# Superinsulation Materials for Energy-Efficient Train Envelopes
## 5 Discussion  
### Thermal Conductivity Measurements  
The thermal conductivity measurements confirm the good declared thermal performance of VIPs and silica aerogel boards. They are the two best commercially available thermal superinsulation materials [26]. Due to the organisation of this project, the thermal conductivity measurement was done on another product (Vacupor NT-B2-S) than the fire and vibration tests (va-Q-vip). However, the thermal conductivity of the second product is comparable to the first, with a thermal conductivity of 4.2 mW/(m\(\cdot\)K) measured at Empa previously (unpublished data). For the VIPs, one should keep in mind that the vacuum degrades over time and with that their thermal conductivity increases. Hence, one should use a value of 7 mW/(m\(\cdot\)K) for calculations of thermal transmittance. The fact that our results for the silica aerogel board product are slightly higher than the declaration value might be due to different measurement conditions. Here, the sample was mechanically supported at its corners, so that not the full weight of the measuring device would rest on it. This is important, since compression can lead to lower thermal conductivities for silica aerogels. We decided to measure without compression, since it corresponds better to the planned application: as a board on a wall under no compression. The results for the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Global Warming** & **Non-Renewable** & **Clobal Warming** & **Non-Renewable Primary Potential for 1 m\({}^{2}\) & **Non-Renewable Primary Energy for 1 m\({}^{2}\)** \\  & **(kg CO\({}_{2}\)-eq./kg)** & **(kWh/kg)** & & **(kg CO\({}_{2}\)-eq.)** & **(kWh)** \\ \hline Vacupor & 9.24 & 43.09 & 190 & 12.3 & 57.3 \\ \hline Spaceloft, A2 & 6.25 & 33.61 & 200 & 23.8 & 127.7 \\ \hline melamine resin foam & 8.02 & 34.00 & 10 & 2.6 & 10.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of environmental product declaration for Porextherm Vacupor VIP [42], Aspen Spaceloft A2 aerogel blanket [43] and melamine resin foam [44] and performance-based global warming potential and non-renewable primary energy. VIPs furthermore show that the thermal conductivity values of the VIP samples before and after the test installation in a train were unchanged, considering the error of the measurement. This indicates that the installation and use in a train--at least short term--do not compromise the envelope of the VIP due to vibration or other mechanical loads. In order to better understand the implications of these results for the thermal properties of a train envelope, one can consider a simplified example of a train wall, consisting of an outside and inside metal layer sandwiching the thermal insulation, without considering any thermal bridges--which are frequent in train walls. Since the metal layers do not significantly contribute to the thermal resistance of such a wall, one can neglect them. The same holds for the external thermal surface resistance, due to the speed of a moving train. Hence, the thermal resistance, R, of such a wall is approximately R = R\({}_{\text{ins}}\) + R\({}_{\text{si}}\), with R\({}_{\text{ins}}\) = d/\(\lambda\) being the resistance of the insulation layer equal to the insulation layer thickness divided by its thermal conductivity and R\({}_{\text{si}}\) being the internal thermal surface resistance, which is set here to 0.13 m\({}^{2}\)-K/W as for buildings. The U-value of the simplified wall would be the inverse of the thermal resistance R, U = 1/R = 1/(R\({}_{\text{si}}\) + d/\(\lambda\)). The values of U and R are shown in Table 5 for a typical insulation thickness in trains of 20 mm and three different thermal insulation materials: VIP, silica aerogel board and conventional thermal insulation material (e.g., melamine resin foam). This simple approximation shows the large improvement that superinsulation materials can bring for a train envelope.