# Effect of Cavity Vacuum Pressure Diminution on Thermal Performance of Triple Vacuum Glazing
## 4 Conclusions  
Long-term durability of the vacuum edge seal has been the paramount requirement, specifically, in the evolution of TVG for the potential retrofitting to existing buildings in achieving progress towards a ZEB (Zero energy Building) target. For this, it has long been estimated, but never been reported quantitatively, the influence of the diminution of vacuum pressure on the thermal performance of TVG. In this study, a 3D finite element model with validated mathematical methods was utilised for the assessment of the influence of vacuum pressure diminution on the thermal performance of a 10-mm wide edge-sealed triple vacuum glazing. The finite element model was validated with mathematical methods and by comparing the results with those in the literature. \(U_{c}\) and \(U_{t}\) values calculated from the current model were in good agreement with those measured by [PERSON] et al. [14] and [PERSON] et al. [33]. The \(U_{c}\) and \(U_{t}\) values in the FEM model of TVG were predicted to be 0.28 Wm\({}^{-2}\) K\({}^{-1}\) and 0.94 Wm\({}^{-2}\) K\({}^{-1}\) at the cavity vacuum pressure of 0.001 Pa. The results suggest that when increasing the cavity pressure from 0.01 Pa to 100 kPa, the \(U_{c}\) and \(U_{t}\) values increased from 0.28 Wm\({}^{-2}\) K\({}^{-1}\) and 0.94 Wm\({}^{-2}\) K\({}^{-1}\) to 2.4 Wm\({}^{-2}\) K\({}^{-1}\) and 2.58 Wm\({}^{-2}\) K\({}^{-1}\), respectively. This indicates that when a decrement of vacuum pressure occurs, it will make the thermal performance inadequate, and the use of nonevaporable getters would resolve the degradation issue. It is concluded that the glass surface temperature gradients between the hot and cold sides increases by decreasing the cavity vacuum pressure from 50 kPa to 0.001 Pa. The influence of cavity vacuum pressure decrement affects the  
Figure 7: (**A**) The surface temperature contours for the hot and cold sides at the cavity pressures of 0.001 Pa, 1 Pa, and 50 kPa, spreading from the vacuum edge seal to the centre-of-pane of one-quarter of the TVG (**B**) along Line A and (**C**) Line B.  
surface heat transfer quite significantly, and the edge effects spread up to distances of 38 mm and 70 mm from the edge to the centre-of-pane on the hot- and cold-side surfaces when contoured along Line A, which is nearer to the edge corners. The edge effects reduce when moving away from the specimen edge. By reducing the width of the edge seal, the edge effects could be further minimised. A negligible difference in temperature gradients was predicted at the cavity vacuum pressure of 50 kPa of the TVG, which effectively increases the heat transmission with less influence of the edge seal. With 50 kPa pressure, the edge effects to the centre-of-pane are smaller along Line A (about 26 mm and 30 mm on the hot and cold sides, respectively) because of the diminution of the vacuum pressure affecting temperature gradients. The ageing and high-temperature exposure issues are important because of the degradation of the vacuum pressure inside the cavity due to ineffective evacuation of the cavity, improper pump-out sealing, and the remaining residues of air or moisture molecules in the cavity (that boost the production of CO, CO\({}_{2}\), H\({}_{2}\)O, and/or H\({}_{2}\) inside the cavity). Such issues could be solved by utilising nonevaporable getters that require activation temperatures of more than 400 \({}^{\circ}\)C and/or preactivated combo-getters. It is recommended that without getters, the stability of vacuum pressure of 0.001 Pa for over 20 years of life span in the cavity of 10-mm wide edge-sealed triple vacuum glazing is difficult to achieve even under a controlled environment. S.M. : conceptualization, methodology, mathematical modelling, and writing of the paper; F.F., S.M., and K.K: software, validation, formal analysis, and review and editing. This research received no external funding and The APC was funded by London South Bank University.