## 4 Discussion  
The similar experiments were performed and summarized by authors [42], but the benefits of our research are results specified for the Central European region, which have not been published yet. They measured the effect of the photovoltaic system installation on the roof to the temperature in the Powell Structural Laboratory in San Diego, California. They used a similar experimental set up as in our research. The temperature differences were also detected by thermal IR camera Fluke TiR1 (Fluke, Washington USA). After the comparison between our results and values in ref. [42] is clear, that ceiling under the roof without photovoltaic module had higher temperature. Ref. [42] presents the time temperature dependencies of ceiling temperatures under the exposed roof and a flat, tilted photovoltaic array. The difference between exposed roof and flat photovoltaic array is in the temperature peak approximately 2 \({}^{\circ}\)C. Results from temperature measurement declared the temperature difference during the temperature culmination was 7.7 \({}^{\circ}\)C. The higher value of temperature difference can be caused by the different roof construction. The model roof had no roof insulation (the authors of [42] had a 200 mm layer of concrete). The authors of [43] performed similar measurements, and they measured indoor temperature under the fiber-cement roof and photovoltaic roof. They obtained approximately the same values in the time range from 8 a.m. to 12 p.m. From the results, it is evident that that the difference between the temperatures inside was between 1.1 \({}^{\circ}\)C-1.2 \({}^{\circ}\)C.  
Our experiment confirmed the same results, that the temperature inside is higher under the roof without installed photovoltaic modules (Figure 19). Average indoor temperature with roof without photovoltaic module was 23.9 \({}^{\circ}\)C. For comparison, we selected the day with a polycrystalline module on the roof. It was for this reason that this day had a closer average ambient temperature. Average indoor temperature with roof with photovoltaic module was 22.8 \({}^{\circ}\)C. The difference between these two values was 1.1 \({}^{\circ}\)C, the same value of temperature difference was presented by authors [43]. Figure 19: The comparison of temperatures under the roof and under photovoltaic module. The heat stress problem is acutely felt in the Central European countries. The weather of these countries is characterized by moderate to high summer temperatures coupled with moderate humidity levels [44; 45]. Hot weather causes heat stress in dairy cows, leading to declines in milk production each summer. These declines can be reduced or eliminated by using open barns for optimum milk production [46]. The heat stress problem is getting worse as production levels continue to rise [47; 48]. The basic condition of management in dairy farms consists in understanding what factors affect milk production the most, i.e., with the exception of nutrition and dairy cow health status, also the parity and season of calving, technological systems, and especially microclimatic conditions [49]. Livestock performance is affected by heat stress because an animal having difficulty in losing heat will decrease its heat production by lowering feed intake [32; 50]. The upper critical air temperature for lactating cows is in the range of 24 to 27 \({}^{\circ}\)C [23]. However, critical temperatures will vary depending on several factors including degree of acclimization, rate of production, pregnancy status, air movement around the animals, and relative humidity [18; 51]. The effect of using the cooling system on thermal comfort was studied by [PERSON] et al. (2012) [52]. The facts presented above are in good agreement with the summary of experimental results obtained in the real cowshed. There were calculated values of the Temperature--Humidity--Index (THI) for the real cowshed without PV modules installed on the roof, and also with PV modules placed on the roof.