This study deals with the investigation into optimum insulation thickness of installed inside building pipe network of VRF (variable refrigerant flow) systems. Optimum insulation thickness, energy savings over a lifetime of 10 years and payback periods are determined for high pressure gas pipelines, low pressure gas pipelines and low pressure liquid pipelines under the heating-only and cooling-only modes of the three-pipe VRF system using R-410A as refrigerant. By using the P-1-P-2 method, the value of the amount of the net energy savings is calculated. Under heating mode of VRF system, while the optimum insulation thickness varies between 16 and 20 mm depending on the pipe sections of high pressure gas pipeline, it varies from 11 to 13 mm for the pipe sections of low pressure liquid pipeline. Under cooling mode of VRF system, the optimum insulation thickness varies between 7 and 8 mm for pipe sections of low pressure gas pipeline and low pressure liquid pipeline. (C) 2015 Elsevier Ltd. All rights reserved.