## 4 Discussion  
The European strategy for reducing energy consumption in buildings sets directions for increasing energy efficiency throughout the life cycle of buildings. The EU is dedicated to developing a sustainable, competitive, safe, and decarbonized energy system to reduce greenhouse gas emissions and save energy in line with existing directions at the Union level [28]. The main purpose of this paper was to determine the OIT value together with the amount of energy savings when using a PUR panel sandwich for cold storage at 0 \({}^{\circ}\)C and frozen storage at \(-23\)\({}^{\circ}\)C in different Romanian cities chosen as representatives of the five climate zones. Heat transfer through building parts represented a maximum of 15% from total cooling load for cold storage and 31% for frozen storage, which is higher due to the increase in the temperature difference between and inside air temperature. Therefore, only in the case of cold storage, as a peculiarity for Romania, the main influence in determining the optimal thickness of the insulation layer is that of electricity, the influence of the climate zone being small. The technical-economic analysis highlights the fact that in Romania, although there are 5 climatic zones, with different maximum average temperatures in summer, this does not cause a difference in OIT values. Subsequently, OIT for cold storage is 150 mm and 180 mm for frozen storage, respectively. On the one hand, this aspect can be caused by the maximum outside temperatures, which are not extremely different from each other (the maximum temperature difference being of 9 \({}^{\circ}\)C). On the other hand, as regards the electrical energy price, this is different in these 5 climatic zones (different energy suppliers). The increase in the investment cost for insulation thicknesses from 100 mm to 150 mm is about 25% and the difference regarding the energy cost is about 13%. The price is higher in climate zones III, IV and V, compared to the climate zones I and II. The energy saving and payback periods were determined for each one of the five climatic zones. The total cost decreases with insulation thickness, increasing from 0.03 m up to OIT, then it decreases. The choice of an insulation thickness higher than the OIT is not justified, because the energy saved (electricity costs) is not significant. Thus, in the case of cold storage, if it is chosen, an insulation thickness of 180 mm, which is higher than the OIT, the investment cost increases by 41%, and the electricity energy costs decrease by 8.3%. The same trend is registered in the case of frozen storage, if it is chosen, where there is an insulation thickness of 200 mm, higher than the OIT, and the investment cost increases by 20% and the electricity energy costs decrease by 6.7%. However, most of the time, the investors decide on insulation thicknesses smaller than OIT for the cold storage, which involves lower initial investment costs, accepting higher electricity costs. For example, in the case of cold storage, when adopting an insulation thickness of 120 mm, which is lower than OIT, the investment saving decreases by 21.45%, but the operating costs will increase by about 11%. If the investors adopt an insulation thickness higher than the OIT (160 mm), the investment costs are higher, but the operating costs are recovered over a longer period. Thus, for the climate zones I and II the payback time is over 6 years and for the other climate zones III, IV and V, they are close to 2 years. It can be generalized that climate zones with higher outside temperatures will have longer payback periods with lower energy savings at 0 \({}^{\circ}\)C reference inside temperatures in comparison with colder climates. In the case of frozen storage, the authors obtained a payback time between 3.8... 5.6 years for an insulation thickness of 200 mm higher than the OIT (180 mm).