daylight autonomy 101

18/02/2015 McGraw-Hill Construction - Continuing Education Center

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Daylight Autonomy 101Specifying useful daylight deeper into the interior

Nov ember 201 3

Sponsored by Lutron Electronics Co., Inc.

Jeanette Fitzgerald Pitts

Throughout history , day light has been considered in the design ofthe built env ironment, although the philosophy of its use and theobjectiv e of its presence hav e changed dramatically ov er the y ears.Before electric light was inv ented, in the late 1 9th century , day lightwas the primary light source av ailable to illuminate the interior ofbuildings, schools, and residences. The houses of Ancient Rome werecommonly found to hav e been planned around a courty ard, withsurrounding rooms positioned so that the av ailable day light couldpenetrate deeper in the interior space. A guiding principle of thedesign of Michelangelo's iconic Laurentian Library , built around1 550, was to maximize the presence of day light from both thenorthern and southern exposures in the reading room. In the mid-1 800s, the one-room schoolhouses that dotted the United States fromNew England through the prairie and into the western frontierrelied on large windows to prov ide the teacher and students withenough light for their lessons.

Almost 1 00 y ears after the inv ention of electric light, and,subsequently , after fluorescent lighting became more widely used,the philosophy of day light's role in the interior changed quitedramatically . Instead of rely ing on large windows and day light asthe primary source of light, the mid-1 960s and 1 97 0s saw schoolsthat were designed with few and no windows, citing significantenergy losses from windows. The v iews to the outdoors were ev enconsidered a distraction for the students, indicating that awindowless room may be thought to actually improv e studentperformance.

Today , the attitudes toward day light seem to hav e come full circle. Study after study has quantified the benefit ofday light exposure to employ ees and students alike. Day light exposure has been linked to improv ed employ eeproductiv ity , student performance, and ev en the regulation of a person's circadian rhy thm, which driv es the all-important wake/sleep cy cle. Day light exposure is considered such a benefit to building occupants that thepreeminent green building rating sy stem in the United States, the Leadership in Energy and Env ironmental Design(LEED) green building rating sy stem contains credits that are specifically awarded for the project's incorporation ofday light and outdoor v iews.

http://construction.com/CE/



The Vista Center was designed to achiev e day light autonomy and is able to illuminate asignificant portion of the interior space exclusiv ely with day light during working hours.

Photo courtesy of Coscia Moos Architecture

Bey ond improv ing the human experience, effectiv ely incorporating day light in the interior, or day lighting, candramatically improv e the operational performance of the building and create energy sav ings. In 1 997 , theIlluminating Engineering Society of North America (IESNA) published a guide entitled Daylighting Design: Smart andSimple in which it posited that a building with a 25 percent window-to-wall ratio could realize a lighting energysav ings of roughly 30 percent by reducing electric light lev els when sufficient day light lev els were av ailable.

Both day lighting know-how and technology hav e dramatically improv ed ov er the past decade, adv ancing the roleand the potential benefits that day light can now deliv er to the built env ironment. Day light harv esting, the practiceof reducing electric light lev els when day light is present, is becoming more and more commonplace and is now, infact, required by ANSI/ASHRAE/IESNA Standard 90.1 201 0 and the California Building Efficiency Standards.Some designers are now interested in taking day light av ailability to the next lev el and using it, when possible, as theexclusiv e light source for a space, in much the same way it was used in the one-room schoolhouses of the past. The



phrase for this new design objectiv e: day light autonomy .

Introducing Daylight Autonomy

Today , designing a space to meet specific day light-related objectiv es is a common practice. The usual day lightinggoals include achiev ing some predefined day light illuminance lev el on the workplane or at the floor, incorporatingsome measure of glare control, or deliv ering a day light zone of a certain size. Achiev ing day light autonomyessentially requires a project to achiev e all of the abov e and more.

The 1 0th edition of The Lighting Handbook, published by the IES (Illuminating Engineering Society of NorthAmerica, formerly IESNA), defines day light autonomy as the percentage of the operating period (or number ofhours) that a particular day light lev el is exceeded throughout the y ear. It is a dramatically different way to thinkabout and measure the presence of day light in a building. One adv antage of using day light autonomy to quantifyday light av ailability in a building, is that the day light autonomy calculations take climate into account, an aspectthat prev ious metrics for quantify ing day light had not included, explained Jack Bailey , Partner at One Lux Studioin New York City . This metric could benefit architects and owners significantly . Architects can use day lightautonomy analy sis to ev aluate different design alternativ es to determine which concept prov ides more usableday light in the interior, and owners will know, definitiv ely , that their building is making good use of day light. It alsoprov ides a consistent metric for comparing the performance of different buildings for building codes and greenbuilding initiativ es.

It should be noted that at this moment achiev ing day light autonomy in a building is not required by anyinternational, federal, state or local building code. It was included in the first public draft of the International GreenConstruction Code (IgCC), but was remov ed in fav or of a simpler metric, explains Bailey , who serv ed on thecommittee that wrote the IgCC. Nor is day light autonomy analy sis required for a project wishing to achiev e LEEDor any other ty pe of green building certification, he continues, howev er, day light autonomy is recognized as anoption for achiev ing the day lighting credit in LEED v 4.

Metrics for Measuring Day light Autonomy

The measurement of day light autonomy (DA) is the percentage of working hours that a particular day light lev el isexceeded throughout the y ear. The DA v alue represents the percentage of the workday that a space could beexclusiv ely illuminated with day light. There are a few other metrics that can be considered along with the DA v alueto paint a more complete picture of the presence of day light in a building throughout the day . Continuous Day lightAutonomy (cDA) giv es partial credit to hours where day light is present, but cannot completely achiev e the targetilluminance lev el. Spatial Day light Autonomy (sDA) refers to the percentage of floor area where 30 footcandles (fc) isachiev ed for at least 50 percent of the workday .

There is a key term used in the definition of day light autonomy , and some of the supporting day light autonomymetrics, that warrants additional attention to ensure that the full concept of day light autonomy is truly understood.

Defining Useful Day light

Where day light autonomy is the goal, it is critical to understand that not all day light is created equal. As thedefinition of day light autonomy implies, there is useful day light and day light that is not usable or practical for theinterior env ironment. Useful day light describes the day light in the interior space that does not cause glare ordiscomfort to building occupants.

In an office setting, the range of useful day light illuminance is generally considered to be between 1 0 fc and 200 fc atthe workplane. This aligns with recommendations dev eloped by the IES that define the optimal light lev els forv arious v isual tasks. The IES recommends that office buildings maintain 30 fc at the workplane in priv ate offices,open office spaces, and conference rooms. Lower light lev els are recommended for circulation areas and higher lev elsare recommended in areas where reading and study ing will be the primary task. There is ev en a metric, entitledUseful Day light Illuminance (UDI), which refers to the percentage of work hours where the illuminance fromday light is between 1 0 fc and 200 fc.



A 2003 study conducted by Heschong Mahone Group linked day light exposure in theworkplace to improv ed employ ee productiv ity and satisfaction.

Photo Bruce Damonte

People can experience v isual discomfort when direct sunlight or ov erly bright sky conditions are present in aworkspace due to the intensity and contrast they create. Two metrics that hav e been dev eloped to help designersidentify and protect a space against this potentially destructiv e and glare-creating day light are referred to as MaxDay light Autonomy (maxDA) and Annual Sunlight Exposure (aSE). The metric maxDA measures the percentage ofwork hours where day light lev els prov ide 1 0 times the necessary lev els of design illuminance. The metric aSEmeasures the number of hours per y ear where the space receiv es direct sunlight. This direct or bright, potentiallyglare-causing sunlight may be considered unusable day light, whereas the diffuse day light that fills the sky on acloudy day is generally considered usable.

The Benefits of Day light Autonomy

In many instances, energy sav ings is achiev ed by sacrificing something else that is deemed v aluable, such asoccupant comfort. One of the reasons that day light autonomy is such an attractiv e design objectiv e is that itmanages to deliv er significant energy sav ings, without negativ ely impacting occupant comfort or the functionalityof the space. In fact, it may improv e occupant comfort by improv ing access to outdoor v iews and ensuring thatoccupants are exposed to optimal amounts of usable day light throughout the day , which improv es productiv ity andsatisfaction, while protecting the space from direct or ov erly bright light that can cause glare and discomfort.Day light autonomy creates energy sav ings by being smarter about the inclusion and management of day light andby eliminating excess lighting energy that is essentially unnecessary .



Day light autonomy can create significant energy sav ings and improv e the quality andcomfort of the av ailable v iews.

Photo courtesy of Lutron Electronics

The Obstacles to Achiev ing Day light Autonomy

If there is a down-side to day light autonomy , it may be that, especially as a relativ ely new design objectiv e, day lightautonomy can be challenging to achiev e. It requires the ongoing, optimal management of a v ery dy namic lightsource and, as such, achiev ing day light autonomy is v ery dependent upon selecting the right day light managementtechnology for the project. Setting a project up to achiev e day light autonomy may also require designers to retooltheir traditional approaches to a new project as it relates to the planning of the building env elope and interior. Inaddition, ev aluating the potential DA factors of different concepts requires the use of complex software programs.

The dynamic nature of daylight . At the center of the day light autonomy challenge is the dy namic and powerfulnature of day light. Day light lev els can range from a few footcandles on an ov ercast day to ov er 8,000 footcandles ona clear, sunny day . It can arriv e at the window in many forms: streaming directly from the sun, gently diffusedthrough the clouds, or harshly reflected off of a surrounding structure. And it arriv es, in some form or another,ev ery day , although its angle and position will v ary as the earth orbits the sun.



The angle of the sun changes constantly throughout the day , affecting day light inside a space.

Illustration courtesy of Lutron Electronics

The potential intensity and daily presence of day light require that, if day light is allowed into a building, it must beeffectiv ely managed or it can wreak hav oc on the v isual env ironment. Some problems commonly experienced as aresult of mismanaged or uncontrolled day light include: glare, hot spots, and thermal heat gain. These problems cancause larger issues of occupant discomfort, loss in productiv ity , loss of usable interior space, and energy waste.



Selecting the right technology . It is often prudent practice to design a building to be functional in the worst-casescenario. As it relates to the lighting sy stem, the worst-case scenario would be something like blackout or midnightconditions, where zero day light is av ailable and all of the illumination must be prov ided by the electric lightingsy stem. In terms of a day light management sy stem, the worst-case scenario would be that the building was subjectedto intense, direct, glare-creating, and unusable day light all day long, so the day light management sy stem wouldneed to be deploy ed to either block or diffuse the day light all day long. Fortunately , the conditions of an actual dayrarely mirror those defined in a worst-case scenario.

It is the technology of the day light management sy stem that enhances or limits the building's ability to allow usefulday light into the interior when it is av ailable. Automated shading sy stems automatically raise and lowerthroughout the day to maximize the useful day light allowed into the building. A manual shade requires that aperson manipulate its position and may not be regularly raised when useful day light is present. Equipping a buildingto maximize the presence of useful day light in a space is a critical step in achiev ing day light autonomy .

A new approach to design. The day light autonomy of a building is affected by v ariables of the building env elope aswell as the interior lay out and furnishings. One of the challenges in achiev ing day light autonomy is that it requiresthe design team to consider how the massing, siting, and orientation of the building impact the av ailability ofday light in the interior floorplate, explains Bailey . It also requires that many of the elements of the interior space bereconsidered in terms of how it affects day light penetration and day light management. This includes the lay out ofthe interior space, the placement and selection of cubicle walls, and ev en the interior color and finish.

Evaluating alternative designs requires a specialist . Perhaps the greatest challenge for early adopters try ingto achiev e day light autonomy is the complexity of the software programs currently av ailable to help designersev aluate the DA factors of their v arious concepts. The complex software often requires that a lighting designer witha specialized knowledge of these programs be included in the design team. The need for this specialized expertise is oneof the factors currently limiting the inclusion of day light autonomy in many of the building codes or wider adoptionof this metric in green building programs, because it does not seem reasonable to mandate that a professional withthis capability be required on ev ery project, regardless of scale, say s Bailey . I'm confident that a more out-of-the-box software solution will be av ailable soon, but it's just not there y et.

Designing to Achieve Daylight Autonomy

The day light autonomy a building can achiev e is affected by so many v ariables, it is critically important that thegoal of day light autonomy be identified and activ ely considered as early in the project as possible. Decisions about theshape and position of the building, interior lay out, furnishings, and ty pe of day light management sy stem specifiedare all critical components in the amount and penetration of usable day light that is ultimately found in a buildingthroughout a y ear.

Building Env elope

Many designers begin creating the conceptual design for a building by considering the general shape and mass thebuilding will hav e, also known as massing, selecting the location on the site that the building will occupy , referred toas siting, and determining how the building will be oriented on the site in relation to the sun.

The initial decisions made with regard to the massing, siting, and orientation of the building all dramatically impactthe ty pe of day light that is av ailable to the project and the potential access that day light will hav e to the interiorspace. For example, the deeper the floorplate of the building, the more challenging it becomes to achiev e target lev elsof day light illumination in the more central spaces. In terms of orientation, in the northern hemisphere, it isgenerally accepted that northern exposures offer the best access to glare-free, ambient light throughout the day ,whereas eastern exposures are often subjected to intense and glare-causing day light as the sun rises, and westernexposures experience direct sunlight exposure as the sun sets.



The massing, siting, and orientation of a building all affect the amount and penetration ofusable day light that will be av ailable in a building throughout a y ear.

Photo courtesy of Coscia Moos Architecture

Interior Lay out and Furnishings

Interior space planning is another opportunity to optimize day light autonomy , especially in office space, explainsBailey . Designing priv ate offices with solid walls around the perimeter of the building used to be common practice,but that approach blocked day light from penetrating deeper into the building. To improv e the day light autonomy ofan office building, priv ate offices are strategically placed in more central locations and equipped with glass officefronts, to prov ide occupants with access to day light and v iews, he adds. Designing a circulation space around theperimeter of an open office area is another way to improv e the potential day light autonomy of the space, by creatinga buffer zone to help limit the potential of glare on the workplane. Lighter colors and lower cubicle walls are twoexamples of interior furnishings that are often specified to help day light reach deeper into the space.

The Need for Day light Management

Achiev ing day light autonomy requires more than filling a space with day light, it must be filled with usable



day light. As a reminder, usable day light is defined as day light within the range of 1 0 fc and 200 fc that will notdisrupt the v isual env ironment or cause glare or hot spots in a space. This range of usable day light represents apretty select segment of the day light that could be av ailable at the window of a building throughout the y ear,especially considering that day light can range from a few footcandles on a cloudy day to ov er 8,000 footcandles on abright, sunny day . Intense day light exposure can trim away the usable space in a building by making areas toobright, too glaring, or too hot to use for parts of the day . Av oid the potential pitfalls of day light exposure by equippinga building to manage the day light as it enters the space and protect the interior from the bright, direct day light thatcan destroy the v isual env ironment and undermine the day light autonomy of the space.

Here's an example of how a space may be negativ ely affected if it does not hav e a way to manage the day lightstreaming in through the windows. Consider a section of an open office bay , positioned on the perimeter of thebuilding where one wall is completely exposed to direct day light.

On a sunny day , the day light lev el may reach up to, or ov er, 8,000 footcandles at its most intense. Today , glass usedin Class A office space may hav e a v isual transmittance v alue (or Tv ) of 0.65, which indicates that 35 percent of theav ailable footcandles are reflected back out into the atmosphere and 65 percent of the v isible energy is transmittedthrough the glass and into the building. In this case, 5,200 fc of the 8,000 fc of day light would enter the open officespace, well outside the range of usable day light.

This scenario was simulated to ev aluate the percentage of working hours where the presence of day light exceeded theusable range of 200 fc on a sunny day . The simulated open office space is 30 feet by 30 feet, the windows are 8 feettall and the working hours were defined as 8 a.m to 6 p.m. The simulation used real NYC climate data and assumesthat no immediate surrounding buildings are present that would cast shadows onto the facade. The DA plotsillustrate the percentage of time, during those predefined working hours, that the anticipated day light lev el willexceed 200 fc.

The simulation of useful day light illuminance resulted in much of the office space beingpotentially uncomfortable for much of the work day .

Images courtesy of Lutron Electronics

The results were shocking. Without any way to manage the av ailable day light bey ond the glass, portions of the office



space were too bright to occupy comfortably throughout the entire workday , shown here in red. Ev en moresurprising was that in the examples with eastern, southern and western exposures, ov er half of the office space wasuncomfortably bright for ov er 50 percent of the workday . In the open office area with northern exposure, nearly athird of the office space would be potentially uncomfortable for half of the working hours. That is a significantamount of usable square footage to lose on a sunny day .

Selecting the Right Day light Management Sy stem

Luckily , designers hav e many options for managing the day light that penetrates into a building's interior. There arepermanent exterior and interior shading dev ices, like fixed ov erhangs and high-performance glazing, which prov ideconstant glare control, but are limited in their ability to offer any v ariety in the amount of day light protection theyprov ide. As a result, these sy stems treat day light on an ov ercast day and day light on a sunny day as if it had equalopportunity to create glare, which it does not. In an attempt to protect the interior from damaging day light,designers may , inadv ertently , minimize the amount of usable day light that is allowed into the work space, reducingthe lev el of day light autonomy that the building could achiev e.

A shading sy stem is a great example of a day light management solution that offers enough flexibility to mitigateglare and heat gain when the outside day light is intense and unusable, but maximize the penetration of usableday light, when it is av ailable. This v ariable lev el of day light control makes a shading sy stem a great tool to help abuilding reach its goal of day light autonomy , without risking day light exposures that would make the spacesunusable or uncomfortable. When selecting the right shading sy stem for a project, the fabric is the key to mitigatingglare and unusable day light and the controllability is the key to achiev ing day light autonomy and greater energysav ings.

It is important to specify shade fabric with the Tv v alue necessary to manage the day light inthe space appropriately .

Images courtesy of Lutron Electronics

Specifying fabric. It is a common practice to select the fabric for a shading sy stem based on its color, instead ofbased on how the shade will need to perform in terms of day light management. While color can impact the v iewprov ided to the outside when the shades are deploy ed, for example, darker fabrics can prov ide a crisper v iew thanlighter alternativ es, selecting a shade based exclusiv ely on its color compromises the ability of the shading sy stem toprev ent glare and maintain an optimal v isual env ironment in the building. It can also negativ ely impact theaesthetic appeal that the shade color was intended to hav e in the first place. If the window is too bright to comfortablylook at, no one will be able to appreciate the carefully selected color of the shade.

A shade manages solar energy in three way s: reflect it, absorb it or transmit it into the interior. The key to glaremitigation is limiting the v isible transmittance of the day light through the shade and into the v isual env ironment,so that it stay s within an acceptable brightness. In terms of achiev ing day light autonomy , the day light passing intoa space should not exceed 200 fc in intensity .

In order to specify a shade fabric that will function as needed, it is important to understand that the shade does not



work alone. Both the window glass and shade fabric impact the total v isual transmittance of day light together andmust be considered in tandem to create a glass/fabric sy stem that appropriately manages the av ailable day lightthroughout the y ear.

The glass used in most buildings today has a v isual transmittance of 65 percent, allowing 65 percent of the light topass through the glass and into the building. Fabric shades are most commonly av ailable in v isual transmittancesranging from 3 percent to 30 percent. If a building has standard, double-paned windows with a 65 percenttransmittance, then on a sunny day where 3 ,000 fc of day light is av ailable, 1 ,950 fc will pass through the glass. If ashade fabric with a 1 0 percent v isual transmittance is specified on these windows, then only 1 95 fc, of the 1 ,950 fcthat were transmitted through the glass, will be ultimately transmitted into the interior space. A day light lev el of1 95 fc supports the design goals of achiev ing day light autonomy and maintaining a comfortable v isualenv ironment. The shade fabric keeps the space comfortable and usable, ev en on a sunny day .

Ty pically , windows with higher v isual transmittance v alues should be paired with fabrics that hav e lowertransmittance v alues. Spaces that receiv e direct sunlight, such as the direct early morning light that an easternexposure receiv es, should keep the combined transmittance of the glass and fabric to less than 1 0 percent. Areaswithout the threat of direct sun exposure can benefit from shades with higher transmittances to maximize thepotential lev el of day light autonomy it could achiev e.

Specifying Controllability: Manual vs. Automated Shading Systems

Specify ing the way that the shading sy stem is controlled can hav e a powerful impact on the lev el of day lightautonomy that a building is able to achiev e. The controllability of shading sy stems can be div ided into twocategories: manual and automated. While the shades in either sy stem can be made from identical fabrics andsimilarly positioned at any height, a person must phy sically deploy or retract the shade fabric of each shade in amanual sy stem. Automated shades are programmed to mov e into their different positions throughout the day inresponse to a pre-determined schedule or in response to sensors that measure the intensity of the day light at thewindow. No manual manipulation is necessary to ensure that glare conditions are being prev ented and usableday light is allowed in.

Manual Shading Sy stem and Day light Autonomy

The primary challenge that a building equipped with a manual shading sy stem will face, when attempting toachiev e day light autonomy , is reliably letting in diffuse, ambient day light, when av ailable. People are relativ elyproficient in closing the shades to reliev e a space from glaring or uncomfortable conditions, howev er, they are not asdiligent at opening the shades back up, when the day light transforms from direct to diffuse. It is quite normal for ashade to be pulled down to block harsh, direct light and then left down for day s, months or longer. A window with amanual shade deploy ed ov er it, day after day , protects the space from glare, but it limits the usable day light allowedinto the interior and significantly reduces the space's ability to exclusiv ely illuminate the space with day light.

Automated Shading Sy stems and Day light Autonomy

An automated shading sy stem is a powerful tool for a building try ing to achiev e day light autonomy . These sy stemsare dedicated to managing the dy namic and ev er-changing nature of day light, ev ery day , all day long. Theautomated shading sy stems use the solar path of the sun as it arcs ov er the building to determine the optimal positionof the shade and continuously adjust the position to accommodate the changing solar angles. The adjustmentsreliably block direct sunlight, while allowing usable day light into the building. These sy stems also hav e sensorsplaced near the windows that can detect the lev el of day light in the space and adjust accordingly , keeping shadesdeploy ed on ov erly bright day s and retracting shades on ov ercast day s to allow the soft, diffuse day light into thebuilding.

Here is an example of how an automated shading sy stem in a glass-clad building may function throughout the day .As the sun rises, the shades on the eastern exposure may deploy to a position halfway down the window to block earlymorning light, while the shades on the northern southern and western exposures may be entirely retracted, to let inthe gentle morning light. As the morning progresses, the shades on the eastern exposure may lower to a full closedposition, while the shades on the northern and southern exposure deploy to one-fourth of the way down the window,and the shades on the western faade remain fully open. As it approaches noon, the shades on the eastern exposureretract to halfway position, while the shades on the southern exposure are deploy ed to the halfway position and thewestern exposure shades lower to cov er the top fourth of the window. In early afternoon, the shades on the easternexposure are retracted completely , letting in the av ailable afternoon light and the shades on the southern andwestern sides deploy further to block the direct light as the day progresses. At sunset, all of the shades are fully open,allowing occupants to enjoy the soft light of early ev ening. Automated shades mov e silently and automatically



through all of these position changes, without requiring any manual manipulation. It should also be mentioned thatautomated shading sy stems mov e shades to precise, aligned lev els along a faade, maintaining the curb appeal of thebuilding, while maximizing the day light autonomy of the interior.

Automated shading sy stems mov e shades to precise, aligned lev els along a window wall.

Photo courtesy of Lutron Electronics

Our firm was recently hired to compare the day light autonomy that could be achiev ed with manual shades andautomated shades on a large, corporate campus, Bailey said. Our analy sis concluded that automated shadessignificantly increased the DA factor of the building and prov ided additional energy sav ings. The owner also installedday light responsiv e controls throughout the project which reduced the building's ov erall dependence on electricenergy .

When deciding which shading sy stem to specify , the most common challenge to selecting an automated sy stem is thecost, howev er, the attitude is changing as the benefits of day light exposure and day light autonomy become morewidely accepted and v alued. As Jack Bailey experienced on a recent project, The owner of this particular projectknew how much they were spending to install glass curtain walls around the buildings. The added cost of anautomated shading sy stem was not significant when compared to the cost of the windows. From the owner's



perspectiv e, this automated sy stem allowed the building to make better use of the windows and prov ided access toimprov ed day light and v iews for a small up-charge. It made a lot of sense to them.

Compare and Contrast the Performance of Manual and Automated Shading Sy stems

These shading sy stems can deliv er significantly different results in terms of the amount of interior space that can beexclusiv ely lit throughout the work day by day light and the energy sav ings that can be generated.

The simulations run on the Vista Center floorplan concluded that automated shades wouldcreate a significantly larger useful day light zone than manual shades.

Images by Coscia Moos Architecture, courtesy of Lutron Electronics

Compare useful daylight zones. The useful day light zone refers to the area of a building that achiev es its usefulday light illuminance lev el at least 90 percent of the working day . It identifies the square footage of a space that couldbe almost extensiv ely illuminated by day light throughout the work day .

For example, Coscia Moos Architecture worked with an outside team to complete the analy sis that compared thedifferent useful day light zones created by manual and automated shading sy stems. The proposed 7 00,000-square-



foot Vista Center would hav e 25 floors, each measuring 28,500 square feet. For the analy sis, 9-foot-tall windows withv isual transmittance v alues of 0.65 were paired with shades that offered a 1 0 percent transmittance v alue.

The DA simulation of the 1 4th floor identified that dramatically different useful day light zones were created bymanual and automated shading sy stems. The manual shading sy stem generated a useful day light zone that was 1 2feet deep around the perimeter of the building. The automated shading sy stem created a useful day light zone thatwas twice as large, measuring 24 feet deep around the entire perimeter of the building.

Compare energy savings. With day light harv esting products now being required in sky lit or day lit areas, theincreased presence of day light in a space can immediately generate energy sav ings. As automated shades are able tomore reliably allow greater amounts of usable day light into a space, the sy stems can also deliv er greater energysav ings when compared to manual shades.

A recent study , completed in collaboration with Lutron Electronics and Purdue Univ ersity , compared the energysav ings that could result from both shading sy stems. An energy simulation of a perimeter priv ate office with alighting power density of 0.9W/square feet, standard, clear, double-pane glass, and a shade fabric with 5 percenttransmittance was conducted. The study av eraged the results of spaces with 20 percent, 40 percent and 60 percentwindow-to-wall ratios. Manual shades were defined as closed shades for the study . The team concluded that anautomated shading sy stem was able to reduce the electric light use in the priv ate office by 83 percent, whencompared to the amount of electric light used if the windows were fitted with manual shades.

In projects of any scale or application ty pe, the effectiv e incorporation of day light is steadily becoming a more andmore common design goal. Armed with more adv anced technology and day lighting design know-how, designerstoday are able to adequately illuminate a space using day light exclusiv ely . Achiev ing day light autonomy sav esenergy and creates a more satisfy ing and productiv e atmosphere for building occupants, which are just twoexamples of how a day lit workspace works harder. And with automated shading sy stems, no one has to lift a finger.

Lutron Electronics, headquartered in Coopersburg, Pennsy lv ania,designs and manufactures energy -sav ing light controls, automatedwindow treatments, and appliance modules for both residential andcommercial applications. Its innov ativ e, intuitiv e products can beused to control ev ery thing from a single light, to ev ery light andshade in a home or commercial building. www.lutron.com

Originally published in the Nov ember 201 3 issue of Architectural Record

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daylight autonomy 101

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presence of day light

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