This study focuses on understanding the heat content within soil-borehole thermal energy storage (SBTES) systems installed in different types of soils in the vadose zone. Temperature fluctuations in the atmosphere can create a variety of heat flux conditions resulting in different temperature gradients in the subsurface, even when a surficial insulation layer is incorporated. A three-dimensional (3D), transient finite element model was built in COMSOL to consider the representative field conditions as well as coupled heat transfer and water flow processes in the unsaturated soil within the SBTES system. The heat content is used to quantify the heat gain above the temperature profile expected above the ambient ground temperature fluctuations. The heat content changes with different type of soils as the hydraulic and thermal properties are specific to soil types. Results indicate that presence of an insulation layer leads to a significant heat gain in the shallow subsurface in all types of soils.