Permeation of Different Gases Through Foils used as Envelopes for Vacuum Insulation Panels

Abstract

Vacuum insulation panels (VIPs) are distinguished by their outstandingly low thermal conductivity. In the evacuated state, the VIPs being examined in this study (which have fumed silica as a core material) have a thermal conductivity of 4 10 3 W/(m K). Gases (N2, O2, H2O,...), which penetrate the foil cover cause an increase in pressure and water content and hence, an increase in the thermal conductivity. To determine these increases, VIPs have been manufactured with laminated aluminum foils (AlF) and aluminum-coated multilayer foils (MFs). The pressure and mass increases are determined at various temperatures, humidity, and with various panel formats. Large differences in the rates of pressure increases (1 -70 mbar/yr) and in the rates of mass increases (0.02-4 mass%/yr) are recorded, depending on the foil type, climatic conditions, and panel formats. From these measurements, the air and vapor transmission rates of the foil covers and their dependence on temperature, relative humidity, and panel size are derived. Using these gas transmission rates, it is possible to estimate which pressure increases are to be expected for panel formats and climatic conditions occurring in building applications. With laminated Al foils and selected Al-coated multilayer foils, rates of pressure increases below 1-2 mbar/yr are achieved. The rates of mass increase for typical climatic conditions for laminated Al foils are significantly below 0.1 mass%/yr, while with Al-coated multilayer foils, depending on the foil quality, mass increases per time of up to 1 mass%/yr are recorded. Increases in gas pressure per time of 1 -2 mbar/yr lead to relatively small increases in thermal conductivity, allowing applications in the construction sector, where service lives of several decades are required. With respect to the humidity-related increase in thermal conductivity, one has to know the climatic conditions, which have a strong influence on the increase in mass, and, above all, the precise dependence of the thermal conductivity on the humidity in the VIP.