## 1 Introduction  
In the view of the current upward trend in electrical energy prices, there are growing concerns in all branches of industry and installations about ways to reduce the costs of energy consumers. These concerns aim at the large-scale use of energy from renewable sources and at improving existing facilities and equipment so that their energy consumption is reduced. Last but not least, the impact on the environment of the various installations must be taken into account both in terms of the emissions they release into the atmosphere during production and exploitation and in terms of the possibility of recycling the component materials at the highest possible percentage too. In the context of those listed above is also the topic addressed in this paper, namely the determination of an optimal thickness of insulation, so that the energy consumption is as low as possible, in the case of cold storage. By balancing the necessary costs for the initial investment to build the cold storage and estimating the necessary costs to ensure the refrigeration load, we established an optimal thickness of the insulation. This was performed for the cold storage placed in different climatic zones on the Romanian territory (according to the national regulations, in Romania there are five climatic zones), insulated with sandwich panels with PUR insulation. Many authors have recently shown interest in determining the optimal thickness of insulation for different applications or constructions in correlation with the climatic zones they are located in. [PERSON] et al. in 2019 [1] performed a study regarding the optimization of insulation thickness in buildings external walls based on a Life Cycle Costing (LCC) approach and analysed the building insulation's effects on annual heating and cooling loads in Palestine. [PERSON] in 2016 [2] determined the optimum insulation thicknesses, total net saving amount, reimbursement period change based on the fuels and insulation materials for 5 different provincial centers (Izmir, Istanbul, Ankara, Sivas and Erzurum) selected according to their degree/day values, placed in Turkey. [PERSON] et al. in 2020 [3] conducted a thermo-economic analysis for determining the optimal insulation thickness for new composite prefabricated wall blocks. Although the study is focused on Iran, it can be generalised to other countries. The above-mentioned examples reinforce the fact that the choice of the optimal insulation thickness is a present concern in the field of constructions as is an extremely important aspect to be considered when analysing the energy performance of the building envelope. This study begins to be widely used in other types of construction, such as cold storage containers or other enclosed spaces that require heating or cooling. [PERSON] et al. [4] highlights the importance of the insulation quality and thickness for an economical operation with minimal energy consumption for a cooled room. The authors developed a rating optimization model of various types of insulations. [PERSON] [5] performed an extended analysis on a cold storage placed in Turkey concerning the optimum insulation thickness effect on costs and savings. [PERSON]. [6] et al. developed a study to determine the optimum insulation thickness of cold storage walls for all climate zones in Jordan. Specific energy consumption values for various refrigerated food cold stores were evaluated by [PERSON] [7] and are considered as a reference. The authors of this paper contribute to the improvement of the current state of knowledge in the field by conducting a complex study on cold and freezing storage rooms placed in different areas in Romania, with the main output data being the optimal insulation thickness and a life cycle cost examination. The thermal insulation panels are according to the national regulations \"EN 14509: 2007 Self-supporting double skin metal faced insulating panels-Factory made products-Specifications\" [8] and they are characterized primarily by a low weight, good rigidity and resistance to mechanical shocks.