# Effect of Cavity Vacuum Pressure Diminution on Thermal Performance of Triple Vacuum Glazing
## 1 Introduction  
Climate change is a global challenge due to an increase of average global annual temperature of 0.99 \({}^{\circ}\)C, with CO\({}_{2}\) levels reaching 400 ppm, breaking the 1950s' level threshold of 300 ppm [1]. In buildings, the thermal transmittance (U value) is the combination of thermal conductance (i.e., the rate of heat flow between the higher-temperature area and lower-temperature area) and the thermal transfer coefficients. The total U value accounts for the effect of frames and edges, and the centre-of-pane U values do not account for the aforementioned effects. Vacuum insulation as a retrofit solution to existing buildings plays a momentous role in the reduction of carbon emissions and bringing our building sector towards a zero-energy building (ZEB) target [2], because buildings are accountable for at least 30% of the overall global final energy consumed [3, 4]. Although a number of retrofitting measures to improve building insulation have already made the building sector more energy-efficient [5], it requires advanced technologies to bring the building sector towards a ZEB target. Triple vacuum glazing (TVG) has the ability to curtail heat flow between the warm side and cold side of a window, i.e., to provide preeminent thermal insulation, so-called thermal transmittance (U value) [6]. TVG consists of three panes of 4 mm thick glass, with an evacuated gap 0.13 mm high, separated by 0.13 mm high and 0.3 mm diameter stainless-steel support pillars. TVG edges are sealed with airtight material such as solder glass, indium alloy, or Cerasolzer CS186. The reason TVG has the potential for retrofitting to existing buildings is because its U value is less than 0.5 Wm\({}^{-2}\) K\({}^{-1}\), compared to triple air-filled glazing, which has a U value of 2.8 Wm\({}^{-2}\) K\({}^{-1}\). It also would allow increasing the window-to-wall area ratios because of its U value being closer to that of cavity wall insulation. Nevertheless, TVG has associated challenges; this paper presents the findings of the effects of diminution of vacuum pressure on the U value. The U value of TVG primarily depends on the thickness of the glass panes, vacuum pressure in the space between the glass panes, emissivity of the coatings on the glass panes, frame, and type of spacers that separate the panes of glass and type of window frame. The total U value consists of the influence of the frame and the area of the edge seal. The edge seal area is dependent on the width and the type of edge-sealing material. The airtight edge seal of a TVG must be able to perpetuate a vacuum pressure below 0.1 Pa for quelling gaseous heat conduction for long-term duration [7]. [PERSON] et al. [8] investigated laser technology in a vacuum chamber to seal the edges of two glass panes. This achieved an airtight edge seal, but the cavity vacuum pressure was not below 0.1 Pa because the laser seal instigated particles such as moisture and notorious gases [9]. The first successful fabrication method of double vacuum glazing (i.e., an evacuated cavity between two panes of glass) relied on a lead-based solder glass material sealed at around 450 \({}^{\circ}\)C (it was recognised as a high-temperature sealing method because of its melting temperature) [10; 11]. This method achieved a centre-of-pane U value of 0.8 Wm\({}^{-2}\) K\({}^{-1}\) and progressed to the commercial scale with a trade name of SPACIA. The problems with the high-temperature method of sealing are: the degradation of soft (low emissivity) coatings, meaning only hard coatings can be used [12]; and if using annealed glass, then loss of toughness, and requirement of higher amounts of heat energy for the construction. Investigations on a low-temperature solder glass material showed a hermetic edge seal, but durability was a problem due to the absorption of moisture [9].