"Ground is the best isolator" is only a misconception that persists for a long time. The truth is quite different, because soil temperature, depending on depth, season and location, can range from 2oC to 13oC. Naturally this applies to depths in which are buildings usually based.
Therefore, with the standards for energy efficient buildings, it is also stipulated to install thermal insulation to the ground and to pay attention on thermal bridges. The heat conductivity of the envelope to the ground, for energy efficient buildings should not exceed 0.4 w / (m2K), while for the Passive House standard this value should not exceed 0.15 w / (m2K).
How, as well as what kind of insulation will be placed under the building and under the foundations must be a subject of separate analysis, but for structural reasons, it should be left to an expert team.