Fenestration of today and tomorrow: A state-of-the-art review and future research opportunities

In this short communication, energy-efficient, eco-friendly and low-cost solutions for existing buildings are considered in a holistic manner toward low/zero carbon building concept of near future. Buildings are still responsible for about 40% of total primary energy consumption in the world, hence decisive measures need to be taken promptly due to growing significance of environmental issues driven by fossil fuel based energy use. Smart solutions addressed within the scope of this research aim at analyzing the energy consumption behaviour of a typical building from building envelope to the appliances utilised, then propose key technologies for urgent mitigation of energy use and drastic reduction of greenhouse gas emissions. For the external walls, aerogel and vacuum insulation panel based thermal superinsulation solutions are recommended owing to the superior thermal resistance ability of such materials (λ<0.01 W/mK for current commercial products). For glazed areas, aerogel glazing, vacuum glazing, solar pond glazing, PV glazing, Transparent Insulation Material (TIM) glazing and Phase Change Material (PCM) glazing concepts are introduced. About 60% of total heat losses from building fabric is attributed to window, which proves the significance of energy-efficient solutions on fenestration products. Depending on environmental and regional conditions, solar, wind and biomass oriented hybrid solutions can be considered for clean energy generation. Current payback period of such systems is below five years, which is promising. Especially thermally resistive PV glazing systems integrated with TiO2 nano coating are capable of producing more than 100 W from per m2 28 of cell area with 100% UV light blockage and self-cleaning feature. Polycarbonate based counter flow waste heat recovery systems are ideal for ventilation purposes with an average COP of about 3.5. For general heating and cooling purposes, air or ground source heat pump systems are found to be favourable for temperate and continental climatic conditions, respectively. Average COP of these systems is found to be greater than 3 in most cases, which is attractive. LED lighting systems are highly energy-efficient and cost-effective compared to the alternatives in market.

Indium alloy-sealed vacuum glazing development and context

Thermal bridging in windows presents a significant challenge for building energy efficiency, particularly at window-wall junctions where material transitions cause heat loss. As energy standards become more stringent, minimising these heat loss pathways is essential for achieving sustainable design objectives. Thermal bridges increase energy consumption, diminish insulation effectiveness, and compromise overall building performance. This review explores the impact of thermal bridges in windows and glazed areas, highlighting advancements such as thin-film photovoltaic (PV) glazing, vacuum glazing, aerogel glazing, low-e coated multilayer systems, transparent insulation materials (TIM), and phase change materials (PCM). It also evaluates high-performance window frame materials, such as fibreglass and composites, alongside advanced installation techniques like thermal breaks and insulation barriers at window-wall interfaces, for their ability to reduce thermal conductivity and heat transfer. Research indicates that thermal bridges increase building energy consumption by 5%-30%. Cutting-edge technologies, such as vacuum glazing with U values as low as 0.2 W/(m2 ·K) and aerogel-filled frame cavities that reduce thermal permeability by 45%, demonstrate considerable energy-saving potential. Furthermore, precise installation techniques lower linear thermal transmittance (LTT) by up to 80%. A holistic approach that integrates advanced glazing technologies, optimised frame materials, and meticulous installation methods offers a powerful solution for enhancing window thermal efficiency, making a substantial contribution to the sustainable transformation of the built environment.

Globally, greenhouse gas emissions from the operational phase of buildings are significantly contributing towards climate change. Global and national efforts, through the Sustainable Development Goals and the UK's 2050 targets, aim to reduce these emissions with net zero energy buildings (NZEBs). A building's glazing plays a significant role in overall building energy consumption due to their traditionally ‘leaky’ nature. This study utilises experimental data from test cells and the International Glazing Database to evaluate the performance of advanced and smart/switchable windows on an existing low energy building (LEB) situated in north Wales, UK, as a step towards making the modelled building a NZEB. A number of glazing constructions were considered in this work; advanced window – vacuum, aerogel, vacuum-aerogel and smart window – PDLC, PDLC-aerogel and PDLC-vacuum, in their fixed and switching states. Results revealed that PDLC-vacuum offered the greatest reduction in building energy, yielding a theoretical U-value of 0.810–0.831 W/m2K and a G-value of 0.257–0.455. Despite its successes, it was notably susceptible to window orientation and window-to-wall ratio. Vacuum and aerogel glazing both offered similar energy savings, with the latter prone to overheating, stressing cooling loads. These advanced windows offered differing daylighting potential with vacuum able to meet 78% of useful daylight illuminance compared to aerogel's 60%. Given the prioritisation trilemma between heating, lighting and cooling needs of a building, PDLC-vacuum presents the best step towards a NZEB. As such, further efforts should concentrate on the development of a PDLC-vacuum window, maintaining smart window functionality and achieving low U-value for cold climates.

Triple vacuum glazing: Heat transfer and basic mechanical design constraints

The effect of glass coating emittance and frame rebate on heat transfer through vacuum and electrochromic vacuum glazed windows

The incorporation of vacuum glazing (VG) products in different window and facade constructions is becoming more viable. The characteristics of the VG products can be summarized as follows: (i) Due to the reduction of conductive and convective heat transfer mechanisms, VG products provide high thermal insulation; (ii) VG products feature low weight and small thickness in comparison to commonly used double and triple-glazing products; (iii) VG is constituted by two parallel glass panes, which are sealed by an airtight edge seal; (iv) The interstitial space between the panes is evacuated; (v) To maintain the parallel positioning of the glass panes, distance pillars are situated within the interstitial space. These characteristics must be taken into consideration when contemplating window and facade construction paradigms that employ VG products. This contribution compares commonly used glass facade constructions (generic post-and-beam constructions with – non-operable – fixed glazing) utilizing insulation glazing with respective facade variations that incorporate VGproducts. Thereby, in a first step, the thermal performance of default facade constructions (equipped with double or triple-glazing) was assessed via numeric thermal bridge simulation. Subsequently, we replaced the insulation glass with a VG product. Thereby the load-bearing post-and-beam details were kept essentially the same. Finally, we slightly modified the facade constructions based on the necessities of VG integration. In this contribution we present the construction paradigms for VG-based facades, the adopted methodology, and the main results of the investigations.

Thermal performance analysis of a new structured-core translucent vacuum insulation panel in comparison to vacuum glazing: Experimental and theoretically validated analyses

Finite element analysis of heat transfer performance of vacuum glazing with low-emittance coatings by using ANSYS

In general, more heat escapes through glass (windows) than through the other materials in a structure. Thus, finding ways to make windows more thermally insulating is important to improving the energy efficiency of a building. We have combined two window construction techniques—vacuum glazing and electrochromic glazing—to provide improved thermal comfort while limiting the use of auxiliary space heating and artificial light. This performance is thanks to the system’s very low heat loss and variable light transmission, which can also control glare from daylighting. A vacuum glazing (VG) comprises two sheets of glass that are separated by a very narrow evacuated space. An array of metal or ceramic pillars holds the sheets apart, and the edges can be sealed with solder glass or indium. The interior faces of one or both glass sheets usually have a transparent, low-emittance coating. Windows made from vacuum glazing are much more thermally insulating than either single pane or conventional double glazed windows. Electrochromic (EC) glazing causes glass to change its tint in response to an applied voltage change. Research in this area and its many potential applications are well documented.1 Visible light transmittance by EC films can be varied between 8% in their colored state and up to 80% in the bleached state by applying a 1–2V DC switching voltage. An ‘EC VG’ combines EC and VG technologies, as shown in Figure 1. Our novel glazing system combines the low-heat-loss properties of VG—a U-value (heat transmittance) of less than 1Wm−2K−1—with the variable transmittance of EC glazing to control solar gain. The first working vacuum glazing2 using a low melt solder glass to form a contiguous edge seal at temperatures above 450◦C was reported in 1989. Many types of soft, low-emittance coatings Figure 1. This schematic diagram shows the various components of an electrochromic vacuum glazing.

Low emittance coatings and the thermal performance of vacuum glazing

Thermal performance analysis of an electrochromic vacuum glazing with low emittance coatings

This document reports the findings of a market and research review related to state-of-the-art highly insulating window frames. The market review focuses on window frames that satisfy the Passivhaus requirements (window U-value less or equal to 0.8 W/m{sup 2}K ), while other examples are also given in order to show the variety of materials and solutions that may be used for constructing window frames with a low thermal transmittance (U-value). The market search shows that several combinations of materials are used in order to obtain window frames with a low U-value. The most common insulating material seems to be Polyurethane (PUR), which is used together with most of the common structural materials such as wood, aluminum, and PVC. The frame research review also shows examples of window frames developed in order to increase the energy efficiency of the frames and the glazings which the frames are to be used together with. The authors find that two main tracks are used in searching for better solutions. The first one is to minimize the heat losses through the frame itself. The result is that conductive materials are replaced by highly thermal insulating materials and air cavities. The other option is to reduce the window frame area to a minimum, which is done by focusing on the net energy gain by the entire window (frame, spacer and glazing). Literature shows that a window with a higher U-value may give a net energy gain to a building that is higher than a window with a smaller U-value. The net energy gain is calculated by subtracting the transmission losses through the window from the solar energy passing through the windows. The net energy gain depends on frame versus glazing area, solar factor, solar irradiance, calculation period and U-value. The frame research review also discusses heat transfer modeling issues related to window frames. Thermal performance increasing measures, surface modeling, and frame cavity modeling are among the topics discussed. The review shows that the current knowledge gives the basis for improving the calculation procedures in the calculation standards. At the same time it is room for improvement within some areas, e.g. to fully understand the natural convection effects inside irregular vertical frame cavities (jambs) and ventilated frame cavities.

This study investigates the linear thermal transmittance coefficient Ψg of vacuum glazing when installed in a wooden window frame. Vacuum glazing is a promising technology for enhancing the energy efficiency of a building by eliminating convective and conductive heat transfer within a vacuum gap. However, thermal losses at the edge seal can significantly impact its overall performance. A 2D computational analysis was conducted to evaluate Ψg values for different vacuum glazing configurations and installation positions. Unlike previous studies, this research systematically analyzes the effect of the depth of an installation on Ψg and the internal surface temperature at the glazing-frame interface. The findings indicate that vacuum glazing with an indium-based edge seal exhibits Ψg values comparable to insulating glazing units with metallic spacers, while vacuum glazing with a glass fusion seal achieves lower Ψg values. Furthermore, the study emphasizes that increasing the depth of an installation significantly reduces Ψg and improves the distribution of the surface temperature, thereby mitigating the risk of condensation. These results provide valuable insights into optimizing the integration of vacuum glazing in wooden window frames and offer practical guidelines for enhancing thermal efficiency in building applications.

To get a slim building envelope with low thermal transmittance, and hence a low energy consumption, vacuum glazing and high-performance insulation material have been combined. Preliminary investigations both experimental and numerical have been carried out to determine thermal, hygric and mechanical properties of these building components according to either existing standards or to best knowledge procedures. This includes measurement of the center-of-glass U-value, the simulation of the influence of support pillars on the glass surface temperature and comparison of measured and calculated results. Several specimens were measured under different temperature gradients to get more information about accuracy and deviation in the production of the vacuum glazing elements. Further, mechanical stability of larger specimens was tested by the standard pendulum tests for flat glass and compared to identical tests on conventional double-glazing elements of the same size. Finally, a one-dimensional hygro-thermic analysis has been conducted to determine the temperature and moisture distribution within a thin wall construction containing high-performance insulation when submitted to climatic conditions in the Swiss Midlands.