The basics of daylighting

A design strategy needs to be considered in order to provide optimal daylighting in a space.

06/20/2017


Learning Objectives

  • How to identify successful daylighting design.
  • Explore the elements of daylighting.
  • Learn how to integrate daylighting and lighting control systems.

Effective daylight design in buildings creates a comfortable and productive environment for occupants. Effective daylighting can reduce energy use, but also must limit HVAC loads. The U.S Green Building Council’s LEED rating system and the International WELL Building Institute’s WELL Building Standard are two well-known rating systems that reward outstanding daylighting design.

 Figure 1: A closed-loop photosensor measures both natural and artificial light in the space. Courtesy: P2S Engineering Inc.

Daylighting design

It's important to first identify the elements of successful daylighting design. Buildings that are daylit provide adequate lighting to perform most visual tasks and limit glare. Luminance is the measurement of task lighting, and Illuminance is a measurement of direct glare. The unit of measurement for luminance is a footcandle (fc), or lux, and the unit for illuminance is candelas per square meter (cd/m2). Uniformity glare is also important, which is the ratio of luminances in adjacent spaces.

People need between 5 and 10 fc for circulation and orientation in a space, between 30 and 50 fc to perform most visual tasks, and between 50 and 100 fc to perform difficult tasks involving low contrast or high accuracy. People also need uniform light. The difference between lighting in task areas and adjacent areas (uniformity glare ratio) should be close to 3:1, and the uniformity glare ratio between task areas and remote areas should be close to 5:1. At no point should the brightest spot be more than 10 times brighter than the darkest spot, or it will become a source of indirect glare.

There are two ways to bring daylight into a building: from the walls or from the ceiling. Once daylight is brought into a building, it’s important to make sure that glare wasn’t introduced.

Sidelighting refers to daylighting from windows. Windows can bring twice as much daylight into a space. For example, if the top of the window is 10 ft above the floor, daylight will penetrate up to 20 ft into the room. Window daylighting has several advantages and disadvantages. Windows allow occupants to look out into the world, contributing to visual comfort. The LEED V4 Indoor Environmental Quality credit calls for view glazing for 75% of the regularly occupied floor area. But, windows reduce insulation efficiency and allow solar heat gain, resulting in increased HVAC energy. Windows on all but the north side of a building present glare conditions unless controlled.

Figure 2: With multiple sources of daylight and user needs, zoning lighting controls is challenging. Courtesy: P2S Engineering Inc.

Toplighting is the application of daylighting from the ceiling. This includes skylights, solar collectors, clerestories, or sawtooth structures. Toplighting can typically supply light to an area equal to the mounting height. A skylight aperture mounted 20 ft above the floor can provide daylight to a 20x20-ft area below the skylight. Skylights get more sun exposure than windows, leading to increased heat gain, but solar tubes can reduce this. Toplighting can penetrate deep into buildings, but it favors only 1- or 2-story buildings and does not provide views to the outdoors.

After bringing daylight into the space, reducing glare is just as important. There are three types of glare: direct glare, indirect glare, and uniformity-related glare. Unshielded light fixtures and low-incident sunlight (such as at sunrise and sunset) cause direct glare. Reflections off of interior surfaces cause indirect glare. Uniformity glare occurs when illuminance varies greatly between adjacent areas of a room.

The only way to limit direct glare is to reduce incident light coming through windows. Static exterior shading reduces heat load and glare throughout the year, but it is not adjustable. Interior shading, such as adjustable blinds or shades, also reduce direct glare and heat gain. Electrochromic glass also is a popular and affordable option that alters the visible transmittance of glaring by up to 90%. The Illuminating Engineering Society of North America (IESNA) and others recommend automatic shading, since occupants often leave shades closed.

Reducing indirect glare hinges on reducing the reflectivity of interior surfaces. Selecting satin instead of glossy finishes, for instance, goes a long way toward reducing glare. Materials with low light-reflectance values (LRV) reduce glare but also reduce daylight penetration. Rotating highly reflective surfaces is also important to reduce undesirable indirect glare. The WELL Building Standard recommends orienting computer screens so they are perpendicular to windows that are within 15 ft of them and allowing a minimum LRV of 50% for furniture.

Daylighting design strategies

There are several design factors that can reduce uniformity-related glare. Providing multiple daylight sources can balance daylight penetration and uniformity. Examples of this are installing windows on different walls or installing a window and a skylight. Reducing the "lease depth"—the distance from the window to the nearest interior partition—allows light to enter the space. Feature 61 in the WELL Building Standard requires that 75% of regularly occupied spaces be within 25 ft of view windows. Balancing surface reflectance and minimizing opaque interior partitions can also improve light uniformity.


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