Balancing Health and Performance Benefits through Natural Lighting

Understanding how to specify glazing systems that balance access to natural light with thermal performance and building code requirements
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Sponsored by Oldcastle BuildingEnvelope®
By Juliet Grable

Comfort and Thermal Performance

Natural light is available during times when building occupants most need it, and good design can ensure that most, if not all, occupants can access this daylight while guarding against issues such as glare and unwanted heat gain. Architects can effectively implement these solutions to maximize occupant benefits and ensure that their designs meet codes related to thermal performance.

Glazing impacts multiple aspects of a building, including aesthetics, daylighting, access to views, occupant comfort, and sound transmission as well as energy demand. All of these should be considered in tandem as the design process progresses. This way, architects can ensure the daylighting goals are met while meeting code requirements and other project goals. Here, energy modeling can be a valuable tool. Glazing system manufacturers often have experts and resources, and therefore can work with designers on the best possible solutions to meet their project goals.

There are some basic design principles to keep in mind that will help ensure a successful daylighting design that does not result in unwanted glare or occupant discomfort.

Orientation

If new construction, the building footprint can be optimized for daylighting by maximizing the north and south exposures and minimizing the east and west exposures. Aligning the building on the east-west axis and keeping the floor depth below 60 feet enhances daylighting opportunities. Although a facade that faces directly south is optimal for solar access and control, the building footprint can deviate up to 15 degrees in either direction.14

Location of Windows

Following are a few basic guidelines from the Department of Energy for locating glazing to ensure effective daylighting without unwanted solar gain or glare.

  • South-facing windows allow most winter sunlight into the buidling but allow little direct sun during the summer, especially when properly shaded
  • North-facing windows admit relatively even, natural light, producing little glare and almost no unwanted summer heat gain.
  • East- and west-facing windows provide good daylight penetration in the morning and evening, respectively, but may cause glare, admit a lot of heat during the summer when it is usually not wanted, and contribute little to solar heating during the winter.

Window Area or Window-to-Wall Ratio (WWR)

Window area or window-to-wall ratio (WWR) is another important aspect of the building envelope that will impact daylighting, ventilation, heating, cooling, and artificial lighting. WWR is defined as the ratio of the total glazed area to the total exterior wall area. Although building codes set prescriptive maximums for the WWR, projects can exceed this maximum by using high-performance glazing systems. Such designs optimize daylighting while reducing undesirable solar gain.15 For example, projects using ASHRAE 90.1 can use the Building Envelope Tradeoff Option to exceed the maximum WWR of 40 percent.

Glare

When it comes to daylighting, there can be too much of a good thing. Excessive daylight can produce glare, which is especially disruptive in workplaces. When there is a significant illumination differential between the surface an occupant is trying to view—such as a desk or computer screen—and the sunlight coming through a window, the human eye works hard to adapt and reconcile the contrasting brightness, which can lead to fatigue and eyestrain.

There are two types of glare. Discomfort glare is glare that is within the eye’s capability to mitigate; disability glare is glare that prevents a person from performing a task. We can also distinguish between direct glare, which occurs when a person directly views the source of illumination, and indirect glare, which is caused when light reflects off surfaces.

Ideally, brightness levels should be kept relatively even across the occupant’s field of vision within a space. The Illuminating Engineering Society (IES) recommends that small patches of sunlight be controlled to less than 79 foot candles. The location of openings and use of awnings and overhangs can help control glare and radiation, as can the use of controls, such as internal or external shading.

Daylighting and Energy Use

Good daylighting design depends on an understanding of the relationships between glazing, daylighting, and building energy use. Electric lighting accounts for 35 to 50 percent of the total electrical energy consumption in commercial buildings. Schools and other institutional buildings have especially high lighting energy use, but good daylighting design can reduce reliance on artificial lighting. In fact, the use of daylight systems and intelligent lighting controls can reduce building electricity use by about 15 percent.16

To effectively reduce lighting energy, the daylighting scheme must be planned in tandem with artificial lighting controls. There are several options for controlling artificial lighting. Manual switches give occupants more control, but automatic controls, which dim or turn off artificial lights once natural or ambient light reaches certain levels, typically save more energy and are a more practical solution in larger, open-plan office buildings or schools.

Even greater savings may be achieved using daylighting and intelligent lighting controls in conjunction with blinds, which can help control glare at lower sun angles. One study of open-plan offices in New York City found that switching a conventional lighting system to a dimmer system and either manually controlled or automated blinds could save between 50 and 60 percent of lighting energy.17

Interior Layout and Partitions

The interior layout is also of critical importance. A building may enjoy good daylighting from glazing, but if that daylight is blocked by walls or partitions, only the occupants on the window side of the partition will benefit from it. One study of an office revealed that 72-inch partitions placed 15 feet from the windows cut daylight levels in half compared to 48-inch partitions.18 Replacing solid partition walls with glass can increase the proportion of the space benefitting from daylighting. If privacy is a concern, decorative glass can be used, which admits some light but restricts views.

In addition, the reflectance of interior surfaces has a significant impact on the distribution of daylight. Incoming light “bounces” off highly reflective and light surfaces, evenly distributing light and reducing glare, while darker surfaces absorb daylight. For this reason, ASHRAE recommends minimum reflectance values for interior ceilings, walls, and flooring for various building types. The ceiling is the most important light-reflecting surface and has a high recommended reflectance values of 80 percent or more. Reflectance values should be at least 50 percent for walls and at least 20 percent for floors. Keep in mind that other surfaces, including workstations and furniture, also have an impact on light distribution.19

Heating and Cooling Energy

In addition to impacting lighting energy use, glazing can also affect the energy required to heat or cool the building. When daylighting replaces electric lighting, less heat is produced. This can be significant in buildings with large cooling loads. The Whole Building Design Guide (WBDG) estimates that the energy savings from reduced electric lighting can directly reduce building cooling energy usage by an additional 10 to 20 percent.

Airtight, well-sealed, and well-insulated glazing systems can reduce heat loss during the heating season. However, glazing can also be used strategically to encourage or discourage solar heat gain.

The type of glass plays an important role in controlling heat gain and loss. Use U-factor, solar heat gain coefficient (SHGC), and visual transmittance (VT) values to make the appropriate selection for the climate, building orientation, and building use. The WBDG provides some general recommendations.

  • For climates with significant cooling loads, specify windows with low SHGC values (less than 0.40).
  • In general, low-SHGC windows should be considered for east- and west-facing glazing as a means of controlling solar heat gain and increasing occupant comfort. For large commercial and industrial structures, specify low SHGC windows on the east, south, and west facades. SHGC for north-facing windows is not critical for most latitudes in the continental United States.
  • Analyze the tradeoffs between standard glazing and spectrally selective glass. Spectrally selective glass has a relatively high VT and a relatively low SHGC.
  • For buildings where passive solar heating is a goal, choose south-facing windows with high SHGC values coupled with low U-factors.

Natural Ventilation

Glazing systems can facilitate the use of natural ventilation, which can reduce reliance on artificial cooling and lower building energy use. Occupants often appreciate access to fresh air. Keep in mind, however, that in most climates, natural ventilation will not keep interior spaces within the “comfort zone” all the time, and the natural ventilation strategy must be planned in tandem with the HVAC system. This may lead to some conflicting solutions, as a naturally ventilated structure typically includes large window and door openings, while one strategy for minimizing mechanical cooling loads is to reduce the window area or keep the windows sealed.

In general, effective natural ventilation requires cross ventilation. Higher ceilings and open layouts facilitate cross ventilation, as does a narrower floor plan. Air can still move across deeper floor plans, but the air temperature will increase as it moves across the room. In addition, operable skylights can allow rising warm air to exit, allowing cooler air to enter through lower windows by taking advantage of the “stack effect.”

The Role of Shading

It’s important to balance the positives of daylighting—including the benefits to occupants and savings on lighting energy use—with potential heat gains and losses. Shading devices can help prevent overheating and glare. Common shading devices include exterior overhangs and sunshades and interior blinds, screens, and roller shades.

Exterior overhangs and sunshades are typically used to block the summer sun, which is higher in the sky, while allowing sunlight to enter the building in winter, when the sun is at a lower angle. Overhangs are fixed elements on the exterior of the building. Sunshades serve a similar function as overhangs but generally refer to metal louvers that are attached above or in front of a window. The brackets, outriggers, and louvers come in many styles to accommodate different aesthetic and shading goals.

Horizontal projections work best on south facades, as they block direct sun at high angles. Vertical projections work best on east and west facades, blocking the sun when it is lower in the sky. Overhangs present an opportunity for visually interesting exterior architectural elements in the form of fins, “egg crates,” (a pattern of alternating vertical and horizontal projections), and brise soleil.

Interior roller shades, blinds, and screens can be manually operated or automated. They can also be part of a building management system programmed to coordinate with electric light controls. One disadvantage of these devices is that they still allow solar heat to enter through the window, where it can become trapped between the shading device and the glazing. A good strategy is to combine exterior overhangs with interior shading controls.

An elegant study conducted in hypothetical classroom spaces in Colorado shows the relationship between shading, daylighting, electric lighting, and cooling energy use. Classrooms were fitted with either roller shades, blinds, or overhangs; all windows consisted of double-paned low-e glazing. Annual electric lighting energy savings for roller shades, blinds, and overhang compared to the base case were 55 percent, 56 percent, and 67 percent, respectively; cooling energy savings for those same configurations were 39 percent, 34 percent, and 51 percent, respectively.

As you can see, the classroom with overhangs saved the most energy. This is because the 4-foot overhang covered the full windows for 35 percent of the “noon time” from April to August, which significantly lowered cooling loads during that period.

During winter, the overhang allowed solar heat gain, which resulted in a lower heating energy demand during those months.

On the downside, the overhang allowed illuminance levels to exceed 2000 lux—the level which prompts occupants to take actions to reduce the daylight level—for more than 52 percent of the total simulation hours. Both blinds and shades keep the illuminance levels below 2000 lux for most of the simulation hours. In other words, the overhang allowed much less control over maintaining proper indoor illuminance levels than blinds and shades. The study concluded that the overhang combined with interior shading controls could save the most energy.20

 

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Originally published in Architectural Record

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