In both residential and commercial buildings, heating and cooling is a major cause of energy waste. 35% of the total building energy goes to air conditioning, mainly due to lacking or outdated design, which causes either hot or cold air leakage. Improving the insulation of the building requires to heat/cool it.
Windows are one of the most difficult but most important ways to improve insulation. The ideal window design should have good thermal insulation, let enough light in, and let heat in when useful but block it when it would add to cooling loads. A building that is more often cooled should have windows that let less heat into the building. There has to be a mixture of current and innovative technologies to achieve these goals. Lowering the U-factor—the rate at which non-solar energy flows—is an important start. Metal frames (the most common frames) have a high U-factor. To lower it, a "thermal break" is necessary, which is a plastic strip in between the frame and the sash. Other materials may also make up the frame, such as fiberglass, which has air cavities that can be filled with insulation.
The second part of window efficiency is the actual glass (glazing). A common technique is called insulation glazing (IG), which is where there are multiple panes of glass that are airtightly sealed (hermetically sealed). As a result, heat gets trapped between the panes, thereby insulating the window. However, there is a new technology emerging. Vacuum insulated glazing (VIG) technologies are systems that have a space entirely devoid of matter between the two panes of glass, maximizing the insulation as no gas can enter the system, and therefore no convection can occur. This technology has a lower U-value than standard IG technologies; it could bring the U-value from 1.2 to 3.3 W/m•m•K down to at most 1.0 W/m•m•K. Traditional IG technologies can not go lower than 1.0 W/m•m•K due to the complicated heat transfer mechanism. One Korean study found that the total annual heating energy demand for each household in the Korean Capital region was 6001, 3273, 2546, and 2182 kWh for IG double-pane windows, triple-pane windows, and quadruple-pane windows, respectively. Meanwhile, the heating energy demand for a 0.7 W/m•m•K VIG system was 1273 kWh, and the heating energy for a 0.2 W/m•m•K VIG system was a mere 364 kWh, a 1,548% decrease from the double-pane windows. These numbers show that a VIG system can significantly decrease the U-value, thereby drastically lowering the energy requirements.
Baldwin, Sam, et al. Chapter 5: Increasing Efficiency of Building Systems and Technologies. In: Quadrennial Technology Review: An Assessment of Energy Technologies and Research Opportunities. United States: Department of Energy, 145-182.
Baek, Sanghoon, and Sangchul Kim. “Potential Effects of Vacuum Insulating Glazing Application for Reducing Greenhouse Gas Emission (GHGE) from Apartment Buildings in the Korean Capital Region.” Energies 13, no. 11 (2020): 2828. https://doi.org/10.3390/en13112828.
“Window Types and Technologies.” Department of Energy. Accessed June 21, 2020. https://www.energy.gov/energysaver/window-types-and-technologies.