The energy slab encases heat exchange pipes as a heat exchanger inside a slab structure of a building to utilize the geothermal energy for heating and cooling buildings. In the energy slab, heat exchange is induced with the surrounding ground by circulating a working fluid through the heat exchange pipes. The heat exchanger should be arranged with a horizontal layout, which are adjacent to underground space, to install in the energy slab. Therefore, when the heat exchanger is assembled in the energy slab, it is important to thermally insulate the heat exchanger from adjacent indoor air, because air temperature inside the underground building space significantly affects the thermal performance of energy slab. In this study, thermal performance of the energy slab was experimentally evaluated with consideration of a thermal insulation layer. Then, an optimal thermal insulation material was proposed from the results of parametric study of CFD analysis. First, two field-scale energy slabs (i.e., floor-type energy slab and wall-type energy slab) consisting of both the thermally insulated and non-insulated slabs were constructed in a test bed. In order to evaluate the effect of air temperature on the thermal performance, a series of Thermal Performance Tests (TPTs) was carried out with consideration of existence of a thermal insulation layer. A CFD model was then developed by calibrating with the TPTs results to estimate the effect of thermal conductivity of the thermal insulation material. The thermal insulation layer can relieve the effect of surrounding thermal environment. Moreover, the thermal performance of energy slabs increased as the thermal conductivity of the thermal insulation material decreased. Note that thermal insulation materials with the similar thermal conductivity to a Phenol Foam (PF) board is most efficient for thermally insulating the energy slab.