Balancing Health and Performance Benefits through Natural Lighting

Understanding how to specify glazing systems that balance access to natural light with thermal performance and building code requirements

By Juliet Grable

Access to natural light is critically important to human health and performance; it has been shown to benefit many building occupants, including employees, students, and patients in health-care settings. By using creative design to maximize access to natural light, architects, designers, and engineers play key roles in impacting the long-term well-being of building occupants.

Using glass in buildings, whether as part of a new project or a remodel, is an effective way to bring natural light into a space. In addition, glass can reduce energy consumption by reducing the need for artificial lighting and, in some cases, cooling required to offset the heat generated by artificial light. Creative solutions utilizing systems such as window walls and curtain walls, interior glass partitions and walls, skylights, and light shelves can help maximize these benefits. Proper design, planning, and application can ensure effective and successful use of glass in buildings to maximize light while offsetting thermal heat gain, and ensuring occupant safety and code compliance.

Health Benefits of Natural Light

Window walls, curtain walls, skylights, and interior glass partitions can all be used to bring natural light into buildings. In addition, ample glass and glazing can provide more building occupants with views of nature and, in some cases, access to fresh air. All of these— natural light, views of nature, and fresh air—are elements of biophilic design. Biophilia refers to the innate human affinity to the natural world, and biophilic design refers to those elements that connect people to nature, whether a window with a view of nature or a dynamic fountain with trickling water. Increasingly, design professionals are recognizing the benefits of biophilic design and incorporating these elements into their projects.

How Light Interacts with the Human Body

Light enables us to perform visual tasks, but it also affects mood, controls the body’s circadian system, and catalyzes critical chemical reactions in the body, such as the production of vitamin D.

Access to natural light helps regulate the human body’s natural circadian rhythm, which supports metabolic processes and leads to more restful sleep. Through the hormone melatonin, the circadian system regulates cycles of wakefulness and sleepiness. The natural human circadian cycle is close to 24 hours; in fact, “circadian” means “about a day.” When people are deprived of exposure to natural cycles of darkness and light, the production of melatonin is disrupted, as are the cycles of alertness and sleepiness, potentially leading to sleep disturbances. It is especially important to access sufficient daylight in the morning to synchronize the body’s “clock” to the earth’s rotation.

Scientists are uncovering more and more links between adequate sleep and almost every aspect of health and well-being. Not only is adequate sleep required to remain alert and perform well, but sleep affects the body’s ability to fight off infections, cancer, and perhaps even Alzheimer’s disease.

High melatonin levels cause drowsiness, while low levels correlate to a state of alertness. In a normal, healthy person, daylight or artificial light activates the pineal gland and suppresses melatonin. When daylight or artificial light is inadequate, the natural suppression of melatonin doesn’t happen; as a result, the person feels tired and depressed.¹

According to researchers, the body responds more strongly to daylight as a cue than to artificial lighting. Daylight includes the full spectrum of wavelengths; by contrast, artificial lighting includes a limited part of the visible light spectrum and typically does not include shorter wavelengths. These shorter wavelengths may be important in regulating the circadian cycles. Full-spectrum light may also provide more efficient lighting for vision, potentially reducing eyestrain.²

The positive impacts of daylighting on building occupants have been documented and quantified in nearly every building occupancy type.

Retail Sales

Imagine shopping in a lofty, light-filled atrium compared to a dingy, low-ceilinged building with fluorescent lighting. Which would you choose? Not surprisingly, studies have shown that daylighting can improve retail sales; what may be surprising is the impact on profits.

In one study, retail giant Wal-Mart built a prototype for a “green” store. Only half of the store was lit using daylighting. However, sales per square foot were significantly higher for departments located in that half. Not only that, but sales in daylit departments of this new store were higher than sales in the same department in other Wal-Mart stores without daylighting.

In a more extensive study, researchers analyzed 73 California chain stores over two years. Of these, 49 were lit with artificial lighting. These 49 stores were retrofitted with skylights and subsequently saw their sales spike by 40 percent. The profits due to the skylight retrofit far outweighed the energy savings.³

Terrapin Bright Green estimates that, in general, skylights statistically increase sales by $1.55 per square foot in grocery stores, clothing outlets, and retail chains across the country. It’s no wonder that successful retailers have “seen the light” and embraced daylighting design.

Healing Benefits

Health-care settings present particular challenges: patients experiencing severe pain or mental distress, staff who work long schedules that are out of sync with the normal human cycles, and an environment characterized by constant noise, artificial light, and interruptions.

Daylight can affect patients through the circadian system, helping reduce depression and improve sleep patterns. Daylight may also lessen agitation, ease the perception of pain, and improve the general well-being of staff.

A seminal study conducted in 1984 by evidence-based design researcher Roger Ulrich found that patients recovering from surgery recovered more quickly if their rooms included views of green space compared to those whose rooms faced out onto a wall.⁴

In a more recent study, researchers found that in-patients suffering from bipolar disorder who had east-facing rooms spent an average 3.67 fewer days in the hospital compared with similar patients who had west-facing rooms, and a study of heart-attack patients showed that female patients who were treated in sunny rooms left a day earlier than patients in “dull” rooms. In addition, mortality was higher among patients staying in the dull rooms.⁵

Interestingly, exposure to daylight may reduce perceived pain. Patients who underwent elective spinal surgeries recovered in either the dim side or the bright side of the same hospital unit. Those staying on the bright side were exposed to an average of 46 percent higher sunlight intensity than those on the dim side. These patients reported less stress and less pain; they also took 33 percent less pain medication.⁵

The Nemours/Alfred I. duPont Hospital for Children in Delaware illustrates how daylighting design can be used in a healthcare setting to benefit patients, staff, and visitors. The hospital offers patient rooms with large windows and views of the outdoors, family solariums for gathering, and an outdoor terrace where patients can enjoy fresh air.

Better Conditions for Learning

In schools, good daylighting may improve student performance, help create a healthier indoor environment, and boost attendance. One often-cited study analyzed the test scores of more than 21,000 students in three school districts in California, Washington, and Colorado. Controlling for other factors, the study found that in one school district, students with the most daylighting in their classrooms progressed 20 percent faster on math tests and 26 percent faster on reading tests compared to students in the least daylit classrooms.⁶

Another study compared test scores for 1,200 students in three daylit schools in North Carolina to scores in the rest of the county’s school system, as well as to other new schools within the county. According to the study, students who attended daylit schools outperformed the students in non-daylit schools by 5 to 14 percent. Perhaps not surprisingly, students stuck in mobile, windowless classrooms during the same study period saw their test scores drop 17 percent.⁷

In another study conducted in Alberta, Canada, children attending elementary schools with full-spectrum light were compared to children in classrooms with conventional lighting. The results suggest that the students in the full-spectrum lit classrooms had fewer days of absence per year and enjoyed positive health impacts. For example, daylighting allowed the HVAC system to be downsized, which in turn reduced the noise levels in both the classrooms and library, enhancing the learning environment.

As the same study notes, in most cases, enhancing daylighting does not necessarily add to capital costs, and, in some cases, can even cut them, thanks to the reductions in HVAC equipment and artificial lighting.⁸

Productivity and Satisfaction

The workplace is often a stressful environment for employees, yet people spend a good part of their waking life working. Being able to access nature while at the workplace may help alleviate some work-related stress and lead to happier, more productive employees.

Heart-rate recovery, or how quickly the heart rate returns to normal after a person is exposed to low-level stress, is one way to measure a person’s reaction to his or her surroundings. For example, in one test, 90 participants were broken into three groups of 30 and instructed to perform mildly stressful tasks such as proofreading. One group was exposed to a glass window with a view of nature; the second group was exposed to a plasma screen bearing an image of that same view; and the third was exposed to a wall that was covered by drapes. The subjects with the view out the window experienced greater heart-rate recovery than the other two groups; in addition, when participants spent more time looking at the glass window, their heart rate tended to decrease more rapidly. Interestingly, the plasma screen didn’t appear to be any more restorative than the blank wall.⁹ A similar study involving an older office space with poor lighting and air quality to a healthy, daylit office showed a similar impact on physiological stress markers. Occupants in the older office showed higher activation of stress hormones, while those in the newer office reported far fewer headaches.¹⁰

Through her extensive research, biophilic design expert Judith Heerwagen has shown that people prefer daylight to artificial lighting sources and that most people would choose to be near a window. In one study, she gathered reasons for this preference: psychological comfort, office appearance and pleasantness, general health, visual health, color appearance of people and furnishings, work performance, and jobs requiring fine observation.¹¹

One study has even quantified the economic benefits of providing employees with views to nature. Employees in the Sacramento Municipal Utility District (SMUD) Call Center showed highly variable rates of productivity, depending on whether they had access to views or not. Those with access to views of vegetation through large windows from their cubicles handled far more calls per hour compared to employees with no view of the outdoors. They also handled calls 6 to 7 percent faster than those with no views. SMUD decided to install operable windows and rearrange workstations so that all employees could access these views. The construction cost $1,000 per employee, but the annual productivity savings averaged $2,990 per employee—an outstanding return on investment.¹²

Good daylighting design could give companies a competitive advantage, helping them attract and retain employees. In one striking example, ING Bank designed a new headquarters building in Amsterdam, which maximized daylighting and integrated organic art and water features. Once the project was completed, absenteeism decreased by 15 percent, and employees self-reported greater job satisfaction. The bank also saved an estimated $2.6 million annually once the energy system and daylighting units were installed.¹³

Comfort and Thermal Performance

Natural light is available during times when building occupants most need it, and good design can ensure that most, if not all, occupants can access this daylight while guarding against issues such as glare and unwanted heat gain. Architects can effectively implement these solutions to maximize occupant benefits and ensure that their designs meet codes related to thermal performance.

Glazing impacts multiple aspects of a building, including aesthetics, daylighting, access to views, occupant comfort, and sound transmission as well as energy demand. All of these should be considered in tandem as the design process progresses. This way, architects can ensure the daylighting goals are met while meeting code requirements and other project goals. Here, energy modeling can be a valuable tool. Glazing system manufacturers often have experts and resources, and therefore can work with designers on the best possible solutions to meet their project goals.

There are some basic design principles to keep in mind that will help ensure a successful daylighting design that does not result in unwanted glare or occupant discomfort.

Orientation

If new construction, the building footprint can be optimized for daylighting by maximizing the north and south exposures and minimizing the east and west exposures. Aligning the building on the east-west axis and keeping the floor depth below 60 feet enhances daylighting opportunities. Although a facade that faces directly south is optimal for solar access and control, the building footprint can deviate up to 15 degrees in either direction.¹⁴

Location of Windows

Following are a few basic guidelines from the Department of Energy for locating glazing to ensure effective daylighting without unwanted solar gain or glare.

• South-facing windows allow most winter sunlight into the buidling but allow little direct sun during the summer, especially when properly shaded
• North-facing windows admit relatively even, natural light, producing little glare and almost no unwanted summer heat gain.
• East- and west-facing windows provide good daylight penetration in the morning and evening, respectively, but may cause glare, admit a lot of heat during the summer when it is usually not wanted, and contribute little to solar heating during the winter.

Window Area or Window-to-Wall Ratio (WWR)

Window area or window-to-wall ratio (WWR) is another important aspect of the building envelope that will impact daylighting, ventilation, heating, cooling, and artificial lighting. WWR is defined as the ratio of the total glazed area to the total exterior wall area. Although building codes set prescriptive maximums for the WWR, projects can exceed this maximum by using high-performance glazing systems. Such designs optimize daylighting while reducing undesirable solar gain.¹⁵ For example, projects using ASHRAE 90.1 can use the Building Envelope Tradeoff Option to exceed the maximum WWR of 40 percent.

Glare

When it comes to daylighting, there can be too much of a good thing. Excessive daylight can produce glare, which is especially disruptive in workplaces. When there is a significant illumination differential between the surface an occupant is trying to view—such as a desk or computer screen—and the sunlight coming through a window, the human eye works hard to adapt and reconcile the contrasting brightness, which can lead to fatigue and eyestrain.

There are two types of glare. Discomfort glare is glare that is within the eye’s capability to mitigate; disability glare is glare that prevents a person from performing a task. We can also distinguish between direct glare, which occurs when a person directly views the source of illumination, and indirect glare, which is caused when light reflects off surfaces.

Ideally, brightness levels should be kept relatively even across the occupant’s field of vision within a space. The Illuminating Engineering Society (IES) recommends that small patches of sunlight be controlled to less than 79 foot candles. The location of openings and use of awnings and overhangs can help control glare and radiation, as can the use of controls, such as internal or external shading.

Daylighting and Energy Use

Good daylighting design depends on an understanding of the relationships between glazing, daylighting, and building energy use. Electric lighting accounts for 35 to 50 percent of the total electrical energy consumption in commercial buildings. Schools and other institutional buildings have especially high lighting energy use, but good daylighting design can reduce reliance on artificial lighting. In fact, the use of daylight systems and intelligent lighting controls can reduce building electricity use by about 15 percent.¹⁶

To effectively reduce lighting energy, the daylighting scheme must be planned in tandem with artificial lighting controls. There are several options for controlling artificial lighting. Manual switches give occupants more control, but automatic controls, which dim or turn off artificial lights once natural or ambient light reaches certain levels, typically save more energy and are a more practical solution in larger, open-plan office buildings or schools.

Even greater savings may be achieved using daylighting and intelligent lighting controls in conjunction with blinds, which can help control glare at lower sun angles. One study of open-plan offices in New York City found that switching a conventional lighting system to a dimmer system and either manually controlled or automated blinds could save between 50 and 60 percent of lighting energy.¹⁷

Interior Layout and Partitions

The interior layout is also of critical importance. A building may enjoy good daylighting from glazing, but if that daylight is blocked by walls or partitions, only the occupants on the window side of the partition will benefit from it. One study of an office revealed that 72-inch partitions placed 15 feet from the windows cut daylight levels in half compared to 48-inch partitions.¹⁸ Replacing solid partition walls with glass can increase the proportion of the space benefitting from daylighting. If privacy is a concern, decorative glass can be used, which admits some light but restricts views.

In addition, the reflectance of interior surfaces has a significant impact on the distribution of daylight. Incoming light “bounces” off highly reflective and light surfaces, evenly distributing light and reducing glare, while darker surfaces absorb daylight. For this reason, ASHRAE recommends minimum reflectance values for interior ceilings, walls, and flooring for various building types. The ceiling is the most important light-reflecting surface and has a high recommended reflectance values of 80 percent or more. Reflectance values should be at least 50 percent for walls and at least 20 percent for floors. Keep in mind that other surfaces, including workstations and furniture, also have an impact on light distribution.¹⁹

Heating and Cooling Energy

In addition to impacting lighting energy use, glazing can also affect the energy required to heat or cool the building. When daylighting replaces electric lighting, less heat is produced. This can be significant in buildings with large cooling loads. The Whole Building Design Guide (WBDG) estimates that the energy savings from reduced electric lighting can directly reduce building cooling energy usage by an additional 10 to 20 percent.

Airtight, well-sealed, and well-insulated glazing systems can reduce heat loss during the heating season. However, glazing can also be used strategically to encourage or discourage solar heat gain.

The type of glass plays an important role in controlling heat gain and loss. Use U-factor, solar heat gain coefficient (SHGC), and visual transmittance (VT) values to make the appropriate selection for the climate, building orientation, and building use. The WBDG provides some general recommendations.

• For climates with significant cooling loads, specify windows with low SHGC values (less than 0.40).
• In general, low-SHGC windows should be considered for east- and west-facing glazing as a means of controlling solar heat gain and increasing occupant comfort. For large commercial and industrial structures, specify low SHGC windows on the east, south, and west facades. SHGC for north-facing windows is not critical for most latitudes in the continental United States.
• Analyze the tradeoffs between standard glazing and spectrally selective glass. Spectrally selective glass has a relatively high VT and a relatively low SHGC.
• For buildings where passive solar heating is a goal, choose south-facing windows with high SHGC values coupled with low U-factors.

Natural Ventilation

Glazing systems can facilitate the use of natural ventilation, which can reduce reliance on artificial cooling and lower building energy use. Occupants often appreciate access to fresh air. Keep in mind, however, that in most climates, natural ventilation will not keep interior spaces within the “comfort zone” all the time, and the natural ventilation strategy must be planned in tandem with the HVAC system. This may lead to some conflicting solutions, as a naturally ventilated structure typically includes large window and door openings, while one strategy for minimizing mechanical cooling loads is to reduce the window area or keep the windows sealed.

In general, effective natural ventilation requires cross ventilation. Higher ceilings and open layouts facilitate cross ventilation, as does a narrower floor plan. Air can still move across deeper floor plans, but the air temperature will increase as it moves across the room. In addition, operable skylights can allow rising warm air to exit, allowing cooler air to enter through lower windows by taking advantage of the “stack effect.”

The Role of Shading

It’s important to balance the positives of daylighting—including the benefits to occupants and savings on lighting energy use—with potential heat gains and losses. Shading devices can help prevent overheating and glare. Common shading devices include exterior overhangs and sunshades and interior blinds, screens, and roller shades.

Exterior overhangs and sunshades are typically used to block the summer sun, which is higher in the sky, while allowing sunlight to enter the building in winter, when the sun is at a lower angle. Overhangs are fixed elements on the exterior of the building. Sunshades serve a similar function as overhangs but generally refer to metal louvers that are attached above or in front of a window. The brackets, outriggers, and louvers come in many styles to accommodate different aesthetic and shading goals.

Horizontal projections work best on south facades, as they block direct sun at high angles. Vertical projections work best on east and west facades, blocking the sun when it is lower in the sky. Overhangs present an opportunity for visually interesting exterior architectural elements in the form of fins, “egg crates,” (a pattern of alternating vertical and horizontal projections), and brise soleil.

Interior roller shades, blinds, and screens can be manually operated or automated. They can also be part of a building management system programmed to coordinate with electric light controls. One disadvantage of these devices is that they still allow solar heat to enter through the window, where it can become trapped between the shading device and the glazing. A good strategy is to combine exterior overhangs with interior shading controls.

An elegant study conducted in hypothetical classroom spaces in Colorado shows the relationship between shading, daylighting, electric lighting, and cooling energy use. Classrooms were fitted with either roller shades, blinds, or overhangs; all windows consisted of double-paned low-e glazing. Annual electric lighting energy savings for roller shades, blinds, and overhang compared to the base case were 55 percent, 56 percent, and 67 percent, respectively; cooling energy savings for those same configurations were 39 percent, 34 percent, and 51 percent, respectively.

As you can see, the classroom with overhangs saved the most energy. This is because the 4-foot overhang covered the full windows for 35 percent of the “noon time” from April to August, which significantly lowered cooling loads during that period.

During winter, the overhang allowed solar heat gain, which resulted in a lower heating energy demand during those months.

On the downside, the overhang allowed illuminance levels to exceed 2000 lux—the level which prompts occupants to take actions to reduce the daylight level—for more than 52 percent of the total simulation hours. Both blinds and shades keep the illuminance levels below 2000 lux for most of the simulation hours. In other words, the overhang allowed much less control over maintaining proper indoor illuminance levels than blinds and shades. The study concluded that the overhang combined with interior shading controls could save the most energy.²⁰

Light Shelves

Daylight typically penetrates a room at a depth that is one and a half to two times the window height. Light shelves are horizontal projections installed onto the interior of a window opening that redirect and redistribute natural light into the interior space. They are typically installed toward the head of a window, or between a window and a transom or clerestory. Light shelves are most effective on south-facing windows, and the top surface is usually bright white to encourage the reflection of light. The depth of the light shelf should not exceed 30 inches, or the height of the clerestory or transom above it, and it should be installed 7.5 to 8 feet from the floor to avoid creating an obstacle.²¹

Skylights

Some of our most inspiring buildings feature atria and skylight systems. Skylights are a particularly effective source of daylight because they bring in light from the brightest part of the sky into spaces not reached by perimeter windows. In addition, skylights don’t often cause glare because they are high above the occupant’s visual field. Skylights can drastically reduce electric lighting energy demand; however, solar heat gain must be controlled.

Holistic Design Strategy

Window and glazing choices should be considered holistically since they can impact so many elements of a building, including lighting design and HVAC sizing. Once the design team and owner agree on the design problem, window and glazing options can be evaluated. Issues to consider include heat gains and losses, visual requirements, including views, privacy, and the potential for glare, shading and sun control, thermal comfort, condensation control, ultraviolet (UV) control, sound transmission, daylighting, energy requirements, and aesthetics.

Types and Properties of Glass

When it comes to glass and glazing systems, there have never been as many options available to the architect as there are today. Architectural glass can be designed or “tuned” to reduce solar energy transmittance, control glare, and screen out UV radiation. For example, transmitted solar energy is reduced by the use of tinted or coated glass, colored interlayers, silk-screened glass, or a combination of strategies that absorb or reflect part of the solar radiation in the UV, visible, and near-infrared ranges. The absorbed energy is converted to heat, and a large portion is dissipated to the exterior.

Some manufacturers offer energy analytic applications that calculate a project’s specific energy requirements and performance needs. These applications can incorporate design changes and immediately calculate the impacts on performance. Before we look at specific applications, let’s review the types of glass and key principles related to energy performance.

Heat-Treated Glass

Glass is heat treated to increase its strength and ability to resist external stresses, such as temperature fluctuations. There are two general categories of heat-treated glass: heat strengthened and tempered. The rate of cooling the glass during the heat-treating process determines the strength of the heat-treated glass. Tempered glass is cooled rapidly, while heat-strengthened glass is cooled more slowly. Heat-strengthened glass is approximately twice as strong, and tempered glass is approximately four times as strong as annealed, or untreated, glass. Both types of heat-treated glasses can exhibit distortion that may slightly distort reflected and transmitted views.

Heat-strengthened glass is treated to have a surface compression of between 3,500 and 7,500 psi. It shows greater resistance to thermal stress and shock. When it breaks, the pattern of breakage is similar to annealed glass; it fractures into large shards and tends to stay in the frame. Heat-strengthened glass is not a safety glazing material; in other words, it can’t be used where safety glazing is required.

Heat-strengthened glass should be specified when additional strength is desired and when it is desirable for the glass stay in the frame if broken. Heat-strengthened glass is appropriate for most applications where the glass may experience temperature extremes (high thermal stress), but there are some applications (such as insulated spandrel conditions) where tempered glass will be required.

Fully tempered glass is treated to have either a minimum surface compression of 10,000 psi or an edge compression of at least 9,700 psi, or meet the requirements for safety glazing set by ANSI Z97.1 or CPSC 16 CFR 1201.

Tempered glass is extremely strong and is a safety glazing material due to its break pattern. It has better resistance to impact, thermal stress, and windloads, and it shatters into small pieces when broken, which reduces the risk of injury from impact. However, it typically does not stay in the frame once shattered.

Building codes define applications that require safety glazing. These include tub and shower enclosures, patio doors and entry systems, glass balustrades, and hazardous areas adjacent to walking surfaces. Tempered glass may be needed to meet the wind-load requirements for buildings in high-wind-load regions, such as hurricane zones.

Once the glass has been heat treated, neither heat-strengthened nor fully tempered glass can be cut or drilled.

Laminated Glass

Laminated glass is a type of safety glass that holds together when broken. It consists of two panels of glass held together by an interlayer, typically of polyvinyl butyral (PVB). When laminated glass breaks, the interlayer holds the broken glass pieces together. Laminated glass can be manufactured using annealed, heat-strengthened, or fully tempered glass. It is used in applications where impact or blast resistance is required, or where security is an issue. Because the interlayer(s) help dampen sound, laminated glass is also a good choice for reducing noise levels in noisy environments.

Laminated glass is also ideal as decorative glass, as colored interlayers can be added. Certain interlayers are good UV blockers, thus laminated glass is often used to prevent fading in homes, museums, libraries, etc.

Insulating Glass Units

Insulating glass units (IGUs) are preassembled units composed of two or more lites of glass that are sealed at the edges and separated by air or gas-filled space(s). Insulating glass, also known as double glazing, enables higher energy performance, as it reduces heat loss through the glazing. Insulating glass typically consists of two glazing lites, but some manufacturers offer triple or even quadruple glazing. The space between the glass lites can be filled with a gas, typically argon, which enhances its insulating performance. The glazing panes are held together by spacers and include sealants and desiccants. The sealants ensure that moisture does not enter the insulating space, and the desiccants ensure the insulating space remains dry. IGUs may incorporate just about any type of glass, including annealed, heat strengthened, tempered, or laminated, and the glass can be coated or treated to achieve specific energy performance goals. IGUs can also receive decorative treatments.

Because of its versatility, insulating glass is used in a wide variety of exterior applications, including vertical and sloped glazing, skylights, and both vision and non-vision, or spandrel, locations. It can also be specified where impact, bullet, and blast resistance are required when constructed with an appropriate laminated lite to the interior of the IGU.

Key Concepts Related to Energy Performance of Glazing

When specifying glazing systems, it is important to have a solid understanding of several key concepts related to energy performance, strength, and impact resistance. Terms related to energy performance include U-factor, SHGC, and VT.

U-factor is a measure of the heat flow through the window due to convection, conduction, and radiation. The higher the U-factor, the more heat is transferred or lost through the window; hence, low U-factors are desirable for reducing heating and cooling loads. U-factors for the entire unit may be significantly higher than the U-factor through the center of the glazing.

SHGC represents the ratio of the solar heat gain entering the space through the glazing to the incident solar radiation. SHGC is represented as a number between 0 and 1; the lower the SHGC, the less heat is transmitted. In general, a low SHGC is desirable when cooling loads are high, and a higher SHGC is optimal for projects that rely on passive solar heating.

VT represents the percentage of visible light entering the space through the glazing system. VT is measured on a scale of 0 to 1; the higher the VT, the more light that is transmitted. Clear glazing has a VT of about 0.90. An adequate VT is desirable for effective daylighting.

VT can be affected by tints and coatings. Tints are usually added for aesthetic reasons, but they can also help reduce solar gain and glare. Coatings are intended to impact thermal performance by limiting the transmission of infrared and near-infrared wavelengths. Because these coatings reduce the emittance of radiant heat, they are often referred to as low-emittance, or low-e, coatings. Standard glass has an emittance of about 0.80, while glass with a low-e coating might have an emittance as low as 0.018. Emittance is typically not represented as a separate value but is factored into the U-factor of the glazing product. Hence, low-e coatings result in lower—aka better—U-factors.

There are two types of coatings: sputtered and pyrolytic. Sputtered coatings are applied as thin metallic coatings to finished glass. Because the coating is applied to the surface of the glass, it may be susceptible to degradation through oxidation or scratching during handling or installation. Some sputter-coated products may be tempered or fabricated after the coating has been applied.

Pyrolytic coatings are applied during glass manufacturing. The coatings essentially become part of the glass, making it extremely durable. Pyrolytic-coated glass can also be heat strengthened or fully tempered.

Coatings can be applied to either side of a glass surface. For laminated glass, the coating can be applied to one of four surfaces: the outside or inside of each of the pieces of glass separated by the interlayer. Similarly, double-glazed insulating glass offers four surfaces available for coating. Some low-e coatings are not recommended to go against the interlayer due to the color shift of the low-e coating after laminating. Low-e coatings can be used to maximize solar heat gain (passive heating) or limit the amount of heat entering the building (solar control). It used to be the case that pyrolytic coated glass was typically used for passive heating, while sputter-coated glass was used to decrease solar heat gain. There are no longer strict delineations like this.

Coatings can be used to create spectrally selective glass. This refers to glass that transmits desirable portions of the energy spectrum and reflects others. This enables the glazing product to be “tuned” for specific energy flows and daylighting.

Dynamic glazing with electrochromic coatings is also becoming more available. Dynamic glass can change its performance characteristics in response to the quantity and quality of light hitting it. It uses low-voltage power to electronically tint the glass to the desired level; when the voltage is removed, the glass returns to its original state.

Low-e coatings are sometimes evaluated using a value called the light-to-solar gain ratio, or LSG. This is calculated by dividing the VT of a glazing system by the SHGC. A higher ratio implies a higher-performance glass.

Point-Supported or Structural Glass

As the term implies, structural glass is designed to withstand loads. One common example of structural glass is point-supported glass, named such because the glass is supported at points where high stress concentrations develop, rather than along a continuous edge. Structural glass systems can utilize structural silicone and embedded laminated elements for the connection of the glass to glass or metal support members. Structural glass systems typically utilize tempered glass. Tempered monolithic, laminated, and/or insulating glass can be used in entrances, building facades, and overhead applications. Structural glass can be described as virtually frameless, as traditional aluminum mullion supports are not used.

There are two typical connection configurations for point-supported structural glass: through bolted and edge clamped. Through-bolted point-supported structural glass utilizes fittings that are attached through holes that are drilled in the glass. Edge-clamped point supported glass utilizes fittings that are clamped to the edges of the glass. Point-supported canopies and facades often utilize stainless-steel “spider” fittings with tempered glass to resist wind and snow loads. The spider fittings attach to the steel structure or glass fins and are typically designed to flex under load.

Structural glass systems require careful and early planning. Engineering is required to ensure that the systems will be able to withstand the complex stresses to which they will be subjected. It’s best to work with a system manufacturer that offers comprehensive services and experts who can work with architects to help translate a vision to reality.

Decorative Glass

Decorative glass enables interesting aesthetic expressions while maintaining performance. It is often used in spandrel applications when non-vision glass is desired. Spandrel glass has many applications. It is often used between floors to hide construction elements such as duct work, or to achieve a desired aesthetic in curtain and window walls. Decorative glass can also be used for solar control.

Decorative glass can be classified according to the type of treatment used to achieve the desired aesthetic effect. Common treatments include ceramic enamel frit, painted coatings, digitally printed glass, and etched and frosted glass.

Ceramic Enamel Frit

Ceramic enamel frit consists of finely ground glass particles mixed with pigments. The frit is applied to the glass surface in a horizontal roller coating process, where heat is used to fuse the frit to the glass surface and create a hard, durable coating.

Painted or Back-Painted Glass

Water-based, elastomeric coatings can be applied to glass by rolling or spraying. Painted glass is opaque; thus, it may block light and views, making it appropriate for spandrel glass. Silicone paint creates a rubber film on the glass, which will help the glass shards stay together if breakage occurs and keep the glass within the framing system.

Digitally Printed Glass

Digital printing technology enables designers to create custom graphics that are applied directly to the glass surface. Images can be applied via decal transfer or direct printing. In decal transfer, digital art is transferred to a ceramic decal and fused to the glass surface using heat. With direct digital printing, ink heads deposit ink directly on the glass, which is then cured using UV light or the tempering process. Multiple colors can be applied simultaneously, opening up nearly endless design possibilities.

The new Parkland Hospital, located in Dallas and designed by HDR, uses digitally printed architectural glass supplied to achieve an interesting graphic effect at the hospital’s entrance. From a distance, the image transforms from a dense summer canopy on the western facade to a delicate branch pattern that covers the south-facing lobby. A closer inspection reveals that the “tree” is formed by the names of donors who contributed to this public building.

Silk-Screened Glass

Ceramic frits can be applied to glass through a screen-printing process. Silk screening can be applied to monolithic, IGUs, and laminated. Colors, patterns, and transparencies can be used for different effects. Light colors and open patterns enhance brightness, while dark colors reduce glare. Silk-screened glass is often used in indoor applications, such as interior partitions, doors, glass ceilings, and enclosures.

Etched and Frosted Glass

Etched glass is either acid-etched using hydrofluoric acid or sandblasted with a machine. The opacity of the glass is determined by the depth of the etching: a deeper sandblasting yields a less translucent and more opaque product. Frosted glass simply refers to etched glass with a translucent, cloudy aesthetic. Stencils, sometimes called masks, can be placed on the glass to produce patterns, and the finished glass can also be back painted. Applications for etched and frosted glass include interior partitions, doors, and balustrades.

Applications for Glass

There are many potential applications for glass beyond windows and doors. These include skylights and skylight systems, storefronts, window walls, curtain walls, and interior glass partitions. In many cases, these solutions can be used in place of opaque or non-glass systems to optimize daylighting.

Skylight Systems

When it comes to design options for skylights, it’s safe to say that the sky’s the limit. Skylight systems can be designed in almost every conceivable architectural shape. Many manufacturers will work with architects to design and fabricate custom solutions. Skylights range from single units and tubular skylights to canopies and walkway covers to systems that essentially replace all or part of a conventional roof. Such systems can take the shape of a pyramid, hipped ridge, barrel vault, or single slope. Because these systems are structural, they typically include light but strong tubular metal framing. The framing is coated with a protective layer of paint that protects the system from corrosion and weathering. Glass for skylight systems is almost always laminated. Low-e coatings can be specified for thermal performance.

Storefronts

A storefront is a non-load-bearing assembly that forms the entry system and windows of a commercial space. This may include sliding or thermal flush glaze options. Storefronts are generally less expensive than other glazing solutions. “Mall sliders” are economical and convenient sliding glass systems designed for storefronts in malls.

Storefronts must be able to withstand heavy traffic, exposure to the street, and possible vandalism or attempted break-ins. The storefront also provides customers with a valuable first impression.

Storefronts range up to 10 feet high and span a single floor. Storefronts are usually found only on the ground floor, though storefront systems can be used on higher floors if they meet code requirements.

Window Walls

As the name suggests, a window wall is wall of glazing that visually connects occupants to the outdoors. Doors may be incorporated into window walls, and the walls can be customized for nearly any desirable configuration. Though they resemble curtain walls, window walls are installed between concrete floor slabs and are significantly less expensive than curtain walls. They do not require fire stopping and can also help attenuate sound.

Window walls are prefabricated in a controlled factory environment and pretested and certified by the manufacturer, which can save time and costs. Often, narrow aluminum frames are used in tandem with glazing to create a clean, contemporary aesthetic. The frames can be thermally broken to improve energy performance. While the daylighting advantages of window walls are obvious, attention must be paid to the increased window-to-wall ratio and appropriate solar control designed in. Window walls can incorporate both vision and spandrel glass to achieve the desired aesthetic. However, architects must incorporate the visible concrete slab of each floor into the exterior building design.

The Cooper Union building in New York City illustrates an original and dynamic strategy for balancing daylighting and energy control using window walls and an exterior treatment of punched stainless steel. The perforated sheets wrap the entire facade, working in tandem with energy-efficient glazing system to control sunlight penetration.

Curtain Walls

Curtain walls are metal-framed expanses of glazing that are anchored to the concrete slabs of multistory buildings. While glazing is typically installed on-site, some manufacturers offer prefabricated systems. Curtain walls are more expensive than window walls, but they may offer improved resistance to wind and seismic hazards and can accommodate larger spans of glazing. Because they “hang” from the outside of the building, curtain walls enable a continuous expanse of glass and striking aesthetic possibilities. The framing can be thermally broken for enhanced energy performance. Curtain walls require fire stopping to fill the voids between floors.

Conclusion

Architects and designers have the opportunity to improve the health and well-being of occupants by ensuring access to natural light. They can consider a range of glazing systems, from interior glass partitions and window walls to soaring skylights and canopies, in lieu of conventional wall and roofing systems. They can also select glazing to meet the specific aesthetic goals and climate conditions. Designers must carefully balance daylighting goals with building code requirements, energy performance goals, and glare control. An integrated design approach that considers daylighting in tandem with energy performance and other project goals can help ensure a successful outcome. Designers can work with glazing systems manufacturers to select the type of glazing, strength, and aesthetic attributes of the glazing system.

Juliet Grable is an independent writer and editor focused on building science, resilient design, and environmental sustainability. She contributes to continuing education courses and publications through Confluence Communications. www.confluencec.com

End Notes

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²Mirrahimi, Seyedehzahra et al. “Effect of daylighting on student health and performance.” Computational Methods in Science and Engineering. 2013. Web. 2 May 2019.

³“The Economics of Biophilia: Why Designing with Nature in Mind Makes Financial Sense.” Terrapin Bright Green. 2012. Web. 2 May 2019.

⁴Ulrich, Roger S. “View through a window may influence recovery from surgery.” Science. 27 April 1984. Web. 2 May 2019.

⁵Joseph, Anjali. “Impact of Light on Outcomes in Healthcare Settings.” The Center for Health Design. August 2006. Web. 2 May 2019.

⁶Plympton, Patricia et al. “Daylighting in Schools: Improving Student Performance and Health at a Price Schools Can Afford.” National Renewable Energy Laboratory (NREL). August 2000. Web. 2 May 2019.

⁷“The Economics of Biophilia: Why Designing with Nature in Mind Makes Financial Sense.” Terrapin Bright Green. 2012. Web. 2 May 2019.

⁸Plympton, Patricia et al. “Daylighting in Schools: Improving Student Performance and Health at a Price Schools Can Afford.” National Renewable Energy Laboratory (NREL). August 2000. Web. 2 May 2019.

⁹Kahn Jr., Peter H. et al. “A plasma display window?—The shifting baseline problem in a technologically mediated natural world.” Journal of Environmental Psychology. 8 May 2008. Web. 2 May 2019.

¹⁰Thayer, Julian F. et al. “Effects of the physical work environment on physiological measures of stress.” Lippincott Williams & Wilkins. The European Society of Cardiology. 2010. Web. 2 May 2019.

¹¹Boyce, Peter et al. “The Benefits of Daylight through Windows.” California Energy Commission. January 2003. Web. 2 May 2019.

¹²Heschong, Lisa et al. “Windows and Offices: A Study of Office Worker Performance and the Indoor Environment.” California Energy Commission. 2003. Web. 2 May 2019.

¹³Romm, Joseph J. and William D. Browning. “Greening the Building and Bottom Line.” Rocky Mountain Institute. December 1994. Web. 2 May 2019.

¹⁴Ander, Gregg D. “Daylighting.” Whole Building Design Guide. 15 September 2016. Web. 2 May 2019.

¹⁵”Windows for high-performance commercial buildings.” Efficient Windows Collaborative. Web. 2 May 2019.

¹⁶Piotrowska, Ewa and Borchert, Adam. “Energy consumption of buildings depends on the daylight.” E3S Web of Conferences 14, 01029. 2017. Web. 2 May 2019.

¹⁷Reinhart, Christoph F. “Effects of Interior Design on the Daylight Availability in Open Plan Offices.” Proceedings from ACEEE Summer Studies on Energy Efficiency in Buildings. 2002. Web. 2 May 2019.

¹⁸“Daylight Dividends: Daylight Resources—Energy Issues.” Lighting Research Center. Web. 2 May 2019.

¹⁹Ander, Gregg D. “Daylighting.” Whole Building Design Guide. 15 September 2016. Web. 2 May 2019.

²⁰Moeck, M. et al. “How Much Energy Do Sidelighting Strategies Save?” Lighting Research Center. 2019. Web. 2 May 2019.

²¹Schneider, Jay W. “Daylighting Guidelines.” Building Design + Construction. 7 January 2011. Web. 2 May 2019.