Basic Sustainable Lighting Concepts: On Daylighting

June 27, 2011 / no comments

Part 2 of an ongoing series outlining design principles for sustainable lighting design: here are a few ideas regarding daylighting, to help navigate the greenwash.

Only a little direct sun, please

Too much direct sunlight increases the indoor temperature, creating higher cooling loads. It also increases the potential for glare. If there’s too much glare, people are likely to pull the shades and leave them that way, which equals no more daylighting! Most interior shades do little to reject the heat load. Consider using exterior overhangs to keep excessive sun outside, and light-shelves to distribute the daylight indoors so it’s more useful.

Don’t add complexity and cost by creating one problem and mitigating it with another technology. The New York Times Building has been criticized for this. Its floor-to-ceiling glass has the potential to let too much light and heat inside, so the ceramic tubes outside the glass were introduced to help block some of it. If you have less glass to begin with, you can use less exterior shading… If you can afford it and don’t care, then have at it, as long as you keep your energy use down. Otherwise, try not to pile on unnecessary complexity chasing an aesthetic.

Installing shades is not daylighting

Simply installing internal glare-control shades or blinds is NOT a form of daylighting. Neither is using a lot of glass just to get more light inside. The façade of a building must engage the sunlight to utilize it in a meaningful way, coaxing the useful light in while controlling excessive light and rejecting heat. This means articulated façades, not flush glass.

If you do use shades, make them automated if you can afford it. Automated shades can adjust for different lighting conditions throughout the day, and they don’t rely on a forgetful occupant to pull them back up. If you can’t afford automated shades, try to design your envelope with external shades or a light-shelf such that you can keep the upper part of the window open all the time and still allow manual shading below it.

Dimming the lights

Daylight switching is no replacement for daylight dimming. Switching has a tendency to irritate occupants, because it flips the lights on and off throughout the day when the ambient light is near the threshold light level. More often than not, if it doesn’t work correctly, it will simply be disabled instead of fixed. You definitely can’t rely on people to make the best choices on an hourly basis either – the lights go on and stay on all day. Flipping a switch is what we’ve been trained to do all our lives.

Rely instead on dimming your perimeter spaces. There are variable levels of savings to be had here, from actual energy savings, to rebates just for putting daylight dimming systems in. Every little bit helps in terms of energy – initial cost is a different matter. There may be legislation or changes to the building codes in the near future that would require you to use daylight dimming anyway.

Digital is in!

All the ballast manufacturers, and a few lighting controls manufacturers, are finally, albeit slowly, switching over from older analog technologies, to digital or hybrid analog/digital systems that operate with greater precision and functionality. If you use one of these emerging technologies, your system is more likely to still be in style in the next decade or so (but don’t jump the gun on a brand-spanking-new product, lest it be discontinued). DALI is one of those technologies; it’s been around for about ten years now, and is slowly catching on in the US.

Don’t go crazy

Just because dimming is warranted in daylit zones and conference rooms, doesn’t mean you should use it everywhere. Some advocates claim additional energy savings by being able to dim the lights everywhere, but that would only be if you’ve over-illuminated your interior spaces to begin with. Design them correctly and you can save a lot of materials and costs. Dimming everything is another example of mitigating a problem that you may have created yourself.

Shortchanging Daylight

June 13, 2011 / no comments

The reason for daylighting in buildings is to save energy, and so the value (“payback”) of that daylighting can be calculated by predicting and pricing the amount of energy saved. That’s a common line of thought which is easy to slip into, but it’s dead wrong.

Let’s look at a simple example of a new office building. A typical office worker’s space, including adjoining corridor, is about 110 square feet. Under today’s codes, we’re allowed 100 watts maximum to light that space. If it’s lighted 10 hours per day, 5 days per week, 52 weeks per year, that lighting will use 260 kilowatt-hours per year. At a high-end cost of $0.20 per kWh, that’s 52 bucks for electricity to light that space for a year. Let’s add another 30% for extra cooling cost due to that electricity, and we’ve got almost 68 bucks.

If our worker is the median American clerical worker, according to the US Bureau of Labor Statistics, his salary rate is about $14.40 per hour. Throw in 25% for statutory fringe benefits, and he’s costing his employer 18 bucks per hour.

So let’s say we have a wonderful daylighting design which uses absolutely no electricity to light our worker’s space. That 68 bucks per year equals less than four hours of his salary. That’s right: four hours.

If we have a wonderful daylighting design which improves their productivity by 1%, that saves their employer 375 dollars per year. A productivity improvement of just 1% creates a “payback” five-and-a-half times greater than the value of saving all of their lighting electricity. Run those numbers for a more highly-paid professional, legal, or scientific worker, and the productivity value will be much higher still.

To put it another way, if we calculate the payback of daylighting based only on electricity, we’re grossly underestimating the real payback; we’re shortchanging the daylighting. And that will lead to incorrect design decisions.

Some sophisticated building owners and managers know this. Savvy retailers know that their sales will go up with daylighting. Knowledgeable educators realize that the performance of students in daylighted classrooms will improve. Daylighting produces known health benefits.

We tend to think of these benefits as intangible, but they actually represent large numbers of tangible dollars on the bottom line. Like productivity, these factors aren’t intangible, they’re just hard to quantify.

By the way, the same calculation applies to good electric lighting as well: it may save a few dollars in electricity each year, but its value is vastly greater than that.

Photo Credit: ©Anton Grassl/Esto

Caveat Metrics

May 11, 2011 / no comments


Daylighting metrics are methods for measuring the quantities of daylight in a space during a period of time. More and more, metrics are becoming the dominant means by which daylighting in a space is evaluated. With the imminent adoption of the International Green Construction Code and other codes mandating daylighting, the use of metrics will become even more integrated into the daylighting evaluation of buildings. While evolving analysis tools provide new and exciting capabilities, they also present new challenges to the designer or consultant.


Metrics have the inherent benefit of providing better information on the performance of a space than traditional rule-of-thumb methods. They are fast, adaptable, and instill confidence in the client, and the flexibility of digital modeling allows many variations of a design to be tested quickly at early stages of the design process. Unlike rules-of-thumb, metrics are more easily capable of evaluating non-orthogonal spaces, and they are becoming more accessible as more and more software provides daylighting analysis tools. And if that were not enough, increasingly, clients demand to see statistics and false-color grids in order to be convinced that their building will perform well, achieve credits, or meet codes.

But, like all things, metrics have downsides: metrics can be deceptively convenient. It seems as if it should be relatively easy to just build or import an architectural computer model into a simulation program and run the metric, but this is not so. Each software has its own rules for producing correct output. These include ways in which geometry should be modeled, whether or not to include a ground plane and how to define materials. Different lighting simulation engines have different ray-tracing methods (e.g. backwards versus forwards), and different simulation settings. The same basic variable likely has a completely different name from one program to another, and of course, the software interfaces are different – certain programs allow control over lighting variables, while other programs keep the user from accessing or modifying those variables.

Conversely, one benefit to the increased focus on daylighting metrics is their increasing accessibility. Plug-ins like DIVA-for-Rhino and the su2rad script allow widely used softwares like Rhino and Sketchup to interface with Radiance, the premiere calculation engine. While this overall accessibility is positive because it allows daylighting analysis to be employed more freely, making it more of a player in design decisions, it also makes education about the proper use of those metrics much more important.

The first step in understanding metrics is to know what metrics currently exist and what information they can provide. A useful guide to daylighting terminology was provided by Kevin Van Den Wymelenberg in an Architectural Lighting article in 2008, where he defines several of the daylighting metrics currently most in use today: Illuminance, Daylight Factor (DF), Daylight Autonomy (DA), Continuous Daylight Autonomy (CDA), and Useful Daylight Illuminance (UDI).

As a quick overview, the main distinction between various metrics is between the so-called “point-in-time” (Illuminance, Luminance) and annual, climate-based calculations (DA, CDA, UDI). Point-in-time calculations measure light levels at a specific date and time, under a specific sky condition. These calculations are more intuitive because they mimic how we experience the world: we see the light levels change from one moment to another. Annual or climate-based calculations, on the other hand, use weather data to simulate lighting levels over the length of an entire year. As such, they are more comprehensive than point-in-time metrics, but are also a more abstract, less intuitive way of measuring lighting. While they provide a more comprehensive performance evaluation, they may not show as clearly why one scheme performs better than another. Daylight factor, which originated in the cloudy climate of Britain, is neither point-in-time nor annual, as it uses an evenly illuminated (overcast) sky condition to measure interior-to-exterior light ratios.

Once designers have some idea of which type of calculation to use, they are faced with the issue of whether or not they can use it. Currently, the majority of lighting calculation software provides only illuminance and luminance calculations on a point-in-time level (for example, a clear day on September 21st at 9:00 AM). In general, there is a movement towards using annual, climate-based calculations rather than point-in-time, but the critical issue is that most commonly used daylighting programs do not support climate-based metrics. At present, 3dsMax and AGi32 only calculate illuminance and luminance (point-in-time). Daysim is the only widely used lighting engine which can perform the annual calculations.

The given metric may not really deliver answers to the questions at hand. From an architect and owner’s perspective, there are usually several critical questions posed to the consultant about daylighting. The first two are: how often will we be able to dim or turn off the electric light, and how will daylighting affect thermal performance? Currently, there is no good metric to directly answer those questions. Christoph Reinhart and Jan Wienold have developed one metric, called Daylight Availability, which perhaps comes the closest. In their paper “The Daylighting Dashboard – A Simulation-Based Design Analysis for Daylit Spaces,” they document the metric. It combines DA (Daylight Autonomy) and UDI (Useful Daylight Illuminance), and shows, in one false-color grid, the assessment of areas that are likely to be overlit (requiring shading), well lit by daylight alone, or partially daylit (requiring supplemental electric light). It is possible that this metric, or one like it, could fill the void.

The final part in the daylighting metrics process is the output. Once a metric has been chosen and run, the programs produce either a rendered image, a false-color image, or a grid of numbers as a result. The job of the daylight analyst is done, right? Of course not. This step can be the most challenging of all. Expressing daylighting analysis results in an intelligible way, and presenting them to a client can be difficult. There is no formula for the best way to do it, and it often comes down to what the particular situation requires. The fact is that it is difficult to synthesize in a single image the variability of lighting conditions over the day and year, and when multiple design options like shading devices, materials, or orientations are added, the complexity expands proportionally. Given this, there is a tendency to become metric-happy and produce copious studies for different times and under different conditions; this often overwhelms the client who, unfamiliar with the format, may barely understand a single false-color grid, let alone a set. Even for sophisticated daylighting designers, the useful conclusions may be hidden in the sheer mass of output.


Outputs produced with DIVA-for-Rhino

There is no single metric which can answer all questions; each provides only part of the story. Annual calculations provide information about lighting levels, but not about glare, thermal costs, or aesthetics. One idea beginning to gain acceptance as a solution is the concept of a “dashboard”. Dashboards, as laid out by Reinhart and Wienold, are meant to show summary results of many metrics in a single side-by-side view, although, it should be noted, that synthesis is still left to the consultant.


Reinhart and Wienold, “Daylighting Dashboard” concept image

Lastly, the architect’s and owner’s question, “What will it look like?” still prevails. False-color grids and numbers don’t read as quickly as does an image, and after all, a large part of the value of daylighting design is improving the visual quality of the space. Images may contain the least amount of hard data, but they tend to go the furthest in illustrating daylighting concepts to clients.

As we enter this new phase of daylighting analysis, it is important to know the strengths and shortcomings of each metric and to be informed as to how to properly use them. The increased predominance of the computer does not change the fact that it is the designer who must know how to use the tools, how to understand the results, and how to effectively communicate the results to team members and clients.

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Radiance Visualization using DIVA-for-Rhino

Images credit: Kera Lagios(1-3,5), Christoph Reinhart and Jan Wienold (4)


A Daylighting Pattern Language: Bilateral Lighting

February 9, 2011 / no comments

In Christopher Alexander’s book A Pattern Language he points out: “When they have a choice, people will always gravitate to those rooms which have light on two sides, and leave the rooms which are lit only from one side unused and empty.” He touts that “this pattern, perhaps more than any other single pattern, determines the success or failure of a room.”

Why is this? Alexander goes on to point out that his experiments had been rather informal and drawn out over many years. But the trend is very real. He also recalls that light on two sides was a tenet of the Beaux-Arts design tradition. What is it about certain patterns of light that attract people or enhance space and volumes effectively? Looking to the past may be the best way to design daylighting for the future.

In last September’s entry, ‘A Daylighting Pattern Language’, Robert Osten used Le Thoronet abbey as an example of how well small apertures were designed for introducing daylight into the interiors of large masonry constructions. Before the use of sophisticated computer models and analytical studies, architects based their designs on common sense, common practice, and a basic understanding of the relationship between architecture and the sun. Why are some spaces much more successful than others? When it comes to light, and especially daylight, it’s a human response: what feels good.

Let’s examine some reasons why a room or space with bilateral light is more successful than one with light only from one side.


The first, seemingly obvious, reason is brightness balance and contrast ratios. For example, sitting in a space and talking to a person in front of a window without the benefit of light on their face (from another window or skylight on the opposite side of the room) is not only uncomfortable, but since their face is in silhouette, it is difficult to read their expression, giving us little or no information about their mood or response.

In work environments, the contrast ratio could cause discomfort, and result in eye fatigue. R. G. Hopkinson in his book Architectural Physics: Lighting, published back in 1963, discusses the issues of glare and contrast in great detail, showing how it affects much more than simply the quantity of light. Today, from simple evaluations to exhaustive studies, we continuously find proof that high contrast and glare will affect productivity in the workplace.

Not only is bilateral lighting preferred for being more comfortable, but it instinctively feels more natural. Balanced light – light coming from more than one direction – is more akin to ‘natural’ light. In the great outdoors we have both direct sunlight and light from the sky itself – light coming from all different directions. This helps provide depth, giving us clear information about shapes and forms. A strong light source from one direction tends to flatten our views, providing less visual information. Dramatic, but a bit unnatural, and uncomfortable over long periods of time.


Notice how the unlit wall, relative to the illuminated walls on either side,
looks dim and cavernous in contrast with the daylit atrium above.

Since many large or multi-story buildings tend to introduce daylight through sidelighting, it is critical to balance the overall lighting in the space. The more daylight introduced from one side, the more light it will take from the opposite side to offset cavernous effects. Of course, introducing more daylight through an additional window or clerestory is the most effective approach since it will maintain the same exterior intensities. If daylight cannot be provided, supplemental electric lighting should be designed to fill in the gradients where daylight is lowest. Washing walls and surfaces with light and using light colors will increase the perception of balanced light in the space.

The relationship between daylighting apertures and electric lighting is key. There is a lot to be learned from historic, daylit buildings. We should not ignore those patterns throughout the history of great architecture that have always met with positive responses from the humans who use them.


Photos: © Lam Partners

A Daylighting Pattern Language: Deep Apertures

September 27, 2010 / no comments

Le Thoronet is one of three wonderful Cistercian abbeys in Provence, built around 1170. In the mid-twelfth century this part of southern France was not a major tourist destination. The monks who built Le Thoronet were avoiding the political intrigues and feudal power struggles of the cities by locating in a remote area, and they weren’t necessarily welcoming company. And they were building for eternity, too, so the walls are thick, sometimes over three feet thick. As with a lot of ancient masonry construction, this has a salutary effect on the way daylight works in the interior. Why?

In contrast to today’s vogue for all-glass buildings, how is it that massive masonry construction can result in wonderful daylighting? This has a lot to do with contrast control, which is related to the deep apertures created through the thick walls.

Because sunlight is such a powerful light source, a major challenge with daylighting is to moderate the contrast between very bright exterior views and the relatively much darker interior surfaces. In particular, the interior face of the wall containing the window often tends to be the darkest surface in the entire space, since it may receive no direct daylight at all. This can result in very harsh contrast at the apertures. But, when the aperture has depth, the sides of the opening provide extensive surfaces with a brightness that is intermediate between exterior and interior, graduating the contrast.

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Without this kind of buffer, the contrast is often more than the eye can comfortably accommodate. If splay or architectural ornamentation is present in that zone, the contrast gradient is even more improved; the ornamentation itself is beautifully rendered by the raking light and brightness gradient from exterior to interior.

In addition, those surfaces, especially the sill, diffusely reflect daylight into the interior – for example, illuminating the ceiling even though most of the original daylight source is heading for the floor.

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These are daylighting principles we would do well to emulate in our designs today. We’re rarely going to have walls three feet thick to work with, but we can accomplish similar effects by, for example, positioning our apertures against flanking walls or piers. In this house by Tadao Ando, the room surfaces perpendicular to the apertures have a brightness intermediate between the view outside and the darker interior surfaces. In addition, they diffuse daylight back onto the inside surface containing the aperture, which further softens contrasts.

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The deep aperture approach lies in stark contrast to just treating daylight apertures like simple holes in the wall. Besides improving contrasts, the deep aperture uses daylight as a powerful expression of the extension of space.

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Photo Credits: Betina (1), Nicola Comodo (2), Glen Craney (3), Lam Partners (4-5)

Dawn of the Daylighting Codes

December 21, 2009 / no comments

It’s pretty safe to say that people like daylight and sunlight. Daylight is good for people, since it sets our biological rhythms, gives us a connection to the weather and time, keeps us physically and mentally healthy, and obviously allows us to perform visual tasks. It’s no wonder then, that architects through the ages have designed architecture to effectively introduce sunshine and daylight into building interiors – not only to sustain human life, but to allow it to flourish.

Daylighting has been an integral part of the built environment throughout architectural history, and structures that are thousands of years old are still revered for their daylighting qualities. “The history of Architecture is the history of man’s struggle for light – the history of the window,” wrote Mies van der Rohe.

It’s only within the last 75 years or so that daylighting has been supplanted by electric lighting as the primary source of interior daytime illumination. Ever since the introduction of air-conditioning, and especially of modular gas-discharge lighting (i.e. modern fluorescent lamps), windows and skylights have been getting smaller and floor plates have been getting larger. Our luminous environments have been deemed adequate and appropriate based on a simple numerical criterion, horizontal footcandles. However, in recent years, especially with the ‘green’ movement, there has been much more pressure to re-introduce daylight back into our interiors and create daylit architecture once again.


But what exactly is ‘Daylit Architecture’? It’s difficult to define. For architects it may be about beauty and ergonomics; for engineers it tends to be focused on energy and economics. Fortunately, with recent studies, we finally have hard evidence showing that daylight in schools improves test scores, and daylight in the workplace improves productivity. In retail, it boosts sales; in hospitals, it reduces recovery time. These studies embolden the stance of the ‘quality’ seekers.

But, on the other side are the energy tyrants. They want to see fewer windows in architecture since windows are terrible insulators. The criticism is real. News stories are unfolding about LEED buildings and how they are not living up to their touted energy claims. But the LEED points for daylighting and views have nothing to do with saving energy. It’s all about interior environmental quality.

So now, there is a bigger push to improve energy usage and enforce ‘green’ building codes. LEED, CHPS, and other programs give you the option of getting daylighting points. A ‘green’ code will require it. There has been overwhelming support for some type of daylighting requirement or code, but the problem seems to be in writing one. Most would agree that, if introduced properly, daylighting can save energy associated with interior illumination. The more difficult aspect is quantifying quality. How do you require architecture to beautifully introduce daylight and sunlight into itself?

Codes requiring access to daylighting are relatively new to the United States. Title 24 in California already requires daylighting in certain buildings. There’s a rich history of codes requiring access to daylight. An English law dating back to 1663, Ancient Lights, is a form of easement that gives owners of a building with windows a right to maintain access to daylight. Justinian Code in the sixth century AD included sun rights, laws to ensure that every homeowner had reasonable access to the sun. And, many modern European codes require daylight and views for workspaces and classrooms.

Get ready for daylighting codes across the United States. Come late spring 2010, ASHRAE will have introduced its new Standard 189.1, which is basically a ‘green’ standard that goes beyond the energy-saving measures published in ASHRAE Standard 90.1. It also contains a lot of language about minimum amounts of windows and required illuminance from daylight. The other big player is the International Code Council, with their new proclamation, the IgCC, or ‘International Green Construction Code’. In that particular code, the daylighting portion will most likely be broken into two sections: energy and indoor environmental quality. This approach makes the most sense for both camps. We want enough daylight and views to elevate the human spirit, but not so much as to cause glare or unnecessary energy usage associated with excessive cooling loads.

It won’t just be footcandles and daylight factors anymore. Relatively new metrics such as Daylight Autonomy, Daylight Saturation Percentage, Useful Daylight Illuminance, and Daylight Glare Probability may become common language within these new daylighting codes.


It’s probably time that we have some sort of code that protects and even encourages our access to our greatest energy source, the sun. How it is written makes all the difference. It cannot reward poor design, or suffocate good design.

Great daylit architecture comes from the brilliant architects and designers who create it, not from a formula or code. But gone are the days of overly-glazed façades used in the name of ‘daylight’. Responsible practice must produce sustainable architecture, even if it has to be mandated.

Photo Credits: Elinnea (1), Roryrory (2), Stephen Lee (3), Lam Partners Inc (4)

Heliodon 4.0

August 10, 2009 / no comments

In an era when computer speed and software capabilities are constantly improving, one might wonder why any lighting design firm would bother with physical model-testing for daylight analysis anymore. There are many different types of software available today that can be useful for studying daylight in architectural spaces, but there are still compelling reasons why studies performed by traditional methods will never be completely replaced by computers.

The cornerstone of these methods is a multi-axis device known as a heliodon. A scale model is first secured to the table, oriented correctly north-to-south, and then the table is angled to match the latitude of the real building’s location on earth. A solar sundial is positioned next to the model to identify time of day and time of year.

Rotating and locking one axis of rotation then establishes time of year, while rotating the angled table recreates the changing time of day. Once these variables are set, one can immediately see daylight interacting accurately with the model any time of day, any time of year, anywhere on earth.

The newest heliodon at Lam Partners is a testament to the belief that physical model-testing continues to be indispensable in evaluating daylight. Easy video integration, fast modifications to the model, perfect rendering of materials, and a comprehensible interface are all reasons why we decided that upgrading our heliodon was worthwhile, and this fourth-generation model incorporates new design features gleaned from 35 years of daylight analysis experience.


The primary feature is the motorized table that allows for precise rotation, used in conjunction with a digital video camera. A heliodon is able to quickly cycle through sunrise to sunset, so smooth rotation at a consistent speed is critical for high-quality video output.

Another feature is a wide, stable, and adjustable base platform that is removable and can be field-leveled for the most accurate results. Part of the overall design objective was to make the entire heliodon easily broken down and portable, with three easily reassembled components that all use the same size bolt. All necessary tools are stored neatly within the frame and easily accessed.


The improved precision of the new heliodon required a sundial that was equally precise. We designed a new one and had it CNC machined from solid aluminum, like the heliodon (yes, the chips were all recycled!). The new construction features an adjustable and lockable latitude angle, as well as a removable and replaceable gnomon that can be safely stored within to prevent damage. A polar sundial design was then laser-etched onto the front for durability.

Together, the new heliodon and sundial provide a level of exactitude never before realized in earlier designs. After the initial set-up, an architectural model can easily run through an entire year’s worth of sun-angles in barely an hour on the heliodon. Since sunlight works the same at any scale, measured footcandle levels inside the model, when factored to account for local latitude and season, are accurate and reliable.

This breadth of valuable information takes far longer to gather using computer software which usually lacks the intuitive feel and quick adjustments that physical modeling offers, and most daylighting software continues to struggle in accurately representing complex materials and textures or multiple reflections.

So, while Lam Partners certainly does use software for some aspects of daylight study such as basic sun-angle geometry, the qualitative advantages inherent to heliodon analysis make it an extremely useful tool for daylight-intensive projects.


Photo Credits: Anna Baranczak / Lam Partners Inc (1, 4), Justin Brown / Lam Partners Inc (2, 3)

Daylighting Reduces Heat Gain – Pantheon Redesign?

June 14, 2009 / no comments

 In our June Photo of the Month article, we talked about the daylighting in the Pantheon. Let’s do some numbers just for fun: on a partly-cloudy March day in the mid-afternoon there will be about 1.2 million lumens streaming through the Pantheon’s 700-square-foot oculus. The interior light levels are fine. If we tried to equal that with large metal-halide indirect floodlights, it would take about 15,000 watts. The heat energy in the daylight will be about 10,000 watts. So if heat gain is a negative, the daylight is winning on that count.

But we can make it even better: let’s renovate the Pantheon, so we have glazing, not an open aperture. The right glass can help. First, we’d need to increase the aperture area to compensate for the light transmission of the glass. So we increase the Pantheon aperture to 1,000 square feet to make up for a light transmission of 70%. Now we still have the original amount of visible light, but what about heat? Using a high-quality low-E insulating glass, the heat gain per square foot would be reduced to about 35% of what the same aperture would admit with no glass. So despite the increased aperture area we actually have only half the heat gain of the original open oculus, with the same amount of admitted daylight.

Let’s compare that to the electric lighting equivalent. All of the daylight is going to eventually end up as heat in the space, and so will the electric light and the electricity used to produce it. So with the new glazing our daylight is adding 5,000 watts of heat gain. The equivalent metal-halide system is adding 15,000 watts. So the daylighting is now creating only about one-third as much heat gain as the electric lighting that would achieve the same light level.

So that’s a big win for the daylight, right? Maybe. It’s a win only if we assume that the daylight light levels and the electric light levels are the same, and that’s tricky. Clearly, if the daylight levels are higher there will be more heat gain; if they’re three times higher the daylight will produce the same heat gain as the electric system. This is not as unlikely as it may sound. It’s easy to design an electric lighting system that maintains virtually the same desired light level all the time – it’s not at all easy to do that with a daylighting system.


For the Pantheon, what happens on a nice sunny May afternoon around 2:00? Now we have well over 5 million lumens coming in through the oculus, and even with the new glass the heat energy in the daylight will be about 22,000 watts, or about 75,000 BTU per hour. But the electric lighting system would still be plugging along at the same 15,000 watts. Sure, the daylight level is now higher than the electric light level, but we don’t need that extra light, and now the daylight is producing more heat gain than the electric lighting would. And that means we’re going to increase the amount of energy we have to use to air-condition that heat away (did I mention we’re adding air conditioning, too?). And by June the problem will be even worse.

This illustrates one of the important challenges of designing daylighting: maintaining reasonably consistent light levels at different times of day and different times of year (and also under different weather conditions). There is a related design challenge: maintaining consistent light levels at a given time but in different parts of the space. Because the Pantheon aperture is at the center and high above the floor, light levels at the floor will generally be very uniform (although occasionally on mid-days in summer direct sun will hit the floor, and all bets are off).

But in multistory buildings the daylight often has to enter from the sides, and we don’t usually have 142-foot-high ceilings, so daylight levels near the windows tend to be much higher than those farther away. The issue is the same for both challenges: if we design for an adequate daylight level for less-than-favorable conditions or locations, we can end up with much higher than needed daylight levels for the favorable conditions or locations.

There is also a tendency to simply overdesign daylight levels under all conditions. Our Pantheon example had a daylight aperture of only about 4.5% of floor area: a glass curtain-wall building could easily have a ratio of 40%. Without extensive shading and low-transmission glass, that is very likely to result in daylight levels much higher than needed. And those higher levels bring higher heat gain. They can also be visually uncomfortable, but that’s another subject.

Photo Credits: Irene (2), OliverN5 (3)