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In porous materials, such as foams or pressed powder boards, the thermal conductivity via the gas phase represents at ambient temperatures a significant or even dominant contribution to the total thermal conductivity. The variation of the thermal conductivity of an open porous material with gas pressure is a function of the pore size. In the pressure range below 1 bar, measurement of thermal conductivity offers a non-destructive probe of pore sizes larger than 100 nm. If the thermal conductivity setup also allows for measurements at higher pressures, the pore size range can be extended to even smaller values.Experimental data for a variety of different porous materials such as open porous foams, granular porous and nonporous materials and aerogels, porous solids with an adjustable mean pore size in the meso-or macro-pore range, are presented. The pore characteristics determined from thermal conductivity measurements are compared to morphological data derived from nitrogen sorption and small angle X-ray scattering data.The potential and limits of this method in terms of the extraction of an average pore size and a pore size distribution are discussed.

For vacuum insulation panels (VIPs) with fumed silica kernels and foils as cover, a calculation model is developed to predict the service life. It is defined as the period during which the thermal conductivity of the VIP has risen 50% due to infusion of air and moisture. Two panel sizes, 50 Â 50 Â 1 cm 3 and 100 Â 100 Â 2 cm 3 are considered. For VIPs with laminated aluminum foils, calculated service lives of many decades are determined. For VIPs with aluminum-coated multilayer foils, shorter service lives still above 20 are calculated. This is due to the higher water vapor transmission through the Al-coated multilayer foils (compared to laminated Al foil) and the humidity-related increase in thermal conductivity. Overall, our model predicts service lives, which are large enough for applications of VIPs in buildings. An open question that remains is the long-term stability of the foil cover.

Vacuum insulation panels (VIPs) have a thermal resistance that is about a factor of 10 higher than that of equally thick conventional polystyrene boards. VIPs nowadays mostly consist of a load-bearing kernel of fumed silica. The kernel is evacuated to below 1 mbar and sealed in a highbarrier laminate, which consists of several layers of Al-coated polyethylene (PE) or polyethylene terephthalate (PET). The laminate is optimized for extremely low leakage rates for air and moisture and thus for a long service life, which is required especially for building applications. The evacuated kernel has a thermal conductivity of about 4 × 10 −3 W · m −1 · K −1 at room temperature, which results mainly from solid thermal conduction along the tenuous silica backbone. A U-value of 0.2 W · m −2 · K −1 results from a thickness of 2 cm. Thus slim, yet highly insulating façade constructions can be realized. As the kernel has nano-size pores, the gaseous thermal conductivity becomes noticeable only for pressures above 10 mbar. Only above 100 mbar the thermal conductivity doubles to about 8 × 10 −3 W · m −1 · K −1 , such a pressure could occur after several decades of usage in a middle European climate. These investigations revealed that the pressure increase is due to water vapor permeating the laminate itself, and to N 2 and O 2 , which tend to penetrate the VIP via the sealed edges. An extremely important innovation is the integration of a thermo-sensor into the VIP to nondestructively measure the thermal performance in situ. A successful "self-trial" was the integration of about 100 hand-made VIPs into the new ZAE-building in Würzburg. Afterwards, several other buildings were super-insulated using 1 1124 Fricke, Schwab, and HeinemannVIPs within a large joint R&D project initiated and coordinated by ZAE Bayern and funded by the Bavarian Ministry of Economics in Munich. These VIPs were manufactured commercially and integrated into floorings, the gable façade of an old building under protection, the roof and the facades of a terraced house as well as into an ultra-low-energy "passive house" and the slim balustrade of a hospital. The thermal reliability of these constructions was monitored using an infrared camera.

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