Investigation on Longer Service Life of Vacuum Insulation Panels by Applying Double Envelopes

The conversion of existing buildings to ZEB requires high thermal insulation. Vacuum insulation panels (VIPs) have a potential to be used for retrofitting thermal insulation of existing buildings from the viewpoint of low thermal conductivity, small thickness, and light weight. However, in order to realize vacuum insulation panels applicable to building insulation, it is necessary to overcome the challenge of maintaining the low pressure at which the high insulation performance can be achieved for several decades. The authors applied the double envelopes to VIPs and investigated longer service life of VIPs. The double envelope VIP is fabricated by fabricating a normal VIP, then wrapping the VIP with core materials, and then inserting it into the envelope for vacuum sealing. By applying the double envelopes, the pressure difference and the permeation of gas in inner VIP can be drastically reduced, resulting in a longer VIP lifetime. In this paper, the gas permeabilities of the gas barrier envelopes were experimentally estimated. Then, the pressure ageing in the double envelope VIP is calculated. The inner pressure after 25 years was 1.6 Pa for the envelope of aluminium composite film and 59.3 Pa for the envelope of transparent gas barrier film, indicating that long-term insulation performance could be improved. Next, the authors carried out acceleration test, in which the double envelope VIPs were put in a chamber at 80oC and remove once a month to measure the thermal conductivity. Even after 136 weeks, one sample showed only a 9.1% decrease in thermal resistance. From this result, it was confirmed that long-term insulation performance can be greatly improved by fabricating double envelope VIPs with the getter agent inside and the desiccant agent outside.