## 4 Discussion
### Effect of the Construction Geometry and Use of Innovative Insulation Material  
As anticipated in the preliminary study [14], thermal bridges originated at the door edge play a key role in determining its fire resistance. To fully appreciate this effect, Figure 10a reports the decomposition of the heat flow along the Cartesian directions calculated for the door presented in the previous study [14] with edges of 5 mm. The heat flow along the x-direction, which represents the thermal bridge, is even higher than the heat flow passing through the door along the y-direction. On the basis of the results obtained in the previous work, a different geometry of the door has been presented, using thin steel plates (1 mm) for the edge as a way to minimize the thermal bridge. The decomposition of the heat flow reported in Figure 8 fully demonstrates the validity of this solution, along with the results of the FTP test. An important observation is the difference in the pattern of the transversal temperature profile at the final stage of the fire test (60 min) observed in Figures 9 and 10b, respectively, for the door presented here and that of the preliminary study. The trend of the profile shown in Figure 9 clearly suggests that the effects of the thermal bridge reach the location of the measuring points, at a relative distance of 0.25, 0.50 and 0.75. On the contrary, Figure 9 reveals an almost constant temperature profile for relative distances between 0.2 and 0.8. Thus, the heat transfer at the measuring points is essentially affected by the heat flow passing through the door, the other contributions being irrelevant, as shown in Figure 8.  
Figure 10: \(Cont.\)  
In summary, by reducing the thickness of the sheets, and specifically the thickness of the edges, it is possible to minimize the effect of the thermal bridge and build a thin and light door. Specifically, the results indicate the feasibility of a door 37% thinner and 61% lighter than the door reported in the preliminary study [14]. Moreover, the thermal performance is better since temperatures as low as 100\({}^{\circ}\)C could be measured in the FTP fire-test. Keeping in mind that the FTP test limit is an average temperature of 140\({}^{\circ}\)C above room temperature (18 \({}^{\circ}\)C in this case), this means that there is a \"reservoir\" of nearly 60\({}^{\circ}\)C before the FTP limit is reached; clear evidence that a very efficient thermal insulating capability may not be the most important criterion for the choice of the insulating material to be used within the door. This observation opens new perspectives in the design of fire-doors as far as the use of novel/innovative insulation materials is concerned. As observed in the introduction section, soundproofing is another important issue on onboard ships, particularly passenger ones [35; 36; 37; 38]. Generally speaking, cellular foam materials are extensively employed as insulating materials, as their structure can be optimized for both thermal and acoustic performance [38]. Open pore cellular foams feature better soundproofing properties compared to closed pore ones [38]. Incidentally, the insulator used in this paper presents an open cell microstructure, as shown in Figure 3b, which features significantly better soundproofing capability compared to rock-wood [14]. The results show that it is classifiable as an incombustible material, which is a major requirement for use in fire doors. It is also lighter in comparison to the rock wool and features better mechanical properties (e.g., no loose fiber), however, its thermal insulation capability is poorer. Despite the approximately 30% higher thermal conductivity compared to rock-wood, the numerical analysis of the door equipped with the foam gives a maximum final temperature of 120 \({}^{\circ}\)C on the measuring points, well below the limits imposed by FTP fire-test [1].