A divided-bar apparatus is deemed to be the most accurate method of measuring the thermal conductivity, λ (W/mK), of intact rock cores in the laboratory. The divided bar is a steady-state comparative method in which the temperature drop across a disk of rock is compared with that across a disk of standard material of known conductivity. Thermal conductivity test results obtained from rock cores can be used in software programs to determine the design requirements for any medium to large-scale ground-source energy system. This paper describes the design and development of a low-cost divided-bar apparatus and compares the values obtained to those achieved by previous researchers and those recommended by EED, a commonly used borehole heat exchanger design software program. The divided bar was designed in accordance with the following principles: keep construction costs low by using readily available materials, develop a simplistic operating procedure to promote continuity of use and cater for the testing of different sized rock cores. As there are currently no recognized testing standards available for operation of a divided-bar apparatus, the sample preparation procedure for samples tested on the UCD divided-bar apparatus was developed as a proposed standard testing procedure. The proposed procedure amalgamates the developments and suggestions of previous researchers in addition to published test procedures in Ulusay and Hudson [Ulusay, R. and Hudson, J. A., 2007, The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 1974–2006, ISRM Turkey], and could possibly contribute toward the development of a standardized procedure for testing on a divided-bar apparatus. The test results presented in this paper demonstrate a strong relationship between thermal conductivity and mineral composition with the effects of porosity also having a notable influence on the thermal conductivity of the tested rocks.