Vacuum insulation panels - From research to market

Vacuum insulation panels (VIPs) have a thermal resistance about a factor of 10 higher than that of equally thick conventional polystyrene boards. Similar to thermos flasks these systems make use of 'vacuum' to suppress the heat transfer via gaseous conduction. While thermos flasks are to be pumped down to a high vacuum, filling material integrated in the flat VIP elements, which bears the atmospheric pressure load, reduces the requirements on the vacuum and thus on the tightness of the vacuum casing. Optimal in this respect is a kernel of fumed silica. This kernel is evacuated to below 1 mbar and sealed in a high-barrier laminate, which consists of several layers of Al-coated polyethylene (PE) and polyethylene terephthalate (PET). The laminate is optimized for low air and moisture leakage rates and thus for a long service life. The evacuated silica kernel has a thermal conductivity of about 0.004 W m(-1) K-1 at room temperature, mainly resulting from solid thermal conduction along the tenuous silica backbone. As the kernel is nanoporous, the gaseous thermal conductivity becomes noticeable only for pressures above 10 mbar. At about 200 mbar the thermal conductivity measures about 0.008 W m(-1) K-1. Such a gas pressure could occur after several decades of usage in a middle European climate. With VIP, slim yet highly insulating facade constructions can be realized. A centre U-value of 0.2 W m(-2) K-1 can be achieved for a VIP thickness of only 2 cm, if optimized kernels and barrier laminates as well as stringent quality control are employed. A successful self-trial using VIPs within a facade of the ZAE-building in Wurzburg in 1999 was the starting point for new applications of evacuated insulations in the building sector. (C) 2007 Published by Elsevier Ltd.