnel environment and ventilation, including vent design, high tunnel orientation, plant canopy height, shoulder-season management, and high tunnel row spacing. The main tool used for this investigation was computational fluid dynamics (CFD) simulations because they can accurately and quickly describe the airflow within a complex system, while allowing for an iterative design process. Field experiments were conducted at the Pennsylvania State University High Tunnel Research and Education Facility (Rock Springs, PA) in order to collect environmental data within and immediately outside of a reference high tunnel. This data was used to validate a CFD model made using commercially available software (ANSYS Fluent), which incorporated the physical processes of energy transfer (convection, conduction, and radiation), turbulence, plant canopy induced drag, plant evapotranspiration, and water vapor transport. The model had a root mean squared error of 1.17 °C (n = 144), showing good agreement between experimental and simulated results since this error is close to the measurement error of the temperature sensors used. Permutations to this base model were made to investigate the research questions posed. These included separate simulations of five roof vent designs, three tunnel orientations, three plant canopy heights, four distinct sets of weather conditions representing the colder periods of the year, and five row spacings for two differently sized tunnels (research size and commercial size). Finally, practical recommendations are presented related to how the findings of this dissertation can be implemented by growers.