Windows are thermal holes. In most cases, the insulating capacity of glazed apertures compares poorly with the performance of the surrounding envelope. The result is a familiar tug-of-war between the affordances of windows—light, views, and sometimes fresh air—and the critical insulating function of adjacent opaque walls (including the insulated portions of curtain walls). Façade designs are heavily influenced by this battle between light and climate control—effectively serving as diagrams representing the story of an uneasy resolution between client preferences and energy code requirements. This conflict is so commonplace that we rarely think about it, nor do we consider the potential of another paradigm. The struggle is unfortunate and counterintuitive, as it pits two indoor environmental quality considerations—daylight and thermal comfort—against one another.
This problem is not new, and manufacturers have been developing products and processes to address it. Strategies primarily include modifying glass to increase insulation performance and mitigate solar heat gain. Simple and common enhancements involve adding special films and coatings, interlayers, and more glass lites with inert gas-filled cavities. More sophisticated changes include adding aerogels, phase-change materials, and other high-performance insulating substances. Today’s commercialized products thus exhibit improvements; however, more significant performance enhancements are necessary to solve the window-wall conflict. Thankfully, a collection of further advances suggests a promising trend in the right direction.
Low-emissivity coatings for solar control in glass reduce the penetration of infrared and UV light—the invisible, unwanted parts of the light spectrum that contribute heat or material degradation, respectively. A recent collaboration between French and Japanese researchers has resulted in a new metallic nanocoating that offers another strategy for eliminating these undesirable bandwidths of light. It consists of compounds of niobium-tantalum clusters dispersed within a polyvinylpyrrolidone (PVP) matrix onto indium-tin-oxide glass. Preliminary studies show promise in the coating’s ability to dramatically reduce the amount of UV and near-infrared penetration while admitting visible light. Although the method is not yet ready for commercialization, it represents an additional encouraging strategy for glazing modifications.
Advancement in aerogels also seem promising. Aerogels offer significant insulation capacity, consisting primarily of air and offering exceptional protection against heat and cold. Early aerogel-based insulated window systems have been translucent, not transparent, making these systems less attractive to potential adopters. However, scientists at the University of Colorado Boulder have devised a transparent, “super-insulating” gel that could accelerate adoption in glazing systems. The flexible film is derived from an unexpected source: waste from beer-making. The research team collected excess wort, the sugary liquid used in the brewing process, to develop their new gel. Then, the scientists added bacteria to convert the material into cellulose, orienting the molecules into a lattice pattern that ensures the consistent passage of light. The result is a pliable material that is approximately 100 times lighter than glass and has the potential to increase windows’ insulating capacity significantly with negligible weight or carbon footprint.
Air insulates, and water offers thermal storage. Scientists at Singapore’s Nanyang Technological University have developed a fluid-filled window that regulates heat throughout the day and night. The innovative aperture utilizes hydrogel, a water-loving polymer that stores significant quantities of liquid, encapsulated between glass lites. When exposed to the sun, the hydrogel becomes more opaque and slows the passage of solar radiation, trapping and storing the heat. The window gradually emits this heat overnight during the cooler hours and regains its transparency. According to the research team, the window saves 30% more energy than conventional Low-E glass and is less expensive to manufacture. In simulated tests, the window reduced HVAC consumption by up to 45%. The fluid-based system also reduces noise transmission by 15% compared with standard insulated glass units.
As indicated by these examples, most insulated window innovations involve modifications to conventional glass. However, some technologies don’t use glass at all. For example, materials scientists at the University of Maryland have pioneered a new transparent glazing material made from wood. The researchers developed a technique to make strips of wood transparent by removing the material’s lignin, which imparts wood’s color and opacity. By adding clear epoxy to provide structure, the team maintained the tell-tale wood grain pattern while providing 80% light transmission. The resulting wood “pane” is more robust and insulating than its glass counterpart and selectively reduces the passage of UV and infrared light.
Although not commercially unavailable, biomaterial-based approaches like these may offer the most significant potential for future windows. Utilizing substances such as wood and cellulose from beer waste may seem outlandish, but their renewability and low carbon footprint deserve further attention. After all, we should aim to create building products that perform well in embodied and operational carbon accounting. Since windows are energy-intensive to manufacture and responsible for up to 30% of buildings’ HVAC energy consumption, they represent an excellent place to start.
The views and conclusions from this author are not necessarily those of ARCHITECT magazine or of The American Institute of Architects.
Read more: The latest from columnist Blaine Brownell, FAIA, includes the rise of holistic interior greening, user experience design in architecture, what we can learn from India about confronting a warming climate, and the future of insulation.
