# Superinsulation Materials for Energy-Efficient Train Envelopes
## 2 Screening of Insulation Materials  
In order to find suitable insulation materials for the insulation of a train envelope, all commercially available superinsulation materials were identified. This was done through internet research as well as expert consultation. When there were several products of the same material type, e.g., silica aerogel boards, the product with the most suitable properties for the application in trains was selected. The superinsulation materials found in our research were vacuum insulation panels (VIPs), silica aerogel boards, polyurethane (PU)aerogels, fumed silica boards as well as polyisocyanurate (PIR) foams. The properties of the most suitable product in each of these material categories are listed in Table 1, and some materials are shown in Figure 1.  
Vacuum insulation panels consist of an open pore core material, most commonly fumed silica or glass wool, and a metallised, air- and vapour-tight envelope that is evacuated during production. The thermal conductivity of VIPs right after production can be as low as 2-4 mW/(m\(\cdot\)K) at the centre of the panel. Taking into account their \"aging\", i.e., the decreasing of the vacuum quality over time, as well as thermal bridging via the panel edge, official declaration values for VIPs with pyrogenic silica core in Europa are typically 7 mW/(m\(\cdot\)K). This value is a mean value over the panel's assumed lifetime of over 25 years and likely up to 50 years, which should not be exceeded within the first 12 years [26]. Vacuum panels are vapour tight and come in fixed sizes. They are the superinsulation materials with the highest insulation performance [27]. Silica aerogel insulators are nano-porous materials, with open pores and thermal conductivities in the range of 14 to 20 mW/(m\(\cdot\)K) [26; 28]. Compared to VIPs, the handling of such fibre-reinforced aerogel boards is more similar to mineral wool, i.e., relatively flexible. Silica aerogel is hydrophobic; hence, its humidity uptake is minimal, making it a suitable insulation material in areas that could be exposed to high humidity. There is also a diffusion-open, rigid polyurethane aerogel board, which is not as brittle as silica aerogels [29]. In the last few years, polyisocyanurate foam insulation (PIR) boards have reached lower and lower thermal conductivities by the use of special filler gases, so that some products are in the thermal superinsulation range, with the best commercially available materials reaching thermal conductivities down to 18 mW/(m\(\cdot\)K). PIR boards are diffusion-closed and can be cut and handled easily. They are significantly cheaper than both aerogels and VIPs. Finally, fumed silica powder can be pressed into board shape, reaching a thermal conductivity of 19 mW/(m\(\cdot\)K) [30]. \begin{table}
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One reservation with silica aerogels and VIPs is their significantly higher density compared to conventional insulators, cf. Table 1. This is a disadvantage, as train manufacturers try to minimise the weight of the trains in order to reduce traction energy needs. Conventional insulation materials such as EPS, mineral wool or melamine resin foam were not considered here, as their thermal conductivities are not in the range of thermal superinsulation, more specifically, above 30 mW/(m\(\cdot\)K). In order to make trains more comfortable and energy-efficient in the future, it is crucial to use space-saving insulators, i.e., thermal superinsulation. The most important properties of the materials apart from their thermal conductivity are fire safety, mechanical stability and workability for an efficient installation. Fire safety was an exclusion criterion in this case. The fire properties of the screened materials are classified in Table 1 according to the building material standard EN 13501-1 [25], as they are mainly used for building insulation. Materials with a high contribution to fire according to this standard--an E-classification--were not considered likely to pass the train-specific fire tests.