Presented in this article are the results of the theoretical research demonstrating the need to consider the changes in the ground heat transfer properties in geothermal borehole heat modelling because of moisture condensation/evaporation in the ground pores. It is our opinion that in the GSHP system design, the quantity of the boreholes is often overestimated and the associated parameters are oversized, while the extent of the ground heat transfer is underestimated. In most cases, the operation and performance of the GSHP design are caused by an incorrect assessment of the ground moisture content, which has the most tangible effect on the ground heat transfer properties. This article demonstrates the need to consider the ground moisture condensation/evaporation in the GSHP system design. Presented in this article is the mathematical simulation of the ground pore moisture condensation at the GSHP boreholes. Additionally, the numerical data derived from the calculations are presented to assess the effect of the ground pore moisture condensation on the borehole heat transfer efficiency. Through analysis and experimentation, it was determined that the ground pore moisture condensation has a substantial impact on the GSHP efficiency. (C) 2014 Elsevier B.V. All rights reserved.