by Helen Sanders, PhD
As long as humans have built shelters, windows have been an important design element to bring in daylight. In the second half of the 20th century—facilitated by advances in structural engineering and the advent of electric lighting—buildings became larger, deeper, and increasingly isolated occupants from the outside world, especially with the arrival of high-walled workstations. As we have now come to realize, having a connection with the outside world and access to daylight is very important for human health and well-being.
The World Health Organization (WHO) predicts that by 2020 cardiovascular disease and mental health disorders will be the top two causes of death worldwide; both are impacted by lifestyle and environmental factors. Since we now spend 90 percent of our time indoors, the built environment is a highly influential factor on human health. As a result, in a century where the impact of global warming will also become acute, the design community is wrestling with the dual, and often conflicting, sustainability challenge of creating energy-efficient buildings to minimize environmental impact, and creating human-centric buildings sustaining the health and wellbeing of occupants.
Daylighting a building not only provides occupants access to needed daylight and a connection with the outside world, but it also offsets the need for electric light and can save significant amounts of energy if automatically dimmable lighting controls are used. The problem with daylight admission involves the accompanying heat and glare—both of which have to be controlled to deliver energy performance and human comfort. To deliver effective daylighting, the envelope’s thermal and solar control performance becomes increasingly critical. Today’s solutions are often insufficient to control the sun’s heat or glare without compromising the daylight design by some means—whether it be by blocking the view, inadequate admission of daylight, or insufficient control of solar gains.
According to the U.S. Department of Energy (DOE), fenestration with dynamic solar control is a key component of a net-zero energy façade solution. (For more information, see D. Arasteh et al’s report, “Zero Energy Windows, Proceedings of the 2006 ACEEE Summer Study on Energy Efficiency in Buildings,” available online at aceee.org/files/proceedings/2006/data/papers/SS06_Panel3_Paper01.pdf.) Electrochromic (EC) glazing provides dynamic solar control since it has the ability to change its visible light transmission (VLT) and solar heat gain coefficient (SHGC) at the touch of a button, or automatically based on the input of sensors (e.g. for light or occupancy). The ability to tune the SHGC allows:
- heat to be blocked during the summer to offset cooling loads;
- heat to be admitted in the winter to reduce heating loads; and
- maximization of daylight admission at all times to reduce electrical lighting loads.
Unlike automated mechanical shading systems, EC glazing also maintains the view to the outside—the original reason for putting in the window.
Current state-of-the-art electrochromic glazing has a range of visible light transmission from 60 to one percent, with a corresponding range in SHGC of 0.41 to 0.09 (Figure 1). At one percent VLT, the glass is sufficiently dark to control the glare in all but the most extreme cases, eliminating the need for mechanical shades or blinds.
Energy performance is important, but a primary benefit of electrochromic glazing is the thermal and visual comfort provided to building occupants. By dynamically controlling solar radiation, EC glass can reduce overheating and the thermal discomfort caused by direct beam sunlight. The use of EC glass instead of manual blinds creates more open, daylit spaces where views to the outside are always maintained—helping to create more human-centric spaces and preventing the ‘blinds down, lights on’ syndrome seen in many of today’s office buildings. With the addition of EC glass, occupants can reclaim the previously uncomfortable floor space next to the façade.
EC glass has become a ‘design enabler’ for architects by removing the tension between having enough glass to admit sufficient daylight and the need to create energy-efficient and comfortable spaces. It also allows more complex and interesting façades (e.g. sloped, segmented curves, non-rectangular shapes) because no mechanical shading systems need to be hung on the façade—inside or outside (Figure 2).