November 2, 2016
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:
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).
Daylight design process
EC glass can help designers balance the need for sufficient daylight admission while managing solar gains and glare.
Daylight design starts as early as building massing and shape determination. Building shape has a significant impact on the size of the floor area that can be day-lit—especially by vertical windows.
For the best daylight penetration from side lighting (i.e. vertical windows), the floor area near the windows should be maximized—meaning those buildings that are long and skinny, have inner courtyards, or are shaped like the letter ‘I’ or ‘E’ (akin to some European hospital designs). Figure 3 shows different building geometries of the same area, but with increasing perimeter area. As the perimeter to floor area increases, the importance of envelope performance becomes increasingly more critical—both from an energy perspective and for occupant comfort. EC glass can help reduce the impact of increased solar loads by attenuating the sun’s heat during cooling periods, and offsetting heating loads during winter. Thus, the impact of increased fenestration to the floor area is less significant. In colder climates, the combination of EC coatings in triple-pane glazing with high-performance, thermally broken framing systems provides additional thermal loss reduction as well as solar control.
Building orientation also has a critical influence on daylight design effectiveness. Ideally, buildings should be oriented with their long sides facing north and south to minimize solar loads (Figure 4). However, designers do not always have the luxury of an unconstrained site to ideally orient the building. High solar gains through windows on east- and west-facing façades are much harder to manage since fixed-exterior horizontal shading systems are not effective at controlling low-angle sun. In many cases, reduced window areas are needed with these orientations due to the high solar gains, or a higher air conditioning system capacity. EC glass can help reduce the negative impact of sub-optimal building orientation.
Figure 5 shows the impact on building energy performance of a three-story prototypical medium office building, used by Pacific Northwest National Laboratories (PNNL), to assess the performance of American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, of the change from an ideal north-south orientation to 45 degrees off optimal orientation in Phoenix for three different fenestration options including:
Even though this building has a small perimeter zone relative to the core, the increase in energy usage with EC glass is much lower (i.e. less than two percent) than with the static glass options (i.e approximately five percent). It is important to note the use of manual blinds (i.e. where they are modelled as pulled and left down for a few hours until re-opening) has a negative impact on energy performance, and is not taken into account in most building modelling during the design process.
Area, placement, light redirection
The amount of fenestration and its placement is also critical to daylighting performance. There needs to be sufficient fenestration to admit enough daylight, but positioning is critical. The higher the glass is located on the façade, the greater the depth of daylight penetration. The rule of thumb is that the depth of daylight penetration is 1.5 to two times the window head height. That is why clerestory fenestration is so effective at daylighting. Glass below desk height is least effective for daylighting. If furniture is moved away from the outer façade in order to create a circulation space and light-colored flooring allows for re-direction and mixing of the light in the room, then some daylight from this area can be harvested.
Optimal daylight designs often have a ‘split-façade’ where there is vision glazing (for views to the outside) where glare must be controlled more often for the occupants and clerestory glazing above which provides more consistent daylighting. Figure 6 illustrates how a light shelf is used to redirect the light coming through the clerestory windows further back into the space, increasing the depth of daylight penetration. In this case, EC glass in the view section is controlled independently from the EC glass in the clerestory to maximize glare control through the vision window and daylight admission through the clerestory and light shelf combination.
There is a trade-off between putting more fenestration in a wall and the perimeter zone’s energy performance. This trade-off becomes more significant for buildings designed for more effective daylighting (i.e. those with larger perimeter zones). EC glass can help relieve that trade-off by reducing peak solar loads during cooling periods, offsetting heating loads during winter, yet letting in enough daylight to turn off electric lights (which are also a heat source) throughout the year. Many studies have shown significant energy savings of EC glass over static glazing solutions, exceeding 20 percent. (There are many studies showing the energy savings of electrochromic glass. These include a study by Paladino and Company, titled “Performance Assessment of … Electrochromic Coatings and Control Strategies,” a study by E. Lee et al called, “The Energy-Savings Potential of Electrochromic Windows in the U.S. Commercial Buildings Sector,” a Public Interest Energy Research (PIER) Report design guide for early-market electrochromic windows complied for the California Energy Commission (CEC), and another Lee study, “Advancement of Electrochromic Windows. California Energy Commission.”) In fact, because of the dynamic solar control performance of EC glass, more glazed area can be used to provide the needed daylight penetration, without taking an energy penalty.
An example is shown in Figure 7, which illustrates the results of energy modeling of the standard three-story medium office building used by PNNL for assessing the performance of ASHRAE 90.1. The performance of the building in Phoenix with EC glass was compared to the performance using the ASHRAE 90.1-2010 baseline glazing and the baseline glazing with manual blinds. A building with a 50 percent window to wall ratio of EC glazing has the same energy performance as 20 to 30 percent window area with conventional glazing. This is significant, given this large difference was shown in a building with a relatively small perimeter zone compared to the core area, in which impacts on building energy consumption from changing the envelope conditions are normally relatively small because of the dominance of the core.
Skylights are an excellent way of providing daylight to the core of buildings, but of course are only effective for daylighting the top floor or where there is an inner courtyard. That being said, skylights come with significantly more challenges around solar heat gain control, as is demonstrated by a skylight installed in 2009 at Ball State University in Muncie, Indiana (Figure 8).
Converted from an open central courtyard into an enclosed space, the university wanted to preserve the open feel of the space and create a general purpose area serving as a lounge, entryway, and a venue for large group audio visual presentations. When first installed, the specified glazing was an insulating glass unit (IGU) with a standard solar control low-emissivity (low-e) coating on the exterior lite with a 50 percent frit pattern on the inboard lite. Within only a few months, the school found there was too much light and heat coming into the space, making it very uncomfortable for occupants. They considered many options, including mechanical systems on both the inside and outside of the skylight, but found EC glass to be the most low-maintenance and aesthetically pleasing solution. (It was also first-cost-competitive.) With EC glass installed a year later, the space immediately became thermally and visually comfortable. The ability to tint the entire roof to one percent means the university is also able to project audio visual (AV) presentations under the roof for entertainment events.
Another benefit of EC glass is it lowers peak loads, which can reduce the size of the HVAC system—in some cases, enough to allow the transition to more innovative and sustainable heating and cooling systems such as chilled beams and radiative cooling and heating. An example is shown in Figure 9, where EC skylights were used to control solar gains as part of a day-lit and naturally ventilated space at Chemeketa Community College in Salem, Oregon.
While glazing choice does not impact interior design per se, interior design can make or break a daylight design. It is critical interior design choices support the effective distribution of daylight in a space. The color (preferably light and neutral) and reflectivity of walls and ceilings are critical choices. The Illuminating Engineering Society (IES) recommends 50 percent reflectivity for walls and 80 percent for ceilings.
Additionally, the height of workstations should be minimized so daylight can penetrate deeper into the space. If perimeter offices are needed, glass walls can be used to share the daylight with the interior.
Glare management is an essential part of a daylight design. If management of visual comfort is not considered, the actions of occupants can completely negate its benefits. Occupants will resort to any means to achieve visual comfort and to continue their work unhindered. Typically, manually operated shades are pulled down when the glare condition is present, and left down long after the glare condition has passed, blocking daylight admission and the view to the outside.
A number of studies have shown manual blinds are not actively managed. (For more information, see K. Van Den Wymelenberg’s article “Patterns of Occupant Interaction with Window Blinds: A literature review,” in vol. 51 (2012) of Energy and Buildings.) One 2013 study by the US Green Building Council (USGBC) of 55 office buildings in Manhattan showed an average of 59 percent of the window area was covered by blinds or shades, while more than 75 percent of buildings had more than half their window area occluded by blinds or shades. (This information was gathered from Urban Green, the New York City Chapter of USGBC’s paper, “Seduced by the View,” published in December 2013) The following year, a similar Swiss study reported an average 57 percent of the window area was blocked by blinds in Lausanne. (B. Paule et al’s report, “Global Lighting Performance,” prepared by ESTIA, for the Swiss Federal Office of Energy in October 2014.)
Most shades are pulled from the top down, which means not only is more than 50 percent of the window area covered, but the most important part of the window for daylighting (the top section) is blocked. On top of this, the view is obstructed and starts to negate the reason for the window. Although the normal solution for managing sunlight glare is the use of manual shades or blinds, in their absence, aluminum foil, umbrellas, and paper have also been used by resourceful occupants to permanently block daylight admission.
One only has to walk around a city and look at the windows in its buildings to see the impact of leaving glare control to the occupants. For example, Figure 10 shows the northeast corner of a building at 5 p.m. in San Francisco. The sun is on the other side of the building, yet the blinds are still mostly down on the east elevation and also, surprisingly, on the north elevation. Studies have shown the negative impact of manual shade use on daylighting and energy performance of a building. (See C. Dyke et al’s paper, “Comparing Whole Building energy Implications of Side-lighting Systems with Alternate Manual Blind Control Algorithms,” published in Buildings [vol. 5, 2015].)Manual blind use might help explain the gap between the expected energy performance of sustainably designed buildings and their actual performance.
An automated response for glare is needed to optimally balance glare control and daylight admission. In this way, the glare can be blocked when present, and daylight re-admitted as soon as the glare condition has passed without relying on occupant intervention.
EC glass can provide an automated response for glare while still maintaining the view to the outside. It can tint and clear in the background without occupant intervention, maintaining a balance of visual comfort, daylight admission, and heat gain control. It can also create a more uniform exterior aesthetic.
Balancing daylight admission and glare control
Unlike mechanical shading systems, EC glass can provide a more effective balance of daylight admission and glare control because of the ability to control groups of glass panes independently (i.e. in horizontal rows [zones]). By fully tinting the zone that the direct sun is shining through, the glare is controlled. However, other zones can be at higher visible light transmissions to let in sufficient daylight while also controlling solar loads.
As the sun moves down in the sky, the transmission state of each zone will change in response to the changing sun angle, each becoming fully tinted when direct sun is seen by the occupant through it, while the others become less tinted. EC glass can also be split into up to three independently controllable zones within a pane (in-pane zoning)—therefore, the architects can still design with large lites of glass because additional mullions are no longer needed to create the best daylighting solution. Figure 11 shows one large pane split into three zones with a single mullion separating it from a second smaller pane at floor level, thus creating a very flexible four-zone system.
Achieving neutral color rendering of light
Zoning is also an important tool for control of light color rendering. As is evident in this article’s images, when at intermediate or fully tinted states, EC glass has a blue tint. If all the EC glass on a façade is held in a tinted state, the daylight coming through the glass will be shifted to a blue color, creating poor color rendering in the room. However, it has been shown (both analytically and in real buildings) that a neutral light color rendering can be maintained in a space with EC glass if 10 to 15 percent of the façade area is kept in the fully clear state (60 percent), which does not change the spectrum of daylight significantly, while the rest can be tinted. (This information comes from J. Mardaljevic et al’s article, “Neutral daylight illumination with variable transmission glass: Theory and Validation,” from Lighting Research and Technology, published in 2016. An additional source can be found in Mardalje’s “Colours of Daylight” presentation at the 2015 Professional Lighting Designers Conference.)
In this scenario, the light coming through the section held at 60 percent—effectively neutral—washes out the small amount of blue light coming through the areas of the façade that are tinted to one percent. This is illustrated analytically in Figure 12 and the effect can be seen in Figure 11, where starting from the window head, the zones are at one, six, 18, and 60 percent VLT, respectively. The light color rendering in the space appears very neutral. Color rendering indices of above 90 have been recorded in spaces with EC glass zoned appropriately. Given that the best compact fluorescent bulbs only have color rendering indices of 80 to 85, EC glass when zoned effectively can provide a good color neutral interior environment at all times.
Electrochromic glazing has the ability to provide designers with a tool to more effectively manage the difficult trade-off between providing enough fenestration to adequately daylight buildings without negatively impacting energy performance or occupant comfort. More glass can be used without energy penalty, orientation dependency is reduced, and the energy impact of designing narrower buildings with higher perimeter-to-area ratio can be mitigated—all without losing the view to the outside.
By providing an automated response for glare, EC glass can also deliver the anticipated daylighting and energy performance while maintaining a uniform exterior aesthetic. Effective zoning of EC glass can optimize the trade-off between glare control and daylight admission and negate any concerns over color rendering, creating comfortable, well day-lit spaces that are as attractive and productive for occupants as they are aesthetically pleasing to architects.
Helen Sanders, PhD, has more than 20 years of experience in the glass industry, with 17 years focused on dynamic glass technology and manufacturing. She is responsible for technical business development at SAGE Electrochromics, a developer and manufacturer of electronically tintable glass and business unit of Saint-Gobain. Sanders is a board member of the Insulating Glass Manufacturers Alliance (IGMA). She can be contacted by e-mail at firstname.lastname@example.org.
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