## 4 Results and Discussions
### The Influence of PCM or SIM Layer Thickness  
The PCM or SIM layer thickness not only affected the energy saving characteristics of green building roofs but also determined its incremental cost during the construction period and the corresponding building's comprehensive incremental benefit. In Figure 9, the effect of PCM or SIM layer thickness on the energy saving amount of a green building's roof is plotted. The energy saving characteristics of both PCM roof and SIM roof improve with the increment of their PCM or SIM layer thicknesses. The results indicate that the energy saving amount, \(\Delta W\), of a SIM roof increases almost linearly with respect to the increasing SIM layer thickness because of the linearly increased thermal resistance. contrary to the SIM layer, the increasing the rate of the energy saving amount, \(\Delta W\), of a PCM roof decreased in terms of the PCM layer thickness and its energy saving amount almost approached a constant value when the PCM layer thickness is large. To clarify the underlying mechanism of this situation, the average liquid fraction of a PCM layer during the summer cooling time is shown in Figure 10. It was found that the energy storage capacity of a latent heat PCM layer increased with its increasing layer thickness. For the PCM layer of \(D=20\) mm, the PCM became fully melted and lost the capability of controlling the roof temperature at 15:15 p.m. As a consequence, the interior roof surface temperature began to rise after 15:15 p.m., which increased the energy consumption of the PCM roof. In comparison, when the PCM layer of \(D=60\) mm was used, the inside roof temperature was modulated by the PCM layer until 21:00 p.m., which guaranteed that the PCM layer had the latent heat characteristic during the air conditioner working time so that the maximum energy saving amount was achieved. However, similar to the discussion in Section 4.2 for the effect of PCM melting temperature, the increment of the PCM layer thickness also had a negative influence on the energy saving capability of the PCM roof. As the environmental temperature became low during the nighttime, the latent heat function of the PCM layer at nighttime, after 18:00 p.m., maintained its temperature nearly at the solidification point, which prevented heat transfer from the internal roof region into the environment. As a consequence, due to the trade-off effect of the PCM layer thickness, the increasing rate of the energy saving amount of a PCM roof decreases. Besides, when the PCM or SIM layer thickness was \(D\leq 45\) mm, the energy saving amount of the PCM roof was more than that of the SIM roof because of the latent heat characteristic of the PCM layer, which attenuated the temperature oscillation inside the roof. However, it is interesting to note that the SIM roof had a better energy saving capability than the PCM roof when their layer thickness, \(D\), was more than 50 mm. The reason for this phenomenon is that a SIM roof with larger thickness could prevent the heat transfer from the environment into the interior room in the daytime and the interior SIM roof temperature could also be efficiently cooled  
Figure 8: The PCM roof interior surface temperature during the summer cooling season at different melting temperatures. down without the effect of latent heat during the nighttime, which makes the SIM roof have a different thermal performance compared to the PCM roof. In order to evaluate the comprehensive incremental benefit of a green roof building during the life cycle in a hot summer and cold winter region of China, the energy saving amount of a PCM roof and a SIM roof during the winter AC heating season should also be investigated. When the PCM layer with a melting temperature \(T_{m}=31\) was inserted into the green building roof, it was kept in the solid state without melting and solidification processes in Hanzhong city during the winter AC heating time.