What makes a facade “sustainable”? All facades create barriers between the exterior and interior environment, providing building occupants with thermally, visually, and acoustically comfortable spaces. Sustainable facades do more. They provide optimal levels of comfort using the least amount of energy. To achieve this high performance, designers need to consider many variables—thermal insulation, daylighting, solar shading, glare, acoustics, and indoor air quality—when designing facades for sustainable interior environments.
THERMAL COMFORT Thermal comfort is defined by ASHRAE as “that condition of mind which expresses satisfaction with the thermal environment” (ASHRAE, 2004). Because it is a condition of mind, comfort is inherently based on one’s experience and perception; there are large variations in physiological and psychological responses for different individuals. Few buildings are designed to meet the unique thermal comfort needs of a single person. Therefore, organizations such as ASHRAE have established standards for thermal comfort that apply to the majority of people most of the time.
Six primary variables affect thermal comfort: air temperature, air movement, humidity, mean radiant temperature, occupants’ metabolic rate, and occupants’ clothing. Although each of these variables can be separately measured, the human body responds to them holistically. When the design for interior space balances these variables correctly, the occupants will feel comfortable. These six variables have specific characteristics and effects on thermal comfort:
Indoor air temperature affects the rate of heat loss from the skin. It can
be controlled by changing the temperature of the air supplied to a space by the HVAC systems, by bringing outside air into a space, or by increasing or decreasing the amount of direct sunlight within a space by adjusting window shading devices. Although indoor air temperature can be measured precisely and objectively, occupants’ comfort perception will differ depending on the outdoor air temperature, the amount of solar radiation at different times of day, and the type of activities being performed. Moving air affects thermal comfort in two ways: it conducts heat from a warm surface, such as skin, to the colder room air and surfaces; and it helps evaporation of perspiration from skin. The faster the air moves, the greater are these effects. Typically, the movement of air within a space cannot be controlled by the occupants. Therefore, to maintain their comfort, occupants respond by adjusting one of the variables they can control, such as the air temperature or their clothing. The measure of the amount of water vapor in the air is the relative humidity (RH). It is given as a percentage of the actual vapor in the air compared to the maximum amount of vapor possible in fully saturated air for a certain temperature. Thus, a relative humidity of 100% describes air that is completely saturated by water vapor, and an RH of 0% describes air that is arid, or completely dry. Under humid conditions, the rate at which perspiration evaporates through skin is lower than under dry conditions. Because the human body uses evaporative cooling of the skin as its primary mechanism to regulate body temperature, people usually find overly humid air to be uncomfortable. However, numerous health problems are caused by excessively dry indoor air, so a balance must be achieved. Depending on the air temperature, most people are comfortable with relative humidity levels ranging from 25% to 60%. The effect of relative humidity on a person’s thermal comfort is usually less than that of air temperature and air movement across the skin. Typically, it cannot be controlled by the occupants. Mean radiant temperature is the measurement of the energy radiated by objects and surfaces. It is different from air temperature, and is perceived as heat on one’s skin. Solar radiation from sunlight is one of the greatest sources of heat. Because radiant energy acts independently of air temperature, a room’s occupants may feel discomfort from the radiant
energy even though the air in the room is at a normally comfortable temperature. The effects of radiation can be minimized by any opaque object that blocks the radiation. In the case of solar radiation, window blinds may be used to block sunlight and create shade. However, if the blinds are inside the window glass, the material of the blinds will absorb some of the solar energy and radiate it into the room. Metabolic rate is the measure of the amount of energy, in calories, expended by a human body and the amount of internal body heat generated through thermogenesis. Metabolic rates can fluctuate depending on a person’s physical characteristics and types of activities. The metabolic rate of a person engaged in strenuous exercise will be higher than that of a person sitting at a desk. Because of their different metabolic rates, each person will experience different sensations of comfort in the same environment. Clothing is the simplest form of personal insulation. It forms a barrier that creates a layer of trapped body heat between the skin and the clothing. This is the variable most within the control of an occupant. People have an expectation for how much clothing is appropriate for a particular type of space and activity. If they need to wear more or less clothing to feel comfortable, they will be dissatisfied with the thermal performance of the space.
Methods of Measurement Thermal comfort is a perceived sensation, and therefore a subjective experience. Several methods have been developed to objectively measure occupants’ satisfaction with interior environmental conditions. ASHRAE’s Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) are calculation methods for statistically predicting the number of individuals who would express dissatisfaction with certain comfort conditions. The PMV index uses a seven-point, hot-to-cold thermal sensation scale, based on responses from a large number of people exposed to a certain environment. The PMV thermal sensation scale spans from –3 for a cold thermal sensation to +3 for a hot thermal sensation, with intermediate stages of –2 (cool), –1 (slightly cool), 0 (neutral), +1 (slightly warm), and +2 (warm). The PPD index predicts the percentage of people who are thermally dissatisfied, based
on data derived from the PMV index. Figure 3-1 shows the relationship between PPD and PMV. PPD assumes that people who vote +3, +2, –2, and – 3 in the PMV are thermally dissatisfied, and that the distribution of votes follows an inverse bell curve, with 0 at the center of the bell. The PPD curve is represented by unitless numbers, with the number being the percentage of people who are thermally dissatisfied. ASHRAE Standard 55-2004 recommends that PMV values be between +0.5 and –0.5 for general conditions, which correspond to a PPD of 10 (i.e., 10% of the occupants are dissatisfied).
Figure 3-1 Relationship between PMV and PPD indices.
For naturally ventilated spaces, where occupants have some control over their environment (e.g., by opening or closing windows to alter air temperature and air movement), the ASHRAE standard provides an optional method for determining thermally acceptable conditions. Indoor operating temperatures can be adjusted up or down, depending on the mean monthly outdoor temperatures, while still maintaining acceptable comfort conditions.
This is illustrated in Figure 3-2, which specifies ranges of operating temperatures for acceptable thermal comfort in naturally ventilated buildings. Higher operating temperatures are acceptable for climates with high mean monthly temperatures, significantly reducing energy consumption by mechanical systems. This allows for 10% of the occupants to experience whole-body thermal discomfort, and for an additional 10% to experience thermal discomfort for some part of their bodies. For example, if the mean monthly outdoor temperature is 95°F (35°C), the indoor operating temperatures can be relatively high, between approximately 75°F (24°C) and 87°F (30.5°C), and still satisfy 80% of the occupants. At the other extreme, if the mean monthly outdoor temperature is 50°F (10°C), the indoor operating temperatures can be relatively low, between approximately 64°F (17.5°C) and 77°F (25°C).
Figure 3-2 Acceptable operating temperatures for naturally conditioned spaces, according to ASHRAE Standard 55-2004.
These criteria can also be used for buildings with mixed-mode ventilation systems (i.e., natural ventilation, or facade openings, combined with HVAC systems). Mixed-mode buildings can be naturally ventilated with outside air
through facade openings when environmental and climatic conditions are favorable and mechanically ventilated when conditions are not favorable. The goal of well-designed mixed-mode buildings is to reduce or eliminate energy consumption by fans and cooling systems when conditions permit natural ventilation.
The Center for the Built Environment (CBE) at the University of California at Berkeley (UCB) has developed a more advanced model for understanding and determining occupants’ thermal comfort. It relies on complex relationships between environmental conditions and the physiological response of an occupant, who is represented in the model by a “thermal manikin” (Huizenga et al., 2001). In the CBE model, thermal comfort is related to the principles of human thermal regulation. To differentiate local thermal comfort, the thermal manikin can be monitored at an arbitrary number of body segments, such as head, chest, arms, and legs. Most applications use sixteen body segments. Figure 3-3 shows how the thermal manikin can be used to reflect characteristics of actual users, such as level of clothing, metabolic rate, and physiological properties. Convection, conduction, and radiation of heat between the manikin and the environment are considered in the calculations.
Figure 3-3 Thermal manikins and comfort responses. Courtesy of Charlie Huizenga, Center for the Built Environment.
The CBE model can be used to predict occupants’ thermal comfort and thermal sensation indices. The thermal sensation index is similar to ASHRAE’s PMV index, with “very hot” (+4) and “very cold” (–4) added to the ends of the scales. This accommodates extreme environments that may be encountered. Thus, the thermal sensation index is based on a nine-point scale: +4 (very hot), +3 (hot), +2 (warm), +1 (slightly warm), 0 (neutral), –1 (slightly cool), –2 (cool), –3 (cold), and –4 (very cold). The thermal comfort scale indicates whether the occupants are comfortable with their indoor environments, and the scale includes “just comfortable” (+0), “comfortable” (+2) and “very comfortable” (+4) on the positive axis, and “just
uncomfortable” (–0), “uncomfortable” (–2) to “very uncomfortable” (–4) on the negative axis. This comfort scale differs from other methods for measuring thermal comfort by differentiating levels of comfort as positive or negative. This forces subjects to be explicit about whether their perceived thermal state falls in the overall category of “comfortable” or “uncomfortable” (Arens et al., 2006). Scales for both thermal comfort and thermal sensation are necessary, because knowing that occupants feel cool or warm does not necessarily tell us if they are comfortable or uncomfortable.
Figure 3-4 Comparison between PMV and CBE Thermal Comfort Model results (Adapted from Huizenga et al., 2006).
Figure 3-4 compares ASHRAE’s PMV with the CBE Thermal Comfort Model for a specific condition. This comparison is based on an occupant sitting 3 feet (1 meter) from a window, and shows occupant’s thermal sensation as the temperature of the glass surface gets hotter or colder (Huizenga et al., 2006). Both methods show that the occupant will be neutral at 0 when the window temperature is 77°F (25°C); however, the CBE
Thermal Comfort Model is more responsive to the changes in glass temperature and its effects on thermal sensation, and can better predict local discomfort caused by the window.
Facade Design and Thermal Comfort Of all the facade elements, windows have the largest thermal fluctuations. Windows are usually the coldest interior surfaces in cold weather and the warmest interior surfaces in warm weather. This is the case even for windows with high-performance glazing and thermally broken frames. As a result, facades with high window-to-wall ratios (WWRs) are more likely to affect the thermal comfort of occupants than those with low WWRs. This effect increases as occupants get closer to the window, and also depends on how active the occupants are. For example, occupants who spend most of the time seated close to the windows are more likely to feel discomfort than occupants seated farther away or moving within the space.
The optimal WWR should be based on the floor plan of a space, the occupants’ positions in the space, and the types of occupant activities. Smaller WWRs should be used for spaces where occupants are typically close to the windows, especially for south-oriented facades. For example, in the design of commercial office spaces where the occupants are seated near windows, the WWR should not exceed 40%, and WWRs as low as 25% should be considered. For spaces where occupants do not spend a lot of time near windows, or for corridors and other circulation spaces, higher WWRs can be used with minimal effect on the occupants’ thermal comfort. Window size is not always entirely the designer’s choice; in some cases, such as hospital patient rooms, building codes or other standards may prescribe minimum window sizes.
The choice of facade glazing materials also influences occupants’ thermal comfort. The effects are different for summer and winter. During winter, the thermal comfort effect is largely driven by inside window surface temperature, which is usually colder than the room it faces. Table 3-1 shows, for six common glazing types, the lowest outdoor temperature at which a person seated three feet (one meter) from a window would feel comfortable. The table shows that double-glazed, air-insulated, low-e glazing units are suitable for climates where winter exterior temperatures are above 21°F (–
6°C), while triple-glazed, air-insulated, low-e glazing units can be used for temperatures as low as –18°F (–28°C).
During the summer, thermal comfort is driven by the combination of the inside surface temperature of the glass and the transmitted solar radiation through the glass. These in turn are significantly influenced by the construction of the glazing units, the material properties of the glass, and the effectiveness of shading elements used with the window. Table 3-1 shows, for the same six glazing types, the maximum amount of solar radiation on the surface of the glass before an occupant seated close to a window starts to feel uncomfortable. As we can see, spectrally selective double-glazed, air- insulated, low-e glazing units are the best choice for climates with high solar radiation. These types of glazing units have a light-to-solar-gain ratio of 1.25 or higher (i.e., they have high visual transmittance and low SHGC), allowing large amounts of daylight to enter the interior while blocking much of the solar heat.
Table 3-1: Glazing systems and winter and summer environmental conditions for meeting thermal comfort of occupants seated close to a window. Glazing type Winter
Minimum allowable outdoor temperature in °F (°C)
Summer Maximum allowable solar radiation Btu/ft2 (W/m2)
Double-lite air-insulated IGU (clear)
45 (7) 150 (469)
Double-lite air-insulated IGU (low-e)
21 (–6) 165 (516)
Spectrally selective double-lite air-insulated IGU (low-e)
16 (–9) 342 (1,069)
Triple-lite air-insulated IGU (clear)
28 (–2) 146 (456)
Triple-lite air-insulated IGU (low-e)
–18 (–28) 196 (613)
Spectrally selective triple-lite air- insulated IGU (low-e)
–22 (–30) 323 (1,009)
Source: Huizenga et al., 2006.
The variation in window surface temperatures, and its effect on occupant comfort, can be tempered by changing the interior air temperature. The cooling or heating effect of cold or warm glass can be compensated for by
raising or lowering the air temperature inside the space. Figure 3-5 shows how the interior air temperature affects the occupant comfort under a variety of glass temperatures, for occupants seated close to the windows. The four curves correspond to different glass temperatures, and each curve shows the interior air temperature that gives occupants sitting close to the glass the greatest comfort. For example, if the glass temperature is 50°F (10°C), occupants near the glass will be most comfortable with an interior air temperature of 78°F (26°C). For a glass temperature of 104°F (40°C), the highest level of comfort is reached with a room temperature of approximately 70°F (21°C).
Air movement can also have an effect on thermal comfort. Facades can cause two forms of undesirable air movement: induced air motion caused by cold interior window surfaces, and infiltration of outside air through gaps in the exterior enclosure. In general, the drafts affect thermal comfort. In the exceptional cases—for example, very tall windows with low-performance glass—a heater under the window sill may mitigate the effects of the draft. In a truly sustainable strategy for interior comfort, this type of glazing system should be properly designed and selected to preclude interior air induction.
Air infiltration can have a much more significant effect on comfort. Although facades cannot be built fully airtight, infiltration resistance can be achieved if the assembly is designed with an appropriate air barrier. Air leakage can be aggravated if the interior air pressures are significantly different from the exterior air pressure. These internal pressure differences can be created by the HVAC system or by the varying vertical air pressures in tall buildings (stack effect). Significant pressure differences between the interior and the exterior of the building can cause air to be driven through the facade if there are penetrations in the air barrier. This will result in outside air being pulled into the space or conditioned interior air being driven out of the building, requiring more internal air to maintain thermal comfort.
Figure 3-5 Thermal comfort and glass temperature (Adapted from Huizenga et al., 2006).
In summary, designers have four design strategies available to improve the thermal comfort of a building’s occupants:
Find the optimal window-to-wall ratio (WWR). In some conditions, it is better to have more windows or larger windows to bring in more daylight, whereas in other conditions it is better to have fewer or smaller windows for increased wall insulation and better acoustics. The optimal WWR strikes a balance between all thermal comfort factors and other factors that give the occupants the best overall comfort. Select glazing materials with the best performance characteristics, especially solar heat gain coefficient and U-value. Design shading elements to reduce interior solar heat gain during warm seasons, while allowing direct sunlight to provide warmth during cold seasons. Provide a facade assembly strategy that will be as airtight as possible,
with all gaps sealed to limit uncontrolled movement of air through the facade. This will keep exterior air from penetrating the exterior wall and conditioned air within the interior space.
DAYLIGHT AND GLARE Daylighting Strategies
Use of natural light has become an important strategy for improving buildings’ overall energy efficiency. By providing occupants with natural light, reliance on artificial lighting to perform daytime activities is reduced. Because even energy-efficient light fixtures can generate significant amounts of heat, extensive use of daylighting can play a major role in reducing cooling loads.
Research has shown that the benefits of daylight extend beyond energy savings to include the positive physiological and psychological well-being of people. Exposure to natural light positively affects people’s circadian rhythms, which can lead to higher productivity and greater satisfaction with the internal environment (Edwards and Torcellini, 2002). Different wavelengths and spectral distributions of light have different effects on the human body, and daylight, unlike most artificial light sources, includes the full spectral distribution of wavelengths needed for biological functions. For this reason, people subconsciously prefer daylight to any type of artificial lighting (Liberman, 1991). Studies have shown that in commercial office spaces, daylight promotes increased productivity, improved health of occupants, reduced absenteeism, and financial savings. In educational facilities, the benefits include improved student attendance and academic performance. Research also suggests that natural light in hospitals and assisted-living facilities improves the physiological and psychological states of patients and staff (Edwards and Torcellini, 2002).
Though few (if any) negative effects result from exposure to daylight, exposure to direct sunlight has both good and bad effects. For example, when the ultraviolet radiation in sunlight hits human skin, the skin produces the essential vitamin D. However, too much sunlight on skin can cause tissue
damage. Window glass usually blocks most of the sun’s ultraviolet radiation from reaching interior spaces.
Designers of naturally lit spaces need to consider the project’s design goals and criteria, and its fixed and variable conditions. Design goals and criteria include subjective qualities, such as privacy and views to the outside, as well as objective and measurable qualities, such as energy use and the intensity of the daylight. They are set either by the project team (for example, views to the outside) or by prevailing codes, zoning ordinances, and standards. When considering visual comfort, designers need to think about illumination levels, daylight distribution, and protection against direct sunlight and glare. Integration of building systems is also important, because facades, lighting, shading elements, HVAC systems, and building controls must function together to have the largest effect on building performance. For example, spaces that use natural daylight for perimeter zones and control artificial lighting with photosensors and dimmers reduce the cooling loads for the HVAC system and, most likely, the sizes of mechanical equipment and ductwork.
Fixed conditions are outside the designer’s control. They include the building’s location and the climate, which determine, respectively, the position of the sun and the outdoor temperature, and the surrounding buildings, trees, topography, and other features that can affect daylight availability and views. Designers can control the variable conditions, such as building geometry and the design of the facade, including material properties, size and orientation of windows, and shading of windows. By understanding the fixed conditions, and carefully manipulating the variable ones, designers can create spaces that use daylighting to enhance occupants’ visual comfort. Table 3-2 shows typical examples of fixed and variable conditions.
Table 3-2: Daylight design considerations. Design goals and criteria Fixed and variable conditions Visual comfort
Illuminance Daylight distribution Exposure to direct sunlight Glare
Visual characteristics Views to the outside
Climate (fixed) Daylight availability Temperature
Site and location (fixed) Latitude Local daylight availability Exterior obstructions and surrounding
Daylight quality: color, brightness Privacy
buildings Ground reflectance
Building energy use/costs Codes and standards Systems and products Integration of systems: facade, lighting, shading, HVAC, and controls
Room and fenestration properties (variable) Geometry Material properties and reflectance Fenestration size and orientation Shading system
Lighting system (variable) Light fixture properties Ambient and task lighting Controls
Occupants’ activities (fixed)
People experience visual comfort when they have the right amount of light —natural or artificial—to perform their tasks. Illuminance measures the intensity of perceived light on a surface. The units of measure for illuminance are foot-candles (fc) in the imperial system and lux in the SI, or metric, system (1 fc = 10.764 lux). The Illuminating Engineering Society recommends ranges of illuminance levels for different types of spaces and tasks, which are published in the IESNA Lighting Handbook (IESNA, 2011). For example, public spaces with dark surroundings require from 2 to 5 fc (20 to 50 lux); work areas where visual tasks are occasionally performed require from 10 to 20 fc (100 to 200 lux); and spaces where detailed visual tasks are performed for prolonged periods of time require from 200 to 500 fc (2,000 to 5,000 lux).
Humans perceive differences in light levels through logarithmic, rather than arithmetic, progressions. Therefore, a change from 25 to 50 fc would appear to be the same increase in light as a change from 50 to 100 fc, as in both cases the amount of light is doubled. Similarly, an increase of 25 foot-candles would appear greater when the light changes from 25 to 50 fc (a 100% increase) than with a change from 50 to 75 fc (a 50% increase).
The orientation and WWR of a building influence the availability of natural light for interior spaces. Analyzing daylight availability during the different seasons is an important part of the design process for high-performance sustainable facades. Lighting simulation software programs, such as Radiance, developed by the Lawrence Berkeley National Laboratory, can be used to simulate and study different design options. The following case study demonstrates the procedure and results. The building under consideration is a
research facility, consisting primarily of laboratories and offices. It is located in a mixed, humid climate (zone 4A). Figure 3-6 shows two of the facades, along with the projected shadows from a neighboring building, on June 21 and December 21. In this case study, we compare two interior laboratory spaces. Both spaces have ribbon windows on the long northeast-facing facade, and Laboratory 1 also has floor-to-ceiling windows on the short northwest-facing facade. The window-to-wall ratios are 65% for Laboratory 1 and 55% for Laboratory 2.
Figure 3-6 Facade orientation, location of laboratories, and projected shadows.
Figure 3-6 (continued) Facade orientation, location of laboratories, and
Using the Lawrence Berkeley National Laboratory’s Radiance software, simulations were performed for both of these laboratories. Figure 3-7 compares the daylight levels for the two laboratory spaces on June 21 at 10:00 a.m., showing floor plans and three-dimensional views of the daylight distribution within the interior spaces. The recommended illumination level for laboratories is 50 fc (560 lux). Laboratory 1 has higher daylight levels than Laboratory 2 as a result of its higher WWR; it has adequate light levels even at the inside wall. Laboratory 1 also has a pronounced “hot spot” in the corner next to the full-height windows. Laboratory 2 has lower, but more
uniform, daylight levels than Laboratory 1. At roughly halfway into the room, daylight levels fall below the recommended illumination level.
Figure 3-8 shows daylight levels for the two laboratory spaces on December 21. Daylight levels during the winter are significantly less than during the summer, due to lower exterior illumination levels during winter for this location. They drop below the recommended levels at 9 feet (approximately 3 meters) from the windows in Laboratory 1 and 6 feet (2 meters) in Laboratory 2.
The primary energy-related design objective of a daylighting system is to provide as much usable daylight as deep in a building’s interior as possible. The secondary objective of daylighting is energy conservation. As a rule of thumb, the depth of the interior daylighting zone in a room is twice the height of the window. Sustainable strategies for improving natural light levels provide ways of increasing that depth without increasing the amount of glazing.
Light shelves have been successfully and economically used to expand the daylighting zone. Light shelves are horizontal fins mounted to the inside of the window framing, usually at least 80 inches above the floor to comply with building codes and accessibility requirements. During winter months, when the height of the sun is low, sunlight can pass above the light shelf to provide radiant warmth to the interior spaces. During summer months, when the sun is higher in the sky, the direct sunlight is blocked by the light shelf. However, the sunlight “bounces” off the top surface of the light shelf to hit the ceiling of the space, and then bounces again deep within the space, as shown in Figure 3-9. This effectively increases the daylighting zone using indirect light with minimal or no direct sunlight penetrating the building. A light shelf can be nothing more than a light-colored painted surface, or it can be more sophisticated. The top surface can be covered with prismatic aluminized films to increase reflectivity. The shelf can be shaped into compound geometries tailored for specific solar altitudes, or the shelf can be moveable and adjusted seasonally to provide the optimal patterns of reflected light. Sloped ceilings can further enhance the bounced indirect light effect.
Translucent glazing materials can be used to provide filtered, uniform, and glare-free daylight. By combining transparent vision glass at eye level with translucent glass above and below, designers can enhance the daylighting
while giving occupants views to the outside.
Figure 3-7 Daylight levels for two spaces on June 21 at 10:00 a.m.
Figure 3-7 (continued) Daylight levels for two spaces on June 21 at 10:00 a.m.
Figure 3-8 Daylight levels for two spaces on December 21 at 10:00 a.m.
Figure 3-8 (continued) Daylight levels for two spaces on December 21 at 10:00 a.m.
Figure 3-9 Diagram of light shelf performance in summer and winter.
Figure 3-10 Diagram showing daylight facade strategies for locations with predominantly cloudy sky conditions (Adapted from Ruck et al., 2000).
A successful daylighting strategy depends on how much daylight reaches the building envelope. Locations with predominantly cloudy skies require different daylighting strategies from those with mostly clear skies. Figure 3- 10 shows strategies that are effective in locations where cloudy skies predominate. Large windows located high and equipped with light shelves can be effective. Where clear skies predominate, strategies that control direct sunlight, such as reduction of window size and the use of shading elements, should be applied. Figure 3-11 shows effective strategies in these locations. Table 3-3 lists different strategies and their applicability for various sky conditions, climates, and design criteria.
Case study 3-1 illustrates how facade design options affect interior daylight levels, and how light shelves or other light-redirecting mechanisms can enhance the daylighting of interior spaces.
Figure 3-11 Diagram showing daylight facade strategies for locations with predominantly sunny sky conditions (Adapted from Ruck et al., 2000).
Table 3-3: Applicability of different daylight facade strategies.
Legend: + Excellent 0 Average – Poor Source: Ruck et al., 2000.
The building is a hospital located in climate zone 4A (mixed, humid). The curtain wall design used for the south and southwest facades is shown in Figure 3-12. These facades enclose an interior public space that is used as a waiting area and a circulation corridor.
The curtain wall materials are low-e insulating vision glass, opaque spandrel glass, and low-e insulating fritted glass. An external horizontal shading element is attached to the curtain wall mullions above eye level. Figure 3-13 shows how the horizontal sunshade reduces incident solar radiation along the south facade. Because fritted glass reduces daylight due to its lower visual transmittance, several different daylighting strategies were investigated as a method to increase the daylight levels without increasing the amounts of incident solar radiation. The considered strategies included a light shelf, modified ceiling geometry, and variation in the placement of spandrel areas and fritted glass.
Figure 3-12 Facade design (south and southwest orientation).
Figure 3-13 Effects of horizontal shading device on incident solar radiation (south facade).
Three design scenarios were analyzed for their effects on daylighting (Figure 3-14). The constants in all three options are the floor-to-floor height, the depths of the mullion extrusions, the depth of the room, and the exterior horizontal sunshade. The characteristics of different scenarios are as follows:
Option 1: Curtain wall with fritted glass at the upper portion of the facade, and the interior space with a level ceiling
Figure 3-14 Diagrams of simulated options.
Option 2: Curtain wall with a 1-foot (0.3-meter) deep interior aluminum light shelf, spandrel glass below sill height, and the interior space with a ceiling sloping down from the curtain wall into the interior space Option 3: Curtain wall with fritted glass near the floor finish, and the
interior space with a ceiling sloping up from the curtain wall into the interior space
Summer and winter daylight levels were simulated and analyzed for these three options using the Radiance simulation program. The results for the south-oriented facade are shown in Figures 3-15 (summer conditions) and 3- 16 (winter conditions). Figures 3-17 and 3-18 show results for the southeast- oriented facade.
Figure 3-15 Daylight levels during summer conditions for south-oriented facade (June 21, noon).
Figure 3-16 Daylight levels during winter conditions for south-oriented facade (December 21, noon).
Figure 3-17 Daylight levels during summer conditions for southeast-oriented facade (June 21, noon).
Figure 3-18 Daylight levels during winter conditions for southeast- oriented facade (December 21, noon).
The light shelf used in Option 2 and the sloped ceiling used for Option 3 both increase daylight levels compared to Option 1. Because Option 2 has a smaller vision area than Option 1, the light shelf and the ceiling sloping down from the curtain wall are good strategies for reducing solar heat gain without negatively affecting the available daylight in the interior space. Option 3, with a ceiling sloping up from the curtain wall and with fritted glass placed at
the lower portion of the curtain wall, is also a good strategy for increasing the daylight levels, improving distribution of light, and reducing potentials for glare.
Case Study 3.1 Centers for Disease Control and Prevention, National Center for Environmental Health The Environmental Health Laboratory Building at the Centers for Disease Control and Prevention is an example of balancing the aesthetic and environmental objectives of the facade design. It is located in Atlanta, Georgia (climate zone 3A). The building’s program includes a variety of functions, including laboratories, offices, conference rooms, and circulation, each having its own daylighting needs. Each facade is designed to balance those needs with its orientation. As a result, there is considerable variation in the design of each facade (Figure 3-19).
Figure 3-19 Building facade enclosing (from left to right) laboratories, atrium, and offices. Courtesy of Nick Merick © Hedrich Blessing
A five-story atrium separates the laboratories and the offices. Sloped ceilings in the open laboratories bounce daylight deep into the interior spaces, as shown in Figures 3-20 and 3-21. The facade uses exterior vertical shades and horizontal louvers to control solar heat gain and glare.
Figure 3-20 Exterior wall section.
Figure 3-21 Sloped lab ceiling as a method to increase daylight levels. Courtesy of Nick Merick © Hedrich Blessing
Glare Glare is caused when there are areas of relatively intense brightness
compared with other darker areas within the field of view. Glare, like thermal comfort, is a subjective physiological response, but it can cause visual discomfort to occupants. It can also impair people’s performance. The human eye can function well across a wide range of illumination levels, but not if an area of extreme brightness is present in the field of view. Good daylighting design controls glare while providing sufficient light for visual performance.
Two methods for measuring glare are the Unified Glare Rating (UGR), developed by the International Commission on Illumination (CIE), and Visual Comfort Probability (VCP), developed by the Illuminating Engineering Society of North America. Both have been developed primarily for artificial lighting rather than for daylighting, but are used in computer simulation programs for glare analysis. UGR and VCP can be predicted using the daylighting simulation software Radiance.
The UGR identifies visual discomfort, and is calculated by a formula that takes into account the position and brightness of each potential glare source, as well as the position of the viewer and angle of sight. CIE has the following recommendations for acceptable ranges (CIE, 1995): Comfort zone:
Imperceptible: < 10 Just perceptible: 13 Perceptible: 16 Just acceptable: 19
Discomfort zone: Unacceptable: 22 Just uncomfortable: 25 Uncomfortable: > 28
VCP is an estimate of the percentage of people who would feel comfortable in a given visual environment. For example, a VCP value of 75 indicates that 75% of the occupants would be satisfied with their visual environment. It is based on an empirical prediction assessment, and considers the number of lighting sources and their locations, the background luminance, the room size and shape, the surface reflectance of materials, the illuminance levels, the observer’s location and line of sight, and the differences in glare sensitivity for different individuals.
Methods for controlling glare start with the proper sizing of windows, as
large areas of glass often result in uncontrolled brightness and unwanted glare. However, small punched openings in otherwise opaque walls can create contrasting bright spots that are uncomfortable to look at. Translucent, light-diffusing glass can be effective in reducing glare. Light shelves, light monitors, and other methods for redirecting daylight can also help reduce glare.
To show the relationship between available daylight, room dimensions, orientation, neighboring buildings, and glare, we use the following case study (Figures 3-22 to 3-25). Figure 3-22 shows two laboratory spaces along the southwest facade of a building. Both laboratories have small, equally spaced punched windows with interior light shelves. In plan, both spaces are deep and narrow. Recommended illuminance levels for these types of spaces are between 60 and 150 fc (667 and 1667 lux).
Figure 3-22 Location of laboratories.
First, we determine the daylight levels in the two spaces. Figure 3-23 shows simulated daylight levels on June 21 at 2:00 p.m. Because both spaces are approximately the same size, face the same direction, and have similar window configurations, it would be expected that these spaces would have similar daylighting patterns. However, an adjacent building partially blocks sunlight from reaching the facade of Laboratory 1, seen in Figure 3-22. Therefore, the daylight levels in this space are less than those in Laboratory 2, and never quite reach the recommended light levels. Laboratory 2 has much higher daylight levels near the windows, but has a significant drop-off as the distance from the windows increases; daylight falls below recommended light levels at around 9 feet (2.7 meters).
Figure 3-24 shows daylight levels for both laboratories on December 21 at 2:00 p.m. Laboratory 1 has usable daylight up to 3 feet (0.9 meter) from the windows, while Laboratory 2 has usable daylight up to around 6 feet (1.8 meters) from the windows.
What are the effects of glare? High contrasts in light levels between windows and opaque surfaces can cause glare, and Figure 3-25 shows the interior views used to calculate UGR and VCP ratings. Calculations were performed for 2:00 p.m. and 5:00 p.m. on June 21, and for 2:00 p.m. on December 21. Results are shown in Table 3-4.
Figure 3-23 Daylight levels for two spaces on June 21.
Figure 3-23 (continued) Daylight levels for two spaces on June 21.
Figure 3-24 Daylight levels for two spaces on December 21.
Figure 3-24 (continued) Daylight levels for two spaces on December 21.
Figure 3-25 Fish-eye perspective of the interior spaces used to determine glare potential.
Table 3-4: Glare indices for the two spaces shown in Figure 3-25. Glare Laboratory 1 Laboratory 2
June 21 (2:00 p.m.)
June 21 (5:00 p.m.)
December 21 (2:00 p.m.)
June 21 (2:00 p.m.)
June 21 (5:00 p.m.)
December 21 (2:00 p.m.)
27.5 22.8 26.9 23.3 23.5 15.4
9.3 35.3 11.6 12.8 21.1 66.4
The glare analysis shows that occupants in both of these spaces would find the glare uncomfortable at certain times. The deep, narrow geometry of the rooms and the small sizes of the windows relative to the opaque walls contribute to this. On June 21 at 2:00 p.m., Laboratory 1, with a UGR of 27.5, is near the uncomfortable end of the discomfort zone. Only 9.3% of occupants would be satisfied with the visual conditions. Visual conditions improve slightly during the late afternoon hours, with the UGR decreasing to a still-unacceptable 22.8. The VCP index indicates that less than half of the occupants (35.3%) would be satisfied with the visual conditions. The UGR for Laboratory 2 at the same date and time is 23.3, which falls within the discomfort zone. This is confirmed by a VCP index of 13%, indicating that few of the occupants would be satisfied.
The glare analysis for the winter months tells a different story. Laboratory 1’s UGR for December 21 at 2:00 p.m. is 26.9, still deep in the discomfort zone. The VCP tells us that less than 12% of the occupants will be satisfied with the visual conditions. In contrast, Laboratory 2 shows considerable improvement. With a UGR of 15.4, the space is well within the comfort zone, with a noticeable but acceptable level of glare. The VCP index of 66.4% shows that roughly two-thirds of the occupants would be satisfied with the visual conditions.
Why the big difference between the two spaces in winter? Quite simply, a neighboring building blocks sunlight from Laboratory 1. In the summer months, the sun is high enough for sunlight to pass over the obstruction and reach the Laboratory 1 windows. In the winter, however, the obstruction blocks most of the lower-angle sunlight. Were it not for the obstruction, Laboratory 1’s winter daylighting and glare levels would probably be similar to those of Laboratory 2.
This case study illustrates how room geometry, window size and orientation, exterior obstructions, and seasonal changes affect daylighting and the potential for glare.
ACOUSTIC COMFORT AND AIR QUALITY
Acoustics Good acoustic design should attenuate unwanted noise and enhance desired sounds. External sources of noise, such as traffic, factories, and airline flight paths, can affect occupants’ acoustic comfort. However, not all noise is undesirable. People unconsciously use ambient sounds to orient themselves within a building, to help make themselves aware of the time of day, and to provide “white” background noise for speech privacy (Reffat and Harkness, 2001). Therefore, they prefer interior environments that are relatively quiet, but not absolutely free of ambient sounds. The normal acoustic environment of an occupied space is a combination of sounds from multiple sources, such
as air diffusers, equipment (refrigerators, computers, telephones, etc.), music, voices (from within the space or from adjacent spaces), and the external environment. As long as those sounds do not become obtrusive, and the occupants are not consciously aware of them, the ambient sounds may be acceptable.
There are a number of established methods for evaluating the acoustic quality of an interior space. These are shown in Table 3-5. Each method evaluates a different aspect of acoustic performance, and not all of them are relevant to facade design.
Table 3-5: Acoustic comfort factors for interior spaces. Acoustic comfort factors Description Metric Background (ambient) noise level Amount of noise generally distributed within the
interior space dB
Noise criteria Relative loudness of a space NC levels
Sound transmission class for walls, partitions, floors
Ability of wall, partition, or floor assembly to block airborne sound
Impact insulation class rating for floors and ceiling assemblies
Ability of floors or ceilings to block impact sounds traveling through the structure
Outdoor-indoor transmission class Ability of exterior enclosure assemblies to block airborne sound
Noise reduction coefficient Sound-absorption efficiency rating for different materials
The sound transmission class (STC) rating system is one way to represent the acoustic experience of an occupant in a room. Partition and floor assemblies are tested to determine their STC ratings. STC is a measure of acoustic performance for a range of frequencies (125 to 4,000 Hz) that encompass most everyday interior sounds, particularly human speech. The International Building Code specifies that walls, partitions, and floor and ceiling assemblies between dwellings, and between dwellings and public spaces, should have an STC rating of 50 or more for airborne sound (ICC, 2012). Impact insulation class (IIC) rating is similar to STC; however, it represents the transmission of impact sounds through the structure, particularly floors or ceilings.
The STC rating system was introduced in 1970, and has become the standard tool for the acoustic design of interior partitions and floor
assemblies. However, because the system is based on the mid- and high- frequency sounds associated with human speech and normal household activities, it has proven inadequate for low-frequency exterior sounds. ASTM, in its E413 standards that govern STC ratings, states that the STC classification method is not appropriate for some sound sources, such as motor vehicles, aircraft, and trains (ASTM, 2010).
Another method for evaluating the acoustic performance of constructed assemblies, the Outdoor-Indoor Transmission Class (OITC), was introduced in 1990 specifically for normal exterior sounds—particularly those generated by an aircraft taking off, a nearby railroad, or a busy freeway (i.e., planes, trains, and automobiles). The OITC’s range of frequencies, from 80 to 4,000 Hz, includes all the STC frequencies, as well as lower frequencies. Like the STC ratings, the OITC system uses a single number to rate the acoustic performance of a product or an assembly of materials. For both STC and OITC, the higher the number, the better a product or assembly will perform acoustically.
Calculations to determine OITC ratings must be based on carefully controlled laboratory or field tests of a product or assembly. Many manufacturers of standard windows, doors, curtain walls, insulation, and joint seals provide OITC ratings for their products. However, custom systems and assemblies often must be tested, either in an acoustic testing facility or in the field, to determine their OITC ratings.
For high-performance facades, ASHRAE recommends a composite OITC of at least 40. Fenestration areas should have an OITC rating of at least 30 (ASHRAE, 2009).
Designers can follow these general principles to improve the acoustic performance of a facade:
Increase the mass of the materials. In general, the more massive a material is, the higher the sound transmission loss will be. Match the resonant frequency of the materials to the predominant sound waves. When the frequency of the sound waves matches the resonant frequency of the materials, energy is absorbed, resulting in higher sound transmission loss. Increase the width of air spaces. Provide acoustic breaks. Similar to thermal bridges, solid materials that
bridge across air spaces will help sound waves pass through a wall. Acoustic breaks will hamper sound transmission. Fill air spaces in opaque walls with insulation materials with desired thermal and acoustic performance ratings. Use layers of different materials. This will create discontinuities in the wall, making it more difficult for sound waves to move from one material to the next. Finally, and perhaps most basically, seal air leaks in the wall assembly. Air leaks give sound waves a continuous path through a single medium (air) from the outside to the inside.
Some of these principles apply only to opaque walls. For glazed facades, designers have other strategies available for improving acoustic performance:
Thicker glass will increase the mass that the sound waves have to pass through. However, unless unusually thick glass is required for other reasons (ballistic resistance, for instance), this approach is not an economical way to significantly improve acoustic performance. Increasing the thickness of the glass from ¼ inch to ½ inch will increase the OITC from 29 to 33, and the STC from 31 to 36. Laminated glass will improve the acoustic performance of single-glazed windows. The laminated inner layer creates a discontinuity of materials that dampens the sound vibrations. A nominal 1/4-inch lite of laminated glass, consisting of two layers of 1⁄8-inch glass and a 0.060-inch-thick laminate interlayer, will have an OITC of 32 and an STC of 35. Standard air-filled insulating glass units will perform better than most single-glazed windows (1-inch-thick insulating unit with 1/2-inch air space will have an OITC of 26–28 and an STC of 31–33). Using laminated glass for one or both lites in a 1-inch-thick, air-filled insulating unit will further increase its acoustic performance. With one lite of 1/4-inch laminated glass, the OITC of the insulating unit increases to 28–30, and the STC increases to 34–36. When both lites are 1/4-inch laminated glass, the OITC and STC increase to 29–31 and 37–39, respectively. Triple-glazed insulating units, with the middle lite being either glass or a laminate membrane, will further enhance the acoustic performance. When the insulating unit is constructed with a “soft” separation between
the lites of glass, the acoustic performance will be improved. Adding a secondary interior lite of glass, separated from the outer insulating unit by a substantial air space, will result in still better acoustic performance. An assembly consisting of a standard 1-inch insulating unit, a 2-inch air space, and an inner lite of 1/4-inch glass will have an OITC of 32–35 and an STC of 42–44. When laminated glass is used for one lite of the insulating unit and for the single interior lite, the OTC is 35–37 and the STC is 44–46.
Most of these measures to improve a facade’s acoustic performance will also improve its thermal performance.
With buildings that use natural ventilation to reduce energy consumption, we can expect more sound to pass through the exterior walls than if the walls were fully sealed. For naturally ventilated buildings, higher indoor noise levels should be allowed (Field, 2008). Research indicates that internal noise levels of up to 65 dB (equivalent to normal conversation 3 feet away) may be acceptable (Ghiaus and Allard, 2005). Calculations can predict the amount of exterior noise that passes through the building facade, including noise coming from openings for natural ventilation. If the predictions show that acceptable acoustic comfort will not be met, attenuating elements, such as acoustic louvers and white noise generators, can be incorporated into the design (De Salis et al., 2002).
Emerging facade technologies, such as double-skin glass facades, can improve the acoustic performance of an exterior wall. For a detailed discussion of double-skin facades, see Chapter 4.
Air Quality Acceptable indoor air quality (IAQ) is defined as indoor air that has no contaminants at harmful concentrations and that satisfies at least 80% of the occupants (ASHRAE, 2007).
IAQ affects the health and comfort of building occupants, and is an integral design element for sustainable, high-performance buildings. The sources of unacceptable air quality include microbial contaminants (such as mold and bacteria), gases (including carbon monoxide, radon, and volatile organic compounds), and particulates and other air pollutants, all of which can affect
occupants’ health. Acceptable IAQ is dependent on many building systems, such as HVAC systems, air-mixing techniques, interior finish materials, and building operations. However, because air infiltration and leakage through the building envelope can affect IAQ, it must be considered when designing the facade.
Air infiltration occurs when the exterior air enters through cracks in the facade. The amount of infiltration will depend on the air pressures across the facade. This can be mitigated by sealing openings and providing air barriers within the assembly. Well-designed and correctly installed air barriers will prevent the movement of air through the exterior wall assembly. Air barriers control the airflow between unconditioned and conditioned spaces, and are intended to resist air-pressure differences, stack effect, and wind loads. They should be impermeable to airflow, and must be continuous over the entire building envelope to be effective in blocking air movement. Air barriers keep airborne pollutants from reaching interior spaces. No exterior wall, however, can ever be designed or constructed to be completely airtight. Some incidental leakage is expected; performance specifications for exterior wall assemblies should specify a maximum allowable air infiltration that is consistent with industry standards.
CHAPTER SUMMARY In this chapter, we discussed how occupants’ comfort should be one of the criteria when designing sustainable, high-performance facades. The objective for any sustainable facade is to provide occupants with thermal, visual, and acoustic comfort while using the least possible energy. Therefore, understanding the principles involved, the methods of measurement, and the available design strategies becomes crucial during the design process.
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