Optimizing Daylight in Different Buildings  

Incorporating daylight into buildings is a fundamental yet sometimes challenging design issue. At the most basic level, sunlight provides natural lighting and a connection to the outdoors for the people inside the building. However, simply letting sunlight into a building means that the light level and quality are largely uncontrolled—the building receives whatever the sun and sky provide. More sophisticated approaches focus on intentional, controlled daylighting, including the locations of the light in the building, the intensity or amount of light, the color properties of the light, and the ability to disperse or diffuse the light so as to control glare. In this course, we address the implications of incorporating a balanced daylighting approach within the design of buildings. We also look at the specific impacts of well-designed daylighting on the people who use the buildings. This includes a discussion of the attributes of light quality and some of the current methods to analyze, predict, and achieve the most appropriate results for a particular building.

Daylighting in Buildings

Bringing daylight into buildings has been a primary design goal of buildings throughout history. In times before the introduction of artificial electric lighting, it was often a critical design element not only for functionality but also symbolism representing life and vitality in some cases. This combination of the practicality and design feature capability of daylight has been used in a wide range of forms and techniques for centuries. This has been dictated, in large part, by the available building technologies of each time period and location, but has included openings to let in sunlight that is direct, reflected, diffused, or otherwise captured and dispersed. In some cases, this has been controlled through the use of shutters, baffles, or windows, while in others, there have simply been openings in the walls and/or roofs of buildings.

In recent decades, additional significance has been placed on daylight. As buildings have become more tightly constructed and the use of mechanical and electrical systems has become integral to design, sunlight has become a friend of buildings in need of heating or lighting energy and a foe of those needing cooling energy. Too little daylight in a building requires more artificial electric lighting. Too much daylight can create unwanted glare that will cause people to block it with shades, blinds, etc. and turn on the electric lights. It can also produce a higher cooling load from the direct solar gain that requires more electricity to address. This has given rise to a considerable amount of effort by code developers, design professionals, construction teams, and product manufacturers to find the best balance between beneficial daylight and overall energy performance in buildings. Adding to the mix is the understanding that the “best” solutions certainly vary based on the geographic location of the building, the climate, the time of year, and even the time of day.

An additional significant feature of daylight is the health and wellness aspects it brings to the building occupants. Working or functioning in the proper amount of light is both a fundamental and reasonable expectation of people in buildings. The Illuminating Engineering Society of North America (IESNA) is the nationally recognized source for determining what is really meant by “proper amount of lighting” in different situations. The organization describes itself as The Lighting Authority® and “seeks to improve the lighted environment by bringing together those with lighting knowledge and by translating that knowledge into actions that benefit the public.” By using data generated by the IESNA based on years of studies by experts and input from users, design professionals can determine how much light (measured in lux) is appropriate, within acceptable ranges, for particular building situations.

Beyond quantity of light, other significant organizations including the National Aeronautics and Space Administration (NASA), different branches of the U.S. Military, major universities, and the National Institute of Health (NIH) have all done work related to the quality of light for human purposes. Based on their work, it is fair to say that simply adding more light is not always the best solution, but rather, factors including the intensity, color, direction (direct or diffuse), and other aspects of the light are equally if not more important. It is often these light quality issues that are tied to the health and wellness of people inside buildings, particularly when they are produced by daylighting schemes.

Studied Benefits of Daylighting

Since the year 2000, a fair number of independent studies have looked at the benefits and consequences of different types of light on people. Recognizing this body of work, the design community has responded either directly or through some well-known organizations. The LEED green building rating system, for example, has included a recognition of the benefits of daylighting since its inception. A number of credits and points in this popular system are directly tied to established knowledge in several areas, including energy optimization and indoor environmental quality.

First, related to the impacts on energy consumption, calculations or computer modeling are encouraged that demonstrate the areas in the building that can benefit from daylighting. Not only should those areas allow artificial lights to be turned off, but they should also lower the cooling load by eliminating internally generated heat from the electric lights. Nonetheless, these benefits need to be balanced with the overall impact that additional solar heat gain into the building will create on cooling requirements. The overall intent is for energy optimization and a notable reduction in the carbon footprint of the building.

Second, related to occupant welfare, LEED and the WELL Building Standard both look at indoor environmental quality issues. WELL delves deeper into the details of the impacts on people, but both programs address similar concerns in regard to daylighting, all based on cited sources, research, and standards. In particular, the intent is to use natural daylight to enhance and stimulate indoor environments while balancing performance, value, and aesthetics. There is specific attention given to prioritizing the appropriate control of light color and intensity, which has been shown to greatly help with natural, circadian, biological rhythms in people.

The focus on biological rhythms is important because people have been shown to respond to the natural cycle of daylight (i.e., dawn, brightening, midday, dimming, dusk, and dark) with their own circadian cycles of alertness and sleepiness. Creating artificial lighting conditions that are static and separate people from the cues that trigger their natural circadian rhythm confuses the body as to whether it should be alert or sleepy. One of the primary differentiators has been determined to be the presence of blue light in the spectrum of light color in buildings. More blue light mimics the light color of daylight (think blue, daylit skies) and signals the body that more alertness is needed. Less blue light mimics the changing color of daylight as sunset approaches and is a signal that more sleepiness is appropriate. Of course, this response is not immediate but is based on the pattern of a daily or multiday cycle, which is why changing time zones or simply changing to daylight savings time can be a challenge for many people until their bodies (i.e., their circadian rhythms) adjust.

Incandescent lighting casts a decidedly non-blue color (more yellow), but fluorescent lighting is also lacking in blue light; this lack of blue light is the condition that tells the body to prepare for sleep. Nonetheless, these have been the primary lamping choices in work environments and school buildings for years. Studies that compared workers and students in buildings with these traditional lighting systems to those in buildings with blue enhanced LED lighting or similar, demonstrated significantly better performance as a result. In some cases, dynamic lighting (i.e., lighting that can be controlled to change color spectrum over the course of a day) was used to more closely match the changing nature of sunlight across the day.

Light Color Related to Alertness and Sleep

Dr. Steven W. Lockley, Ph.D., is a neuroscientist affiliated with Brigham and Women’s Hospital and Harvard Medical School, both in Boston, and Monash University in Melbourne, Australia. He has studied and written about the implications for light color and quality related to alertness, sleep, and health. presenting on this topic at the 2018 IESNA Annual Conference and elsewhere. He sees the direct connection between lighting as designed into buildings and people’s responses to that lighting. He has also worked with NASA since the astronauts and scientists in the International Space Station (ISS) can see as many as 16 sunrises in a day, meaning that their circadian rhythms, sleep patterns, and alertness are all affected. In response, he has helped them to change out the lighting inside the ISS so it can be electronically controlled to better suit human needs. His recommendations based on this work apply directly to the design of buildings as follows.

• For buildings where people do not sleep (e.g., work environments, schools, colleges, outpatient clinics, day centers, etc.), it is appropriate to use high-intensity, blue-enriched light continuously all day. This can be applied to overnight conditions too if the building is in full operation overnight or works through shifts. This provides a safe, short-acting, and non-pharmacological stimulant to improve alertness, performance, productivity, and safety. Because there is no need in these facilities to promote sleep, dynamic lighting or the ability to tune the lighting color is not needed.
• For buildings where people sleep and live 24/7 (e.g., residences, hospitals, extended care facilities, prisons, etc.), it is appropriate to use high-intensity, blue-enriched lighting during the day to mimic natural daylight. As the day wanes, then it is appropriate to dim lighting and remove the blue enrichment gradually for as long as practically possible before sleep. (Avoiding sources of blue light for several hours before sleep has also been promoted by computer and cell phone manufacturers since their devices typically produce a blue-enriched light). This approach can help improve sleep patterns and even some common disorders, thus improving overall wellness.

In short, Dr. Lockley is noting that people respond directly to the amount, color, and timing of light as found in natural daylight. His recommendations apply to artificial lighting systems that are trying to mimic daylight and capitalize on the positive effects to help people achieve the desired results. Of course, designing appropriate daylight into buildings will achieve the intended results naturally.

Daylight and Real Estate Value

A study conducted through the Department of Architecture and the Center for Real Estate at the Massachusetts Institute of Technology (MIT) looked at “The value of daylight in office spaces” from another standpoint.¹ Specifically, while the team recognizes that the presence of natural daylight in indoor workplace environments improves human health, well-being, and productivity, it wondered if those benefits translated into economic value—that is, would office tenants pay more for a naturally daylit space than one that is not? To answer that question, the team looked at more than 5,100 different office spaces in the Manhattan borough of New York City. Specifically, it paired urban daylighting simulations of these spaces with an economic valuation model to determine if there was any correlation.

After completing the analysis, the MIT team found that, holding all other factors constant, occupied spaces with access to high amounts of daylight garnered a 5–6 percent premium in rental rates over spaces that did not have high amounts of daylight. The threshold for “high” amounts was based on spatial daylight autonomy, which is defined as a space that can function on daylight alone. In this case, if the tenant space had at least 55 percent of the area that could be modeled to function solely on daylight, it was considered to have a high amount of daylighting; less than 55 percent was deemed to have low amounts of daylighting. In the process of doing this study, the team determined that an estimated 74 percent of the office spaces throughout the dense, urban environment of Manhattan had low daylighting. Therefore, finding a space with high daylighting values becomes a differentiator, one for which the study suggests tenants are willing to pay a bit more.

Daylighting Performance Considerations

With an understanding of the significance and beneficial impacts of daylighting, we now turn our attention to the ways to achieve it in buildings. Fundamentally, there are two types of daylighting strategies: side lighting or top lighting. As the names suggest, they relate to the part of the building where daylight is designed to enter. Side-lighting uses windows, glazed walls, punched openings, or other architectural techniques to allow daylight to be directed inward from the side walls of a building. That means the users of the building will experience the daylight from one side or the other and at a height that is determined by the location of the opening in the walls. There may be some additional items such as light shelves, baffles, or diffusers that help daylight penetrate deeper or become more evenly distributed in the building sides.

Top lighting is focused on bringing daylight in from above the occupants or users of the building. Strategies to achieve top lighting can include roof openings with specifically sized unit skylights or large, monumental glazed portions of a roof. Top lighting can also be achieved through the use of saw-tooth profiles, roof monitors, or other architectural features and offsets to capture and re-direct daylight downward.

In any given building, side lighting, top lighting, or both should be reviewed as potential daylighting sources consistent with the other design parameters of the building. In all cases, there will be some common considerations to take into account as discussed in the following sections.

Glazing Materials

There are numerous choices when it comes to the actual glazing material used. Transparent glass has been considered the default choice by many, but in fact there are many different types of glass with different properties and characteristics. Some of those traits apply to things like material strength, durability, and suitability for different applications. Many are relevant to daylighting, including things like the percentage of visible light transmittance (VLT) and the ability of the glass to diffuse light or not. Keeping in mind that more light is not always better, the LT rating of different glass products allows designers to choose how much daylight to let in or not. Similarly, transparent glass allows for good views but may not always be the best for daylighting. In many cases, translucent glazing that creates a more even, diffuse light distribution within the building is often better, as glare is more easily eliminated since direct sunlight shining through transparent glass creates high contrasts that can be visually distracting or counterproductive. Hence, the most successful daylighting strategies commonly combine a mix of transparent glazing where views to the exterior are needed or desirable plus translucent glazing for more uniform, even daylighting overall.

Glass is not the only glazing material available today. In fact, glass brings limits and some challenges to daylighting in terms of weight, vulnerability to breakage, low thermal performance, and cost. As such, manufacturers of glazing products have emerged with two prominent alternatives to glass. The first is the use of polycarbonate panels that are readily available as a commodity. Polycarbonate glazing is typically formed as an extruded product with two smooth outer faces separated by continuous ribs that provide strength and rigidity to the panels. The thickness of the panel can vary based on the manufacturer and the structural needs of the panel. The ribs give it a general translucent quality, even when clear polycarbonate is used. As a design material, it is also available in a variety of colors, such as whites, reds, blues, greens, and greys. From a thermal performance standpoint, double-glazed polycarbonate is about the same as double-glazed glass, although polycarbonate may have limitations in high-impact situations.

Fiberglass reinforced polymer (FRP) is another common glazing option that is very effective for daylighting applications. It is typically comprised of a thin sheet of material that is enhanced as needed to suit the conditions it may be exposed to either on the outside or inside of buildings, including high-impact and windborne debris. This can include formulations for general performance issues, such as UV blocking, weathering, structural reinforcing, or fire resistance. It can also include options particularly suited to daylighting, such as light transmittance levels, color, texture, or finish. In particular, it is possible to specify FRP that is specifically manufactured with integral resin that is tuned to be spectrally accurate in terms of light coloration related to daylight, thus assuring a greater correlation to sunlight and circadian rhythms.

In comparing these two glazing materials, polycarbonate and FRP, there are a few similarities for daylighting in that they both provide lighter weight, attractive, translucent alternatives to glass. However, there are some very notable distinctions between the two of them as well. Some of those distinctions start with the way the products are installed. Polycarbonate panels are typically a single extrusion that is inserted into an aluminum frame and secured in place in a manner similar to glass. FRP panels are commonly used in a built-up panel that includes an internal frame between two layers of FRP sheets. The thickness, properties, and materials used for the frame can be selected to produce differing performance capabilities. These include the strength of the resulting panel, which allows the creation of different sizes and span capabilities for the panels. It also includes the creation of a space between the FRP layers that can be filled with translucent insulation to boost the thermal performance of the panel. That means that the resulting U-factor of the glazing can be selectively varied with capabilities that far exceed typical glazing. Available thermal capabilities range from U-0.53 (R-1.88, comparable to double-paned glass) all the way up to U-0.05 (R-20, comparable to an exterior wall). Of course, the higher insulation levels tend to decrease the VLT levels, (except for some light-transmitting aerogel-type translucent insulation), so the project criteria need to be clear to select the most appropriate combination.

In addition to structural and thermal performance, there are some other notable differences between FRP and polycarbonate. Only FRP can demonstrate the full color light transmission that preserves the natural sunlight characteristics—all based on third-party verification of testing in this regard. FRP is also available in different colors for situations where altering the color of the light somewhat is preferred, including a range of clear, white, greens, blues, greys, and warm color variations. The FRP is also more easily manufactured to provide strong UV and fade protection to the interior of the building.

Since both products are translucent, they can both claim glare control, but the range of options in FRP for light transmittance allows for a greater degree of control in that regard. Similarly, both products can claim resistance to impacts and offer durability and longevity as a feature. Note that the degree of such capabilities can vary considerably between manufactured products and should be looked at carefully for any glazing product under consideration. In terms of fire resistance, polycarbonate is a thermoplastic that is regulated by building codes. FRP has addressed this issue directly in many cases through the use of thermoset formulations that have been independently tested to demonstrate that they do not drip or melt when exposed to a fire. Thermoplastic polycarbonate, on the other hand, will in fact melt when fire is present and drip onto people and property below. When this condition is a consideration, be sure to be very clear on what the capabilities and/or limitations of the glazing products are in this regard.

While both polycarbonate and FRP can be effectively used for daylighting, reviewing the full depth and breadth of their other capabilities is clearly important for an overall successful building design. In particular, keep in mind that FRP panels on an integral frame creates a composite product with higher spans and strength compared to pure glazing products. This composite construction usually means the panels are self-supporting.

Total Product Performance

While the glazing is the first factor that everyone thinks of in daylighting, the reality is that the other components of a glazing panel are contributing factors to its performance as well. This point has been recognized by most fenestration professionals and has been the focus of the work of the National Fenestration Rating Council (NFRC). This not-for-profit trade organization is an independent source for data on all manner of fenestration products, including those used for daylighting. It offers standardized testing and documentation of the total performance of manufactured glazing products so that objective comparisons can be made between products. This applies to products that use all types of glazing, including glass, polycarbonate, and FRP, whether the glazing is transparent or translucent. It includes the total product performance taking into account the frame or other supports in a product, the glazing, and any accessory materials, such as thermal spacers, insulation, etc., that are used. The result of the NFRC testing is a label (or report in some cases) that allow all stakeholders to learn the following about a particular fenestration product:

• U-factor: The overall thermal performance of the unit is reported based on the total unit. In some cases, the U-factor of the glazing only may also be reported related to the center of the glazing or other locations.
• Visible light transmittance (VLT): This key element of daylighting is critical for predicting how well the daylighting strategy will work. Knowing what percentage of light is actually penetrating into the building helps to determine how much usable light is available. It can also help with an understanding of how much glare potential may exist if a high VLT is observed.
• Solar heat gain coefficient (SHGC): A fenestration unit that allows daylight to pass through will also induce solar heat gain. This is an important design consideration for the overall performance of the building, so being able to have reliable information on this point to compare products is invaluable.

In addition to the above points, the American Architectural Manufacturers Association (AAMA) addresses testing fenestration products for some specific qualities. These include air and water resistance and impact resistance. The preferred level of resistance to impacts (referred to as “missiles”) is missile D, which is readily achieved by FRP products and less so by some others.

Advanced Computer Simulations for Daylighting Design

With an understanding of the significance of daylighting coupled with a grasp on the ways to optimize it in building design and construction, the question now becomes: How do we know if it will give us the desired results? The best way to answer that is to analyze options and determine outcomes using advanced computer modeling specifically for daylighting. Many architects are familiar with some architectural design software (CAD, BIM, etc.) that will do some rudimentary daylighting modeling showing shadow determination and radiance illumination inside buildings. Typically, these programs provide some basic information and have been used by many to get an overall sense of lighting in a building. Other stand-alone or add-on software programs are also used that look specifically at daylighting in a building.

All lighting design and analysis is fundamentally based on the ability to measure quantities of light. In this regard, the international standard is a lumen, which is a measure of luminous flux: the total quantity of visible light (energy) emitted from a source (the sun, artificial light, etc.) for a given unit of time. Since light that is visible to the human eye is actually made up of different wavelengths (hence different amounts of energy), the lumen is actually a weighted average of the makeup of the visible light. Lumens are great for a single point of light, but for architectural purposes, it is more important to know how much light is spread over a specific area (i.e., a square foot, square meter, etc.). Therefore, the more useful standard unit of measure is a lux, which measures the luminous flux (lumens) over a given area. It is standardized so that 1 lux is equal to 1 lumen of light per square meter.

Using lux as the basic unit of measurement, direct sunlight has been measured to provide between 32,000 and 100,000 lux depending on time of day and local conditions. We all know that direct sunlight is very bright to the eye, so levels well below that are considered much more useful inside buildings. Generally, the range of ambient light should be between 300–3,000 lux to be considered desirable. Anything below this level generally requires supplemental electrical lighting, while above 3,000 lux is considered too bright for comfort. There are other standard terms and units of measure that are important to know as well when conducting daylighting simulations.

Daylight Simulations

Using the above as a basis, there are three basic types of simulations that have been commonly performed.

• Radiance illumination simulation: This analysis can show the pattern of daylight on a floor plan with spot indicators and colorized shading to indicate the range of lux levels. It is common to look at specific points in time, such as the fall and spring equinoxes at early morning, noon, and late afternoon. Noon is most helpful for top lighting (most illuminance), while morning and afternoon are most telling for side lighting when the east/west exposures create a worst case for glare. This type of simulation can be done by selecting either sunny or overcast conditions, but overall, it provides limited information to make design decisions. Typically, sunny skies are chosen to see the most light available in a scenario.
• Glare pattern analysis: It is possible to simulate the ‘hot spots’ where glare may become a problem if the lux levels get too high in any areas. A glare pattern analysis helps identify potential problem areas where visual acuity is critical. This is particularly useful for designers when a mix of translucent and transparent fenestration is used.
• Daylight autonomy: This simulation is based on determining the average daylight values and the amount of time that the simulated space can be operational with daylight alone at a targeted light level. It is expressed as a percentage of the time that autonomy is achieved. Spatial daylight autonomy (sDA) coupled with annual sunlight exposure (ASE) are the more specific aspects of this type of simulation.

While all of the above have been useful, the newest generation of daylighting software combines all of the above and provides the designers with the most flexibility in setting up simulations. It also provides the best information back for making design decisions and adjustments. It is based on input from drawings or models (electronic or physical) and can help identify ways to create optimal daylighting conditions that benefit the users or occupants of the buildings. It can also provide information related to the thermal performance of the daylighting systems and how they impact the total building energy performance. The latest software allows for a variety of simulations including, annual illuminance, point-in-time, and glare distribution simulations such as UDI, sDA, ASE, and DGP. The results can be used for analysis and documentation to qualify for points under LEED and other green building standards. By including simulations for EML, the needed documentation for WELL points can be achieved based on spectral sky calculations for virtually any location.

The progress made in the software and techniques to simulate daylighting in buildings mean that some excellent tools are available to design professionals to accurately predict daylighting performance in all of its forms. However, there remains a critical point that must not be missed. All simulations are based on the data and values that are input. Weather and climate data generally come from reliable outside sources through the U.S. Department of Energy; the correct location just needs to be entered into the simulation.

The information related to the specifics of the daylighting systems, however, need to come from the design team. This means the data must be properly selected from the intended materials to be used. Often, this requires getting the data from manufacturers. In these cases, it is critical that the glazing materials in particular have been tested and independently validated for the quality of light that comes through those materials. In addition to simply the percentage of VLT, the spectral distribution (coloration) and the diffusion characteristics need to be known and accurately put into the simulation. Keep in mind that any given manufacturer usually has a range of different products or options that have different daylighting characteristics, which is good to allow customization and fine-tuning of the design. However, it also means that if different products are being used in different parts of a building that the correct values are put into the simulation. Further, when using a combination of products from different manufacturers, recognize that not all manufacturers test and validate their products the same way. Each one needs to be assessed for the correct values to place into a simulation.

As an aid, some manufacturers will offer a service to architects to perform the daylighting simulation using their products and compare it to other commonly known products. This helps assure accuracy of the simulation and saves the architects from needing to maintain a separate daylighting software program. Discussions and review of manufacturer’s capabilities in this regard should be undertaken before using such a service, of course.

Conclusion

Daylighting in buildings has gained increasing significance in recent decades. Human benefits of health and wellness have been documented, and systems have been advanced that help optimize energy performance. At the same time, advancements in the ability to simulate the performance of daylighting designs in buildings now allows architects, owners, and building users to all benefit from sophisticated, balanced, and appealing daylighting design. This all means that designers can put science behind the art of daylighting to show results in advance.

End Notes

¹Turan, I., Chegut, A., Fink, D., and Reinhart, C. “The value of daylight in office spaces.” Building and Environment. 15 January 2020. Web. 24 March 2020.

Peter J. Arsenault, FAIA, NCARB, LEED AP, is a nationally known architect, consultant, presenter, and author of more than 200 continuing education courses focused on creating better buildings.