The optimum insulation thickness for external walls of different orientation of a piggery building was determined accounting for both the heating and the cooling period. Two types of composite, thermally insulated walls were selected. Walls were either constructed from plaster in the interior, brick, extruded polystyrene and plaster in the exterior (PBEP) or from polyurethane foam sandwich panels (PSP) of various thickness. Annual heating and cooling transmission loads were calculated based on transient heat flow through the external walls and for hourly climatic data of the region of Athens, Greece. An economic analysis based on the Life Cycle Savings (LCS) method was performed for both type of walls and for various insulation thicknesses and different orientations. The optimum insulation thickness was found to be 1.5 cm and 2.5 cm for the PBEP and the PSP wall, respectively and was orientation independent. The PSP wall offers the greatest economic benefit and has the shortest payback period, compared to the PBEP wall, for all orientations and insulation thicknesses. (C) 2014 Elsevier B.V. All rights reserved.