A transient three-dimensional numerical solution is developed to analyze the thermal performance of thermo-active foundations used to heat and cool commercial buildings. Using laboratory testing data, the numerical solution is validated and used to carry out a sensitivity analysis to assess the most important design and operating parameters that affect the thermal performance of thermo-active foundations. It is found that the foundation depth, the shank space, the fluid flow rate, and the number of U-tube loops in each foundation pile are the main parameters that affect the thermal performance of a thermo-active foundation system. Based on the validated numerical solution, thermal response factors for a thermo-active foundation are developed, and implemented into a detailed building energy simulation program. These thermal response factors are then used to estimate the impact of installing thermo-active foundations on the total energy use of typical office buildings in representative US climates.