The energy slab is a slab structure in a building that encases heat exchangers to utilize geothermal energy for heating and cooling of the building. In the energy slabs, the presence of thermal insulation layers is crucially important compared to those of the conventional Ground Heat Exchanger (GHEX). In this paper, a computational fluid dynamics (CFD) model was developed by calibrating with the result of the in-situ thermal performance test (TPT) to comprehensively assess the effect of influential factors on the thermal performance of energy slabs. Note that the thermal conductivity of ground formations and the flow rate of working fluid significantly influence the thermal performance of energy slabs. Especially in the energy slabs, the existence of a thermal insulation layer with appropriate thermal properties is significantly important to relieve the thermal interference induced by ambient air temperature. Considering the performance of thermal insulation and economic feasibility, a PF (phenol foam) board is found to be the suitable material for the energy slab considered in this paper.