A transient, three-dimensional finite volume method (FVM) numerical model was developed specifically for the purpose of modeling thermo-active foundations. Validation against a physical experiment was first carried out using measurements obtained from an experimental set-up of a thermo-active foundation. The model was then used to investigate three design parameters as they apply to thermo-active foundations including tube spacing, pier depth, and flow velocity. The model was also used to investigate the thermal interaction between the thermo-active foundation and the building above it using annual simulations based on hourly weather data for Boulder, CO. In particular, it is found that thermo-active foundation can increase a building's ground-coupled slab heat loss for 57% of hours during the heating season when compared to a standard foundation. A decrease in slab heat loss or increase in slab heat gain was seen for 32% of hours during the cooling season. (C) 2014 Elsevier B.V. All rights reserved.