# Superinsulation Materials for Energy-Efficient Train Envelopes
## 5 Discussion
### Application Aspects  
Taking into account the differences for the application of VIPs and silica aerogel boards in terms of their handling, thermal conductivity and cost per insulation performance as described in the Results section, the following recommendations can be given. If there are no space constraints, conventional materials should be used due to their lower cost and lower embodied energy and lower global warming potential. If space is constrained and a higher performance is desired, for all large, repetitive and regular areas inside the envelope of a train, VIPs are the preferred superinsulation materials due to their better performance and their cost efficiency. Given the right dimensional tolerance for the application area, VIPs can be manufactured to size and easily installed. Care should be taken during installation that the VIPs are not damaged. Quality control of the installation can be performed by subsequent thermal imaging to identify compromised VIPs in the train's envelope. For the insulation of areas that are smaller or not regular (e.g., not rectangular or of varying shape), aerogel boards should be used. These can be drilled and cut according to the needs, either by the manufacturer or on the installation site. Hence, aerogel boards can be used with much higher flexibility. Aerogel boards can also be used to \"frame\" VIPs if dimensional tolerance on the areas to insulate are high or if there are insulation areas of slightly varying size. For that, a VIP is installed in the middle of the area, and pieces of aerogel board are cut to size to go around it. Since it is more costly to achieve the same thermal performance with a superinsulation material compared to a conventional one, superinsulation materials should only be used if there is an additional benefit due to the space savings achieved with them. Whether these space savings are worth the additional cost needs to be decided individually for specific applications. For example, if an additional row of seats or wider seats for more travel comfort are possible due to the space savings, the economic benefits of carrying more passengers or charging a higher ticket price can outweigh the cost premium of superinsulation materials. One can also get an idea of the financial and environmental impact of using superinsulation materials in trains by estimating the energy savings compared to conventional insulation. As an example, a typical U-value reached with about 20 mm of conventional melamine resin foam insulation would be 2.0 W/(m\({}^{2}\cdot\)K), taking into account thermal bridges in the train envelope (as presented in Table 5, an \"unperturbed\" wall without thermal bridges would have a U-value of 1.32 W/(m\({}^{2}\cdot\)K) for an insulation material with a thermal conductivity of 32 mW/(m\(\cdot\)K); so, the difference to the U-value of 2.0 W/(m\({}^{2}\cdot\)K) is due to thermal bridges, which are more frequent in trains than buildings due to constructive reasons.). A conservative assumption for using VIPs instead of conventional insulation would be a U-value of 0.8 W/(m\({}^{2}\cdot\)K) and a U-value of 1.4 W/(m\({}^{2}\cdot\)K) for using silica aerogel boards instead of conventional insulation. The difference of these U-values and the one for conventional insulation multiplied by the heating degree days for the operating region of the train is equal to the yearly energy savings. Heating degree days can be calculated by summing the difference between the target inside and mean daily outside temperatures, counting each day in a year on which the mean outside temperature falls below a defined base temperature. In Zurich, for example, the mean heating degree days for buildings over the time period of 2011 to 2020 are 3125\({}^{\circ}\)d, with respect to the base temperature of 12 \({}^{\circ}\)C [46]. Since trains are not operative all day--and hence, do not need to be heated all day--a conservative value for train heating degree days is to take half of the value for buildings.