# Effect of Cavity Vacuum Pressure Diminution on Thermal Performance of Triple Vacuum Glazing
## 1 Introduction
The second successful fabrication method of double vacuum glazing used indium or an indium alloy, which was sealed in a low-temperature process, at about 160 \({}^{\circ}\)C. It required a secondary adhesive seal to prevent moisture access [13]. A low-temperature sealing process enables the use of low-emittance soft coatings, reducing radiative heat transfer between glass panes, and annealed glass allows to increase support pillar spacing, reducing conductive heat transfer [14]. The problem with the low-temperature fabrication method is the use of semiprecious indium alloys. It is a hindrance in advancing indium-sealed vacuum glazing at a commercial level [15]. Recently, a new low-temperature composite material and method of fabrication were reported by [PERSON] et al. [16], in which the CS-186 type of Cerasolzer and steel-reinforced epoxy resin [17] were used for the development of composite edge-sealed triple vacuum glazing. Despite the aforementioned successful constructions of double and triple vacuum glazing, there is always an uncertainty of the degradation/leakage of the cavity vacuum pressure. It is also pertinent that during the evacuation process or after sealing the pump-out tube of the TVG, there is a propensity of some gas molecules remaining in the cavity to react when exposed to sunlight and/or under extreme climate conditions for longer time periods due to the production of carbon monoxide inside the cavity that degrades the vacuum layer. It is obvious that TVG is exposed to sunlight and thus must be designed to withstand different climates and temperatures in order to avoid degradation of the vacuum. The heat conductance and internal pressure due to ageing (for over 250 days) of a vacuum glazing, with a temperature increased from ambient (21 \({}^{\circ}\)C) to 150 \({}^{\circ}\)C, was reported by [PERSON] and [PERSON] in 1997 [18]. It showed that both pressure inside the cavity and heat conductance rose upon heating. It identified that the increase in the thermal conductance from the glass bulk due to the water vapour evolved into the gap was negligible; it was less than 0.1 Wm\({}^{-2}\) K\({}^{-1}\) over a period of 25 years at 30 \({}^{\circ}\)C for the samples baked out at about 180 \({}^{\circ}\)C for 1 h. The result from mass spectroscopy showed that 95% of the total remaining gas on the internal glass surfaces is water vapour, whereas the remaining 5% includes H\({}_{2}\), H\({}_{2}\)O, CO/N\({}_{2}\), O\({}_{2}\), and CO\({}_{2}\). [PERSON] et al. [19] investigated the extreme temperature cycling effect, for which the heat conductance at the centre-of-pane was found to be increased by about 10.2% (1.18 Wm\({}^{-2}\) K\({}^{-1}\) to 1.30 Wm\({}^{-2}\) K\({}^{-1}\)) when the cavity vacuum pressure inside the vacuum glazing had increased from 0.1 Pa to 0.16 Pa. [PERSON] et al. [20] conducted an investigation of gases evolved into the evacuated gaps in vacuum glazing, in which the samples of vacuum glazing were aged by exposing them to high temperatures and sunlight for a long period of time, and their internal pressure and the existing gases were studied using a vacuum gauge and a mass spectrometer, respectively. However, there is no literature available on the influence of cavity vacuum pressure diminution on triple vacuum glazing (TVG). Thus, this paper reports the effects of vacuum pressure on the thermal transmittance of TVG by developing a mathematical model using the finite element method in predicting their thermal performance. This paper contributes to the new understanding of the effects of cavity vacuum pressure diminution on the thermal performance (heat-transfer characteristics) of triple vacuum glazing. In this paper, a 3D finite element method with details of the governing equations utilised and modelled heat-transfer mechanisms of the 10-mm wide edge-sealed triple vacuum glazing is reported.