Daylighting is a foundational consideration in human-centred architectural design, right up there with structural integrity and material selection. However, natural light in interiors is not as well understood as it should be, especially when you consider that modern humans spend more than 90% of their lives indoors.1 We talked to daylighting expert Lisa Heschong, author of Visual Delight in Architecture, about the importance of great daylighting in architecture, what it looks like, and how to achieve it. Read on for the conversation, including seven of her rules for great daylighting in architecture.
Acid-etched glass for optimal daylighting
Using acid-etched glass is one of the most effective ways to diffuse sunlight and optimize natural daylight in a space. Architects can achieve a great variety of lighting conditions just by choosing the right combination of substrate, etched finish, and glass placement.
Learn more about daylighting with acid-etched glass in our article, How acid-etched glass can brighten your day.
You can also explore our selection of acid-etched glass.
Walker Glass – Let’s start with talking about daylighting and views. We benefit from both. We need both. What does each one contribute to human well-being?
Lisa Heschong – I spent the first 25 years of my career focusing on daylight as a source of illumination in buildings, looking at it strictly from the point of view of visual performance using criteria developed by the Illuminating Engineering Society and other groups. I was studying what was needed from electric light or daylight, which were assumed to be equivalent sources, and how to meet those visual needs as efficiently as possible. It was really only 20 years ago that I began to realize that we were leaving the impact of view out of the equation when it came to the human experience of a space. Since then, my thinking about view has evolved and is still evolving rapidly.
1. Use multiple sources of natural light
LH – Certainly, daylighting as a source of illumination is very important. I am a big proponent of using skylights, which bring daylight into the center of space. I also recommend designing buildings so that all the glazing contributes toward balanced illumination. That includes having daylight come from more than one direction. Multi-directional daylighting reduces shadows, reduces contrast, reduces glare, and allows the whole space to be more evenly illuminated. You could use a window plus a skylight, view windows from more than one wall in a room, or high clerestory windows, which bring daylight deeper into a space. All of those are techniques that an architect should know and understand.
Examples of multi-directional daylighting.
Left: windows plus a skylight. Centre: windows in multiple walls. Right: full-height windows.
2. Room with a view: the importance of windows
LH – People tend to take window views for granted, unless of course they’re real estate agents and they know the enormous value that good views can bring to a building. We haven’t been trained to think about or discuss the merits of views very much.
However, from my research, it’s become apparent to me that view windows are the primary source of circadian stimulus inside buildings. That’s because they’re both brighter and more interesting than the rest of a daylit interior, and people are looking at them more often. Views, therefore, are an essential component of the well-being provided by daylighting in a space.
3. Reduce contrast and glare
WG – What about managing daylight itself? How can architects use materials like acid-etched glass to improve natural daylighting?
LH – Used correctly, diffusing glass can provide very even illumination. I always recommend diffusing glass for skylights. In this application, the glass can take very strong sunlight and spread it evenly, broadly, and gently through the space, much like putting a sprinkler head on a hose. A light-diffusing skylight prevents direct sunlight from creating harsh sunspots, which can create glare with reflections or thermal discomfort if they land on top of somebody’s head.
When using diffusing glass, it’s important to remember that it can become very bright in direct sunlight. This kind of glass essentially takes the beam of sunlight and spreads it out, so the sunlight fills the whole room. This also makes the whole window surface brighter, and it can create silhouettes with excessive contrast in certain circumstances. Thus, the use of diffusing glass requires careful consideration of who will be looking at it, and if it will be in full sunlight.
WG – Glare is a serious problem. Why does it bother us so much?
LH – Glare is painful. That, and it prevents you from seeing what it is you want to see. Glare is like noise. Noise prevents you from hearing what you want to hear. In the same way, glare prevents us from comfortably looking at what we want to see.
With that said, young people have eyes that adapt very quickly. Humans have a huge range of vision, and when you’re young you take it for granted. Even if something is glaring, young eyes recover very quickly, within seconds. However, the older you get, the longer it takes for your eyes to recover. For an older person, glare can remain disabling for many minutes afterwards. Similarly, for older people driving at night, the contrast between headlights and darkness can also be disabling. We naturally try to protect our eyes to avoid those problems.
It’s important to note that it’s not the brightness that tires our eyes, per se. It’s the contrast. For example, when my mother got into her 90s, she would keep all the blinds in her house closed. However, she also enjoyed sitting on the porch to look out onto the garden in full daylight. That’s because the garden had uniform illumination too, even though it was probably a thousand times brighter than in the house. She was avoiding excessive contrast.
4. The 90/10 rule for sunlight in interiors
WG – What does great daylighting look like, in your opinion?
LH – In a room with great daylight, all the humans look great, all the plants look great, and colours are bright and vivid. It’s easy to see everything. It’s like being in the shade of a big tree in summer, or walking outside on a slightly overcast day. There’s plenty of diffused daylight, but there’s still enough direct beam sunlight to provide sparkle and highlights.
Note the distinction here between diffused daylight, and direct beam sunlight. Diffuse daylight creates soft shadows; beam sunlight creates sharp shadows. Direct sunlight can be ten to a hundred times brighter than diffuse daylight from the sky.
I have a rule of thumb for designers: allow direct sunlight onto about 10% of an interior, judged relative to the floor area. This can create sparkle and interest, while not overwhelming anyone with too much sunlight. Within the daylighting metrics that we’ve created through the Illuminating Engineering Society2, somewhere between 2% and 20% of ‘Annual Sunlight Exposure’ in a space seems to be tolerable, depending on the design. At more than 2% of the floor area in sunlight, we think people are likely to start pulling blinds, and at more than 20%, they are often overwhelmed and lose control.
5. Give occupants control over their daylighting
WG – Then people start pulling blinds and darkening the space, right? At this point, they’re losing the benefit of views and daylight.
LH – That’s right, and that’s okay if it’s temporary. It’s nearly impossible to anticipate all visual conditions in a space because daylight is so dynamic. Therefore, you must allow the occupants to draw blinds, curtains, or shades. Even if you think you know exactly where the sun is going to be, reflective surfaces can appear and create intolerable conditions on sunny days. For example, there can be temporary, but intolerable, sunlight reflections off puddles, other windows, or windshields. Pretty much any building will eventually get retrofitted with some form of operable sun control if it’s not there at the beginning. You might as well think about it at the start and get it right.
6. You can’t fake natural daylight
WG – Could we talk about timing and circadian rhythms? Daylight changes throughout the day, and now we can get artificial lighting programmed to mimic natural daylight cycles. Are these types of lighting equivalent in terms of circadian rhythms and human well-being?
LH – Engineers are constantly trying to figure out the perfect formula so that they can mimic nature. Then, once they think they have it, biologists find something else important about natural systems that was left out of the engineering formula. I’ve watched this go back and forth, back and forth for decades. We tend to have a lot of hubris that humans can figure it all out and engineer the perfect solution to replace nature and make a product to sell. There are a lot of people working very hard on that right now.
However, we’ve been learning recently that the health impacts of daylight are not just about its timing and intensity, but also the subtleties of its wavelengths and how those change throughout the day. Those changes are especially noticeable at dawn and dusk, when there are the most rapid shifts in daylight’s spectral content. Besides this, it’s very difficult to match daylight’s intensity with electric lighting, and it’s expensive in terms of energy.
Daylighting also gives people a direct visual understanding of their environment, supporting our internal ‘cognitive map.’. Weather, time of day, and daylight’s natural progression over time give us a sense of temporal and spatial orientation. Daylight makes you feel connected to life and the real world. Even if we could simulate daylight perfectly, people would still know that they were in a simulation. It is unnerving and disorienting. It would be like putting yourself in a movie 24/7 instead of in real life.
WG – That’s a timely comment, considering how much of our lives has gone virtual over the last few years. No matter how much science progresses or how much we’re “connected” via virtual technology, we still crave a connection with the real world.
LH – Yes, indeed. There’s nothing much more real than sunlight!
7. Location, location, location
WG – Is there anything else essential that architects should know?
LH – Well, to do a good job with daylighting, an architect really needs to understand the sun’s daily and seasonal position relative to the building’s location and orientation. This is as fundamental as understanding where gravity is. You wouldn’t design a building without understanding gravity, and you should not design a building without understanding solar movement and local climate. For example, where, when and why is too much sunlight a problem? And where and when is a bit of sunlight delightful?
WG – Thank you very much for your time, and for sharing your expertise with us.
As we’ve seen, good daylighting is crucial for human comfort and well-being. It’s also more complex than many realize, combining factors like diffusion, glare, timing and occupants’ behaviour in often-unpredictable ways. Great designers and architects understand how to harness these elements to create a symphony of harmonious, channelling natural daylight to inspire delight instead of creating discomfort.
Rules for Great Daylighting
Want to design beautifully daylit buildings? Here’s a checklist of the seven essential considerations for a well-daylit space:
- 1. Use multiple sources of daylight: For example, design windows in combination with a skylight, or windows on multiple walls to fill out shadows and reduce contrast.
- 2. Room with a view: Include views of the outdoors in your daylighting design strategy.
- 3. Reduce contrast and glare: Diffusing materials like acid-etched glass can spread daylight evenly through a space, reducing harsh contrasts and making light easier on the eyes.
- 4. The 90/10 rule: Aim for 90% diffused daylight in interiors, and 10% direct sunlight. This keeps the overall lighting gentle and uniform, while allowing some sparkle and highlights to add visual interest.
- 5. Give occupants control: Include blinds, curtains, or other tools to let occupants manage the sunlight and daylight in their spaces.
- 6. You can’t fake natural daylight: There’s much more to circadian rhythms than timing, and natural daylight is still far better for human health than clever facsimiles of it.
- 7. Location, location, location: Above all, you must understand the dynamics of solar positions and climatic conditions at your particular building site.
About Lisa Heschong
Lisa Heschong was a Founding Principal of the Heschong Mahone Group (HMG), and a licensed architect for 30 years. At HMG, a building science and energy efficiency consulting firm based in Sacramento, California, Ms. Heschong led the research team that found a correlation between daylight in classrooms and improved student performance, and daylight and retail sales.
She is a Fellow of the Illuminating Engineering Society (IES) and a newly elected member of the Daylight Academy. She has served as Chair of the IES Medals Committee and the IES Daylight Metrics Committee, where she helped establish a new set of climate-based daylight performance metrics, including sDA and ASE.
Her latest book, Visual Delight in Architecture: Daylight, Vision and View (Routledge 2021), explores new findings on the physiological, cognitive, social and cultural importance of daylight and view in our everyday environments.
Learn more at lheschong.com
- Heschong, Lisa. Visual Delight in Architecture: Daylight, Vison and View (New York: Routledge, 2021), 1.
- LM-83-23, Approved Method: Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE), Illuminating Engineering Society, www.ies.org (Note: These metrics require annual hourly analysis. Thus, they consider all yearly solar positions and local weather conditions.)