Passive Cooling PDF

10 C H A P T E R PASSIVE COOLING Optimizing the building envelope for the climate can substantially reduce the size of the mechanical system. ASHRA Energy D...

10 C H A P T E R PASSIVE COOLING Optimizing the building envelope for the climate can substantially reduce the size of the mechanical system. ASHRA Energy Design Guide for Small to Medium Office Buildings True regional character cannot be found through a sentimental or imitative approach by incorporating either old emblems or the newest local fashions which disappear as fast as they appear. But if you take, for instance, the basic difference imposed on architectural design by the climatic conditions of California, say, as against Massachusetts, you will realize what diversity of expression can result from this fact alone. Walter Gropius Scope of Total Architecture, 1955 285 286â ‡ â ‡ Passive Cooling 10.1 INTRODUCTION of energy, thereby saving both money and light colors are used to minimize TO COOLING and the environment. the heat gain. Closed shutters further reduce the daytime heat gain, while To achieve thermal comfort in the still allowing good night ventilation summer in a more sustainable way, 10.2 HISTORICAL AND when they are open. one should use the three-tier design INDIGENOUS USE OF In urban settings and other places approach (Fig. 10.1). The first tier PASSIVE COOLING with little wind, wind scoops are consists of heat avoidance. At this sometimes used to maximize ventila- level, the designer does everything Examples are sometimes better than tion. Wind scoops were already used possible to minimize heat gain in definitions in explaining concepts. several thousand years ago in Egypt the building. Strategies at this level The following examples of historical (Fig. 10.2b), and they are still found include the appropriate use of shad- and indigenous buildings will illus- in the Middle East today. When there ing, orientation, color, vegetation, trate what is meant by passive cooling. is a strong prevailing wind direction, insulation, daylight, and the control Passive cooling is much more as in Hyderabad, Pakistan, the scoops of internal heat sources. These and dependent on climate than passive are all aimed in the same direction other heat-avoidance strategies are heating. Thus, the passive cooling (Fig. 10.2c). In other areas, where described throughout this book. strategies for hot and dry climates are there is no prevailing wind direction, Since heat avoidance is usually not very different from those for hot and such as Dubai on the Persian Gulf, sufficient by itself to keep indoor tem- humid climates. wind towers with many openings peratures low enough all summer, the In hot and dry climates, one usu- are used. These rectangular towers are second tier strategies of passive cool- ally finds buildings with few and divided by diagonal walls, which cre- ing should also be used. With some small windows, light surface colors, ate four separate air wells facing four passive cooling systems, temperatures and massive construction, such as different directions (Fig. 10.2d). are actually lowered and not just min- adobe, brick, or stone (Fig. 10.2a). Wind towers have shutters to imized, as is the case with heat avoid- The massive materials not only keep out unwanted ventilation. In ance. Passive cooling also includes the retard and delay the progress of heat dry climates, they also have a means use of air motion to shift the com- through the walls, but also act as a of evaporating water to cool the fort zone to higher temperatures. The heat sink during the day. Since hot incoming air. Some wind towers have major passive cooling strategies will and dry climates have high diurnal porous jugs of water at their base, be discussed in this chapter. temperature ranges, the nights tend to while others use fountains or trickling In many climates, there will be be cool. Night ventilation can then be water (Fig. 10.2e). times when the combined effort of used to cool the indoor mass, which The mashrabiya is another popu- heat avoidance and passive cool- will then act as a heat sink the next lar wind-catching feature in the ing is still not sufficient to maintain day. To prevent the heat sink from Middle East (Fig. 10.2f). These bay thermal comfort. For this reason, the being overwhelmed, small windows windows were comfortable places third tier of mechanical equipment is usually required. In a sustainable design process, as described here, this equipment must cool only what heat avoidance and passive cooling could not accomplish. Consequently, the mechanical equipment will be quite small and use only modest amounts Figure 10.2a Hot and dry climates typically have buildings with small windows, light colors, Figure 10.1 Sustainable cooling is achieved and massive construction. Thera, Santorini, Greece. (From Proceedings of the International by the three-tier design approach. This Passive and Hybrid Cooling Conference, Miami Beach, Florida, Nov. 6–16, © American Solar  chapter covers tier two. Energy Society, 1981.)  10.2 Historical and Indigenous Use of Passive Coolingâ ‡ â ‡ 287 Figure 10.2d The wind towers in Dubai, United Arab Emirates, are designed to catch the wind from any direction. These substantial wind towers evolved from the one shown in Fig. 1.2d. (Photograph by Mostafa Howeedy.) Figure 10.2b Ancient Egyptian houses used wind scoops to maximize ventilation. (After a wall painting in the Tomb of Nebamun, circa 1300 b.c., at the Metropolitan Museum of Art, New York City, #30.4.57.) Figure 10.2e Some wind towers in hot and dry areas cool the incoming air by evaporation. Figure 10.2f A mashrabiya is a screened bay window popular in the Middle East. It shades, ventilates, and provides evaporative cooling. Figure 10.2c The wind towers in Hyderabad, Cairo, Egypt. (Photograph by Mostafa Pakistan, all face the prevailing wind. Howeedy.) 288â ‡ â ‡ Passive Cooling to sit and sleep, since the delicate climates, where cross ventilation is tering is discussed in more detail in wood screens kept most of the sun desirable. Chapter 15. out yet allowed the breezes to blow Massive domed structures are an In Cappadocia, Turkey, thousands through. Evaporation from porous appropriate strategy in hot and dry of dwellings and churches have been jugs of water placed in the mashrabiya regions. Besides the thermal benefit excavated from the volcanic tufa cooled not only drinking water but of their mass, their form yields two cones over the last 2000 years (Fig. the houses as well. The mashrabiya different benefits. During the day, the 10.2i). Many of these spaces are still also satisfied the cultural need to give sun sees little more than the horizon- inhabited today, in part because they women an inconspicuous place to tal footprint of the dome, while at provide effective protection from view the activity of the outside world. night almost a full hemisphere sees extreme heat and cold. Wherever the humidity is low, the night sky. Thus, radiant heating A structure need not be completely evaporative cooling is very effec- is minimized while radiant cooling is earth-sheltered to benefit from earth- tive. Fountains, pools, water trickling maximized. Domes also have high contact cooling. The dwellings abut- down walls, and transpiration from spaces where stratification will enable ting the cliffs at Mesa Verde, Colorado, plants can all be used for evapora- the occupants to inhabit the cooler make use of the heat-sink capacity tive cooling. The results are best if lower levels (Fig. 10.2h). Sometimes of both the rock cliff and the massive the evaporation occurs indoors, in the vents are located at the top to allow stone walls. The overhanging, south- incoming air stream, or in a courtyard the hottest air to escape. The most facing cliffs also offer much shade dur- that is the main source of air for a dramatic example of this kind of ing summer days (Fig. 10.2j). In areas building (Fig. 10.2g). dome is the Pantheon, in Rome. Its where rock was not available, thick Small and deep courtyards or oculus allows light to enter while the earthen walls were used. The Navajo atriums are beneficial in hot and dry hot air escapes. The same concept was people of the dry Southwest built climates, not only because they are used in the U.S. Pavilion at Expo 67 hogans for the insulating effect of their self-shading most of the day but also in Montreal. The upper panels of the thick earthen walls and roofs (Fig. because they block the hot wind that geodesic dome had round openings 10.2k). Spanish settlers used adobe for would blow away the cool air. This to vent the hot air (see Fig. 9.15a). the same purpose (Fig. 10.2l). benefit is a liability in hot and humid A large quantity of earth or rock is an effective barrier to the extreme In hot and dry climates, maximize temperatures in hot and dry cli- shading and thermal mass while mini- mates. The deep earth is usually near mizing daytime natural ventilation! the mean annual temperature of a region, which in many cases is cool In hot and humid climates, we enough for the soil to act as a heat find a different kind of building, one sink during summer days. Earth shel- in which the emphasis is on natural Figure 10.2g Many traditional courtyard houses have all of their windows and doors facing the courtyard in large part for thermal comfort. The courtyard stays relatively cool because it is self-shading most of the day and because it is protected from hot winds. Figure 10.2h The trulli are conical stone houses in Apulia, Italy. Their large mass and high Further comfort comes from transpiration ceilings, with the resultant stratification of air, make these houses comfortable in summer. (From from plants and evaporation from fountains. Proceedings of the International Passive and Hybrid Cooling Conference, Miami Beach, Florida, Marrakesh, Morocco. Nov. 6–16, © American Solar Energy Society, 1981.)  10.2 Historical and Indigenous Use of Passive Coolingâ ‡ â ‡ 289 ventilation. In very humid climates, mass is a liability and very lightweight structures are best. Although the sun is not as strong as in dry climates, the humidity is so uncomfortable that any additional heating from the sun is very objectionable. Thus, in very humid regions, we find buildings with large windows, large overhangs, and low mass. Where possible the best solution was to minimize the walls (Fig. 10.2m; see also Fig. 17.2g). Sometimes build- ings are set on stilts to catch more wind and to rise above the humidity near the ground. High ceilings allow the air to stratify, and vents at the gable or ridge allow the hottest air to escape (see Fig. 1.2c). In hot and humid regions, maximize shading and natural ventilation while minimizing thermal mass! Much of Japan has very hot and humid summers. To maximize natural ventilation, the traditional Japanese house uses post-and-beam construc- tion, which allows the lightweight paper wall panels to be moved out of the way in the summer (Fig. 10.2n). Large overhanging roofs protect these panels and also create an outdoor space called an engawa. Large gable Figure 10.2i Dwellings and churches are carved from the volcanic tufa cones in Cappadocia, vents further increase the ventilation Turkey. (Photograph by Tarik Orgen.) through the building (Fig. 10.2o). Gulf Coast houses and their elab- orate version, the French Louisiana Figure 10.2j The cliff dwellings at Mesa Verde, Colorado, benefit Figure 10.2k The Navajo hogan, with its thick earthen walls, from the heat-sink capacity of the stone walls and rock cliff. provides comfort in the hot and dry Southwest. 290â ‡ â ‡ Passive Cooling plantation houses, were well adapted damp and muggy ground on a brick very high (sometimes as high as 14 ft to the very humid climate (Fig. structure. Higher up, there was more [4.2 m]) to permit the air to stratify 10.2p). In that region, a typical house wind and less humidity. The main liv- and the people to occupy the lower, had its main living space, built of ing spaces had many tall openings to cooler layers. Vents in the ceiling and a light wood frame, raised off the maximize ventilation. The ceiling was high dormers with operable windows allowed the stack effect and wind to exhaust the hottest air from the build- ing. Deep verandas shaded the walls and created cool outdoor areas. The common open central hall- way was derived from the dogtrot houses of the early pioneers of the South, who built one roof over two log cabins spaced about 10 ft (3 m) apart (Fig. 10.2q). In the summer, this shady, breezy outdoor space became a desirable hangout for dogs and peo- Figure 10.2l Spanish ple alike. missionaries and set- Many of these same concepts were tlers used thick adobe walls for thermal incorporated in the Classical Revival comfort. architecture that was so popular in the South during the nineteenth cen- tury. As was mentioned in Chapter 9, the classical portico was a very suit- able way to build the large overhangs needed to shade the high doors and windows. These openings were often as high as 12 ft (3.6 m), and the win- dows were frequently triple hung so that two-thirds of the window could be opened. Louvered shutters allowed ventilation when sun shading and privacy were desired (Fig. 10.2r). The classical image of white buildings Figure 10.2m These “chickees,” built by the Indians of southern Florida, respond to the hot was also very appropriate for the hot and humid climate by maximizing ventilation and shade while minimizing thermal mass. The diagonal poles successfully resist hurricane winds. climate. Figure 10.2n Ventilation is maximized by movable wall panels in Figure 10.2o The movable wall panels open onto the engawa traditional Japanese houses. (Courtesy of the Japan National Tourist (veranda), which is protected by a large overhang. Also note the large Organization.) gable vent. Japanese Garden, Portland, Oregon.  10.2 Historical and Indigenous Use of Passive Coolingâ ‡ â ‡ 291 Figure 10.2p This Gulf Coast house incorporated many cooling Figure 10.2q The breezy passage of the dogtrot house was a favor- concepts appropriate for hot and humid climates. Note the large ite for both man and beast during the hot and humid summers. shaded porch, ventilating dormers, large windows, and ventilation under the house. Figure 10.2s The Waverly plantation near Columbus, Mississippi, has a large belvedere for view, light, and ventilation. (Photograph by Paul B. Watkins. Courtesy of the Mississippi Department of Economic Development. Division of Tourism.) designed to respond to either hot or Figure 10.2r Shutters with adjustable cold conditions alone. Rather, they louvers were almost universally used in the old South. must be designed for both summer and winter, which often make oppos- ing demands on the architect. The Governor’s Mansion in Williamsburg, The Waverly plantation is a The author visited this non-air-con- Virginia, is located in such a climate. good example of the classical idiom ditioned building on a hot, humid The building is compact, and the adapted to the climate (Fig. 10.2s). summer day and found it to be com- windows are medium-sized (Fig. It has a large, many-windowed bel- fortably cool inside. 10.2u). The brick construction allows vedere, which offers a panoramic Often the temperate climate is the passive cooling during much of the view, light, and a strong stack effect hardest to design for. This is partly summer when the humidity is low through the three-story stair hall. true because many so-called tem- enough. The massive fireplaces act as Since every door has operable tran- perate climates actually have very additional heat sinks in the sum- soms, all rooms have cross ventila- hot summers and very cold winters. mer, as well as heat storage mass in tion from three sides (Fig 10.2t). Buildings in such climates cannot be the winter. Every room has openings 292â ‡ â ‡ Passive Cooling it and is mainly for decoration or image, and it is a “monitor” if ven- tilation is most important. In the Governor’s Mansion, the tower’s main purpose was to create the image of a governmental building, but it also served all of the other functions. Chapter 17 on tropical architec- ture has many more examples of pas- sive cooling for dry and humid hot climates. 10.3 PASSIVE COOLING SYSTEMS The passive cooling systems described here include not only the well-known traditional techniques mentioned above but also more sophisticated modern techniques. As much as pos- sible, passive cooling uses natural forces, energies, and heat sinks. When some fans and pumps are used, the Figure 10.2t A strong stack effect is created by the octagonal belvedere over the open stair hall of the Waverly plantation. All interior doors have transoms. From Mississippi Houses: systems are sometimes called hybrid. Yesterday Toward Tomorrow, by Robert Ford (copyright). However, the author uses the word “hybrid” only for those systems that use large pumps or fans that use sig- nificant amounts of energy. Since the goal is to create thermal comfort during the summer (the over- heated period), we can either (1) cool the building; or (2) raise the comfort zone sufficiently to include the high indoor temperature and humidity (see Fig. 4.9b). In the first case, we have to remove heat from the build- ing by finding a heat sink for it. In the second case, we modify one of the other factors of the thermal envi- ronment (i.e., humidity, MRT, or air speed) so that the comfort zone shifts to higher temperatures. In this second case, people will feel more comfort- able even though the building is not actually being cooled. There are five main methods of passive cooling: Figure 10.2u The Governor’s Mansion in Colonial Williamsburg, Virginia, is well suited for a temperate climate. Types of Passive Cooling Systems I. Cooling with Ventilation A. Comfort ventilation: Ventilation on all four walls for maximum cross a “belvedere” if the panoramic view during the day and night to ventilation. The little tower on the is most important, it is a “lantern” increase evaporation from the roof has several different names, if it acts primarily as a skylight, it is skin and thereby increase ther- depending on its main function. It is a “cupola” if it has a small dome on mal comfort.  10.5 Basic Principles of Airflowâ ‡ â ‡ 293 B. Night-flush cooling: Venti to increase evaporative cooling on the four types by means of lines lation to precool the building the skin. Although thermal comfort representing airstreams. These for the next day. might be achieved, the daytime air is diagrams are similar to what one II. Radiant Cooling actually heating the building. would see in a wind-tunnel test A. Direct radiant cooling: A build- Night-flush cooling is quite dif- using smoke streams. Airflow ing’s roof structure cools by ferent. With this technique, cool changes from laminar to turbu- radiation to the night sky. night air is introduced to flush out lent when it encounters sharp B. Indirect radiant cooling: the heat of the building, while dur- obstructions, such as buildings. Radiation to the night sky ing the day very little outside air is Eddy currents are circular airflows cools a heat-transfer fluid, brought indoors so that the heat induced by laminar airflows (Fig. which then cools the building. gain to the building is minimized. 10.5e). III. Evaporative Cooling During the day, the mass of the rela- 3. Inertia. Since air has some mass, A. Direct evaporation: Water is tively cool structure acts as a heat moving air tends to go in a straight sprayed into the air entering sink for the indoor air and the people line. When forced to change direc- a building. This lowers the inside. Before these techniques can tion, airstreams will follow curves air’s temperature but raises its be explained in more detail, some but never right angles. humidity. basic principles of airflow and their 4. Conservation of air. Since air is nei- B. Indirect evaporative cooling: applications in buildings must be ther created nor destroyed at the 1. Evaporation cools the discussed. building site, the air approach- incoming air of the building ing a building must equal the air without raising the indoor leaving the building. Thus, lines humidity. representing airstreams should be 2. Water is sprayed on the roof 10.5 B  ASIC PRINCIPLES drawn as continuous. to cool the roof. OF AIRFLOW 5. High- and low-pressure areas. As air IV. Earth Cooling hits the windward side of a build- To design successfully for ventilation A. Direct coupling: An earth- ing, it compresses and creates pos- in the summer or for wind protection sheltered building loses heat itive pressure (Fig.10.5c). in the winter, the following principles directly to the earth. â …â ‡ At the same time, air is sucked of airflow should be understood: B. Indirect coupling: Air enters away from the leeward side, the building by way of earth 1. Reason for the flow of air. Air creating negative pressure. Air tubes. flows either because of natural deflected around the sides will V. Dehumidification with a desic- convection currents, caused by generally also create negative pres- cant: Latent heat is removed. differences in temperature, or sure. Note that these pressures because of differences in pressure are not uniformly distributed. A combination of these tech- (Fig. 10.5a). The type of pressure created over niques is sometimes necessary. Each 2. Types of airflow. There are four the roof depends on the slope of of these techniques will now be dis- basic types of airflow: laminar, the roof (Fig. 10.5d). These pres- cussed in more detail. separated, turbulent, and eddy sure areas around the building currents. Figure 10.5b illustrates 10.4 COMFORT VENTILATION VERSUS NIGHT-FLUSH COOLING Until recently, ventilation was the major cooling technique throughout the world. It is important to note that there are two very different ventila- tion techniques. Although they can- not be used at the same time, they can be used at different times of the year in climates that are too humid only part of the summer. Comfort ven- tilation brings in outdoor air, both during the day and at night. The air is then passed directly over people Figure 10.5a Air flows because of either natural convection or pressure differentials. 294â ‡ â ‡ Passive Cooling Figure 10.5b The four different kinds of airflow are shown. (After Art Bowen, 1981.) Figure 10.5c Air flowing around a building will cause uneven positive and negative pressure areas to develop. (After Art Bowen, 1981.) Figure 10.5d The pressure on the leeward side of a roof is always negative, but on the windward side it depends on the slope of the roof. (After Art Bowen, 1981). determine how air flows through currents (Fig. 10.5e). Note how 6. Bernoulli effect. In the Bernoulli the building. these currents reverse the airflow effect, an increase in the velocity â …â ‡It should also be noted that in certain locations. For simplicity’s of a fluid decreases its static pres- these high- and low-pressure areas sake, turbulence and eddy currents, sure. Because of this phenome- are not places of calm but of airflow although usually present, are not non, there is negative pressure at in the form of turbulence and eddy shown on all diagrams. the constriction of a venturi tube  10.5 Basic Principles of Airflowâ ‡ â ‡ 295 Figure 10.5e Turbulence and eddy currents occur in the high- and low-pressure areas around a building. For simplicity, turbulent air is not shown in any of the diagrams (After Art Bowen, 1981). (Fig. 10.5f). A cross section of an Figure 10.5f The venturi tube illustrates airplane wing is like half a venturi the Bernoulli effect: tube (Fig. 10.5g). As the velocity of air â …â ‡ A gabled roof is also like half increases, its static pres- a venturi tube. Thus, air will be sure decreases. Thus, an sucked out of any opening near opening at the constric- tion would suck in air. the ridge (Fig. 10.5h). The effect can be made even stronger by designing roof openings to be like Figure 10.5g An full venturi tubes (Fig. 10.5i). airplane wing is like half â …â ‡There is another phenome- of a venturi tube. In this non at work here. The velocity of case, the negative pres- air increases rapidly with height sure is also called lift. above ground. Thus, the pressure at the ridge of a roof will be lower than that of windows at ground level. Consequently, even without the help of the geometry of a ven- turi tube, the Bernoulli effect will exhaust air through roof openings Figure 10.5h The (Fig. 10.5j). venturi effect causes air 7. Stack effect. The stack effect can to be exhausted through roof openings at or near exhaust air from a building by the the ridge. action of natural convection. The stack effect will exhaust air only if the indoor-temperature difference between two vertical openings is greater than the outdoor-temper- ature difference between the same Figure 10.5i Venturi two openings (Fig. 10.5k). To max-  passive ventilators imize this basically weak effect, the with adjustable lou- openings should be as large and as vers are used at the far apart vertically as possible. The (–) (–) (–) Strasbourg, France, airfreight terminal. air should be able to flow freely Architects: Jockers from the lower to the higher open- Architekten, 2001. ing (i.e., minimize obstructions). 296â ‡ â ‡ Passive Cooling Figure 10.5j Because the air velocity increases rapidly with height above grade, the air has less static pressure at the roof than on the ground (the Bernoulli effect). Figure 10.5k The stack effect will exhaust hot air only if the indoor-temperature difference is greater than the outdoor- temperature difference between the vertical openings. (+) NEUTRAL AXIS (–) a. Figure 10.5l The central stair and geometry of this design allow effective vertical ventilation by the combined action of stratification, the stack effect, and both the Bernoulli and venturi effects. (+) NEUTRAL AXIS (–) b. (+) NEUTRAL AXIS (–) Figure 10.5m The stack effect causes negative pressure in the lower part of a space, positive pressure in the upper part, and zero pressure in between (top drawing). If this space were the atrium of a multistory building, the hot air would enter the upper floors (middle drawing). To avoid this (–) problem, the neutral axis must be raised by increasing the height of the atrium, using wind, and/or exhaust fans (bot- tom drawing). c.  10.6 Airflow through Buildingsâ ‡ â ‡ 297 the air above the building, thereby it is a rare site that has winds blow- creating a negative pressure, which ing mainly from one direction. Even pulls out indoor air (Figs. 10.5n). where there are strong prevailing direc- Thus, the stack effect is increased tions, it might not be possible to face without additional heating of the the building into the wind. building. In most climates, the need for summer shade and winter sun calls for a building orientation with the Maximize the vertical height between long axis in the east–west direction, openings to promote the stack effect! and Figure 10.6b shows the range of Figure 10.5n A solar chimney increases the wind directions that works well with stack effect without heating the indoors. that orientation. Even when winds are 10.6 AIRFLOW THROUGH east–west, the solar orientation usu- â …â ‡The advantage of the stack BUILDINGS ally has priority because winds can effect over the Bernoulli effect is be rerouted more easily than the sun that it does not depend on wind. The following factors determine the (Fig. 10.6c). The disadvantage is that it is a pattern of airflow through a build- very weak force and cannot move ing: pressure distribution around the Window Locations air quickly. It will, however, com- building; direction of air entering bine with the Bernoulli and ven- windows; size, location, and details Cross ventilation is very effective turi effects mentioned above to of windows; and interior partitioning because air is both pushed and pulled create particularly good vertical details. Each of these factors will be through the building by a positive ventilation on windy hot sum- considered in more detail. pressure on the windward side and a mer days. Figure 10.5l illustrates negative pressure on the leeward side how stratification, the stack effect, (Fig. 10.6d). Ventilation from win- Site Conditions dows on adjacent walls can be either the shape of the roof (the venturi effect), and the increased wind Adjacent buildings, walls, and veg- good or bad, depending on the pres- velocity at the roof (the Bernoulli etation on the site will greatly affect sure distribution, which varies with effect) can combine to ventilate a the airflow through a building. These wind direction (Fig. 10.6e). building naturally. Roof monitors site conditions will be discussed in Ventilation from windows on one and ventilators high on the roof Chapter 11. side of a building can vary from fair are especially helpful: because of to poor, depending on the location stratification, the hottest indoor of windows. Since the pressure is Window Orientation greater at the center of the windward air is exhausted first. and Wind Direction wall than at the edges, there is some â …â ‡The stack effect causes the lower part of a building with an Winds exert maximum pressure when pressure difference in the asymmetric atrium to have a negative pres- they are perpendicular to a surface, placement of windows, while there is sure and the upper part to have a and the pressure is reduced about 50 no pressure difference in the symmet- positive pressure. Somewhere in percent when the wind is at an oblique ric scheme (Fig. 10.6f). between will be the neutral axis angle of about 45°. However, the (Fig. 10.5m, top). Consequently, indoor ventilation is often better with Fin Walls hot air from the lower stories the oblique winds because they gener- enters the upper floors (Fig. 10.5m, ate greater wind motion indoors (Fig Fin walls can greatly increase the ven- middle). To avoid this problem, 10.6a). Consequently, a fairly large tilation through windows on the the neutral axis must be raised range of wind directions will work for same side of a building by changing above the top floor (Fig. 10.5m, most designs. This is fortunate because the pressure distribution (Fig. 10.6g). bottom). An interesting applica- tion of the stack effect is the solar chimney, which can pull hot air out of the building. Since the stack effect is a function of temperature differences, heating the indoor air would increase the airflow, but of course that would conflict with our goal of cooling the indoor air. Figure 10.6a Usually indoor ventilation is better from oblique winds than from head-on winds Therefore, the solar chimney heats because the oblique airstream covers more of the room. 298â ‡ â ‡ Passive Cooling Figure 10.6b Acceptable wind directions for the orienta- tion that is best for summer shade and winter sun. Figure 10.6e Ventilation from windows on adjacent sides can be poor or good, depending on wind direction. Figure 10.6f Some ventilation is possible in the asymmetric placement of windows because the relative pressure is greater at the center than at the sides of the windward wall. (After Art Bowen, 1981.) Figure 10.6c Deflecting walls and vegetation can be used to change air- flow direction so that the optimum solar orientation can be maintained. Figure 10.6d Cross ventilation between windows on oppo- Figure 10.6g Fin walls can significantly increase ventilation site walls is the ideal condition. through windows on the same wall. (After Art Bowen, 1981.)  10.6 Airflow through Buildingsâ ‡ â ‡ 299 Figure 10.6j A fin wall can be used to direct the airstream through the center of the room, as seen in this plan view. Figure 10.6h Poor ventilation results from fin walls placed on the same side of each window or when two fins are used on each window. Figure 10.6i The greater positive pressure on one side of the win- dow deflects the airstream in the wrong direction. Much of the room Figure 10.6k The solid horizontal overhang causes the air to deflect remains unventilated. upward. (After Art Bowen, 1981.) Note, however, that each window must Horizontal Overhangs ventilation because the air passes have only a single fin. Furthermore, fin and Airflow over people. walls will not work if they are placed A horizontal overhang just above the on the same side of each window window will cause the airstream to (Fig. 10.6h). Fin walls work best for Window Types deflect up to the ceiling because the winds at 45° to the window wall. solid overhang prevents the positive The type and design of windows have Casement windows can act as fin pressure above it from balancing the a great effect on both the quantity walls at no extra cost. positive pressure below the window and direction of airflow. Although The placement of windows on a (Fig. 10.6k). However, a louvered double-hung, single-hung, and slid- wall determines not only the quan- overhang or gap of 6 in. (15 cm) or ing windows do not change the direc- tity but also the initial direction of more in the overhang will allow the tion of the airstream, they do block the incoming air. The off-center place- positive pressure above it to affect the at least 50 percent of the airflow. ment of the window gives the air- direction of the airflow (Fig. 10.6l). Casement windows, on the other stream an initial deflection because Placement of the overhang higher on hand, allow almost full airflow, but the positive pressure is greater on one the wall can also direct the airstream they can deflect the airstream (Fig. side of the window (Fig. 10.6i). To down to the occupants (Fig. 10.6m). 10.6n). They also act as fin walls, as better ventilate the room, one should The design shown in Figure 10.6k is described above. deflect the airstream in the oppo- appropriate for night-flush cooling For the vertical deflection of the site direction. A fin wall can be used since it sends the air up to cool the airstream, use hopper, awning, or jal- to change the pressure balance and, structure, while the designs shown ousie windows (Fig. 10.6n). These thus, the direction of the airstream in Figures 10.6l, 10.6m, and 10.6o types also deflect the rain while (Fig. 10.6j). would be appropriate for comfort still admitting air, which is very 300â ‡ â ‡ Passive Cooling 6” MIN. block the sun and view. The large amount of crack and resultant infil- (+) tration makes these types of windows and louvers inappropriate in climates with cold winters. (+) Vertical Placement of Windows Figure 10.6l A louvered overhang or a gap in the overhang will permit the airstream to straighten out. The purpose of the airflow will deter- mine the vertical placement and height of windows. For comfort ven- tilation, the windows should be 12” MIN. (+) low, at the level of the people in the room. That places the windowsill between 1 and 2 ft (30 and 60 cm) above the floor for people seated or (+) reclining. Additional high windows or ceiling vents should be consid- Figure 10.6m A solid horizontal overhang placed high above the window will also straighten ered for exhausting the hot air that out the airstream. (After Art Bowen, 1981.) collects near the ceiling (Fig. 10.6o). High openings are also important for night-flush cooling where air must pass over the structure of the building. Traditional buildings in the South had windows that were almost the full height of the room. Thomas Jefferson’s home, Monticello, in Virginia, had triple-hung windows that went from the floor to the ceil- ing. By being triple-hung, the win- dows’ upper and lower sashes could Figure 10.6n All but double-hung and sliding windows have a strong effect on the direction of be opened for maximum ventilation. the airstream. Jefferson could also raise the lower two sashes to create a door to the porch. It is often advantageous to place windows high on a wall in very tall places where they are too high to reach for direct manual operation. Mechanical devices are readily avail- able for both manual and automatic operation. Some work with mechani- cal linkage (Fig. 10.6p) and others Figure 10.6o For comfort ventilation, openings should be at the level of the occupants. High with electric motors (Figs. 10.6q and openings vent the hot air collecting near the ceiling and are most useful for night-flush cooling. 10.6r). (After Art Bowen, 1981.) Inlet and Outlet Sizes and Locations important in hot and humid cli- keeps the rain out, then the slats of mates. Unfortunately, with this kind the jalousie windows can be set to be Generally, the inlet and outlet size of inclination, the windows deflect horizontal. should be about the same, since the the wind upward over people’s heads, Movable opaque louvers, fre- amount of ventilation is mainly a which is undesirable for comfort ven- quently used on shutters, are like jal- function of the smaller opening. tilation. However, if a large overhang ousie windows except that they also However, if one opening is smaller, it  10.6 Airflow through Buildingsâ ‡ â ‡ 301 Figure 10.6q Each motor opens a bank of awning windows by rotating a shaft that extends the whole length of the window wall. WINDOW RACK GEAR Figure 10.6p One type of mechanical linkage for operating high win- dows. (Courtesy of Clearline Inc.) Figure 10.6r A common way to open remote windows is with the rack-and-pinion mechanism. An electric motor turns a shaft with gears (pinions), which are meshed with a rack on each side of each window. Thus, the windows can be closed or opened by rotating the shaft clockwise or counterclockwise. The same motor and shaft can open and close windows the whole length of the building. should usually be the inlet, because However, because many locations with the lowest resistance for a head- that maximizes the velocity of the have no prevailing wind, most natu- on wind. To compensate for the effect indoor airstream, and it is the veloc- rally ventilated buildings must be of the screen, larger window open- ity that has the greatest effect on designed to function under many dif- ings are required. A screened-in porch comfort. Although velocities higher ferent wind directions. Thus, it is usu- is especially effective because of the than the wind can be achieved ally best to have inlets and outlets the very large screen area that it provides indoors by concentrating the airflow, same size. (Fig. 10.6t). the area served is, of course, decreased (Fig. 10.6s). The inlet opening not Insect Screens Roof Vents only determines the velocity, but also determines the airflow pattern in the Airflow is decreased about 50 per- Passive roof ventilators are typically room. The location of the outlet, on cent by an insect screen. The actual used to lower attic temperatures. the other hand, has little effect on the resistance is a function of the angle If, however, local winds are high air velocity and flow pattern. at which the wind strikes the screen, enough, and the ventilator is large 302â ‡ â ‡ Passive Cooling 100% 100% enough or high enough on the roof, these devices can also be used to 70% 130% ventilate habitable spaces. The com- mon wind turbine enhances ventila- Figure 10.6s Inlets and outlets should be the same size. If they cannot be the same size, the tion about 30 percent over an open inlet should be smaller to maximize the velocity. (After Art Bowen, 1981.) stack. Research has shown that other designs can enhance the airflow as much as 120 percent (Fig. 10.6u). Although the BRE office building uses the simpler open-stack ventila- tors, significant ventilation is achieved by the height of the opening (Fig. 10.6v). A very sophisticated type of ventilator called a cowl is used at the BedZED housing development Figure 10.6t The resistance to airflow by insect screens can be largely overcome by means of (Fig. 10.6w). A large vane aligns the larger openings or screened-in porches. cowl with the wind so that air is both pushed and pulled through the build- ing. The windward opening experi- ences a positive pressure, while the leeward opening experiences a nega- tive pressure. Complex ducting from the cowl sends the air throughout the building for ventilation. Although cupolas, monitors, and roof vents are often a part of tradi- tional architecture (see Figs. 10.2t and 10.2v), they can also be integrated Figure 10.6u The design of a roof ventilator has a great effect on its performance. The very successfully into modern archi- percentages shown indicate relative effectiveness. To maximize negative pressure, the deflector rotates so that the opening is always leeward. (After Shubert and Hahn, 1983.) tecture (Figs. 10.6v–10.6z). Some Figure 10.6v The Building Research Establishment (BRE) office build- ing uses open stacks with great height to maximize both the stack Figure 10.6w The Beddington Zero Energy Development (BedZED) effect and wind effect to ventilate the building naturally. Architect: in London, England, uses rotating ventilators to maximize the air Fielden Clegg. Location: Garston, England, 1991–1997. (Photo: Bruce exhausted from the building by the wind. Architects: Bill Dunster Haglund.) Architects. Engineers: ARUP. (Photo courtesy of ARUP.)  10.6 Airflow through Buildingsâ ‡ â ‡ 303 Figure 10.6x The Animal Foundation Dog Adoption Park uses monitors placed high and ori- ented to the local winds to maximize the natural ventilation. Location: Las Vegas, Nevada, 2005. Architect: Tate Snyder Kimsey. Figure 10.6y Monitors on the roofs of the bathing pavilions at Callaway Gardens, Georgia. Photovoltaics Skylights Fan CORRIDOR Fan CLASSROOM Radiant floor heating Figure 10.6z This extension to the Portland Community College uses a series of ventilation stacks as a passive cooling strategy. Ceiling fans are used for additional comfort. kind of shutter or trapdoor is required needed, and usually there is less wind and is discussed in Chapter 15. The to prevent unwanted ventilation, at night than during the day. Thus, second is to bring in outdoor air to especially in the winter. fans are often required to augment either cool people (comfort ventila- the wind. tion) or cool the building at night There are three quite different pur- (night-flush cooling). The third pur- Fans poses for fans. The first is to exhaust pose is to circulate indoor air at those In most climates, wind is not always hot, humid, and polluted air. This is times when the indoor air is cooler present in sufficient quantity when part of the heat-avoidance strategy than the outdoor air. 304â ‡ â ‡ Passive Cooling Separate fans are required for each purpose. Window or whole-house fans are used for comfort ventilation or night-flush cooling (Fig. 10.6aa). Ceiling or table fans are used when- ever the indoor air is cooler and/or less humid than the outdoor air. Because ceiling fans with flat blades are inefficient, use only fans Figure 10.6aa Whole-house or window fans are used to bring in outdoor air for either with airfoil blades, which move more comfort ventilation or night-flush cooling. Ceiling or table fans are mainly used when the air temperature and humidity are lower indoors than outdoors. air more effectively with less energy. Also, large, slow-moving ceiling fans are much more efficient than small, fast-moving ones. In winter the ceil- ing fans can be run in reverse to bring warm air down. By blowing cooler air up to the ceiling, the warm air collecting there is pushed sideways to the walls and then down to the floor. However, This antistratification Figure 10.6bb In regard to natural ventilation, single-loaded corridor plans (right) are far supe- method increases comfort and reduces rior to double-loaded plans (left), even if transoms are used. heat loss through the roof the winter use of ceiling fans will not work in very high or wide spaces. Partitions and Interior Planning Open plans are preferable because Venetian partitions increase the resistance blinds Section at window to airflow, thereby decreasing total ventilation. When partitions are used but are in one apartment or one tenant area, cross ventilation is often possible by leaving doors open between rooms. Cross ventilation Section at window is almost never possible, however, when a public double-loaded cor- ridor plan is used. Before air-condi- Door tioning became available, transoms (windows above doors) allowed for some cross ventilation, but with the loss of acoustic privacy and odor Section at door containment. An alternative to the double-loaded corridor is the open single-loaded cor- Figure 10.6cc By dropping the outdoor corridor several steps below the apartment floors, privacy ridor, since it permits full cross ven- is improved while maintaining cross ventilation. tilation (Fig. 10.6bb). One important drawback of single-loaded corridors is the lack of privacy, since half of the corridor by using clerestory windows apartment is a duplex with an open- windows in each apartment face the instead of transoms (Fig. 10.6ee). ing to the corridor as well as the oppo- public corridor (balcony). Two inge- Le Corbusier came up with an inge- site sides of the building (Fig. 10.6gg). nious modifications provide privacy nious solution for cross ventilation in The balconies have perforated para- in single-loaded designs (Fig. 10.6cc his Unité d’Habitation at Marseilles pets to further encourage ventilation, and 10.6dd). Single-story buildings (Fig. 10.6ff). The building has a corri- and they form a giant brise-soleil for can improve on the double-loaded dor on only every third floor, and each sun shading. Le Corbusier opened the  10.6 Airflow through Buildingsâ ‡ â ‡ 305 Shutter Windows with light obscuring glazing Figure 10.6dd A narrow but continuous slot above the outdoor corridor can allow cross ventilation while providing complete privacy. The corridor windows can contain light-transmitting but image-obscuring glazing. Figure 10.6ff The Unité d’Habitation at Marseilles, France, was designed by Le Corbusier to provide cross ventilation for each apartment. (Photograph by Alan Cook.) Figure 10.6ee In single-story buildings, a double-loaded corridor plan can use clerestory windows instead of transoms for cross ventilation. Figure 10.6gg Only every third floor has a corridor, and the apartments are all duplexes with exposures on each side of the building for cross ventilation. 306â ‡ â ‡ Passive Cooling area under the building to the wind by Also show the vertical move- enough ventilation, relocate win- resting the building on columns that ment in a section of the building dows, add fins, etc., to change the he called pilotis. In a hot climate, such (Fig. 10.7c). airflow pattern as necessary. an area becomes a cool, breezy place 6. Since spaces that are not crossed 7. Repeat steps 2 to 6 until a good in summer, but in a cold climate the by airflow lines might not receive airflow pattern has been achieved. same area becomes very unpleasant in the winter. The wind patterns around buildings will be discussed further in Chapter 11. 10.7 E  XAMPLE OF VENTILATION DESIGN Ventilation design is greatly aided by the use of airflow diagrams. These dia- grams are based on the general prin- ciples and rules mentioned above and not on precise calculations. They are largely the product of a trial-and-error process. The following steps are a guide Figure 10.7a The initial steps for drawing to making these airflow diagrams. an airflow diagram. Airflow Diagrams 1. Determine a common summer- wind direction from local weather data or from the wind roses given in Figure 5.6f. 2. On an overlay of the plan and site, draw a series of arrows paral- lel to the chosen wind direction on the upwind side of the build- ing and their continuation on the downwind side. (Fig. 10.7a). These arrows should be spaced about the width of the smallest window. 3. By inspection, determine the pos- itive- and negative-pressure areas around the building and record these on the overlay (Fig. 10.7a). 4. By means of a trial-and-error pro- cess, trace each windward arrow through or around the build- ing to meet its downwind coun- terpart. Lines should not cross, Figure 10.7b The completed airflow diagram has connected all windward and leeward arrows. end, or make sharp turns. Airflow through the building should go from positive- to negative- pressure areas (Fig. 10.7b). 5. When the airstream is forced to flow vertically to another floor plan, show the point where it leaves any plan by a circle with a dot and the return point by a Figure 10.7c Airflow should also be checked in section. (This technique is based on work by circle with a cross (Fig. 10.7b). Prof. Murray Milne, UCLA.)  10.8 Comfort Ventilationâ ‡ â ‡ 307 thermal comfort. This passive cooling technique is useful for certain periods of the day and year in most climates, but it is especially appropriate in hot and humid climates, where it is typical for the coincident air temperature and relative humidity to be outside the comfort zone. Often, air motion can move the comfort zone sufficiently to create thermal comfort (see Fig. 4.9b). Also see Figure 4.12 for the conditions under which comfort ventilation is appropriate. Comfort ventilation can rarely be completely passive because in most climates winds are not always suf- ficient to create the necessary indoor air velocities. Window or whole- house attic fans are usually needed to supplement the wind. See Table 10.8 for the effect on comfort due to various air velocities. For comfort ventilation, the airflow techniques mentioned above should be used to Figure 10.7d This water table at Chiang Mai University in Thailand allows effective ventilation studies by using streams of colored water to simulate airflow through a building. See Appendix maximize the airflow across the occu- G for more information about the water table. (Photo of and by Prof. Ruht Tantachamroon) pants of the building. If the climate is extremely humid, if little or no heating is required, and This technique is based on work by Prof. Murray Milne, UCLA. Table 10.8â … Air Velocities and Thermal Comfort Ventilation can also be designed and tested with a water table appara- Air Velocity Equivalent Temperature tus, as shown in Figure 10.7d. Water I-P SI Reduction* flow is used as an analogy for wind. A section of the building is modeled fpm mph m/s kph °F °C Effect on Comfort in slight three dimensions about ¾ in. 10 0.1 0.05 0.2 0 0 Stagnant air, slightly (2 cm) thick. Water is thus allowed to uncomfortable flow through the model. When narrow 40 0.5 0.2 0.8 2 1.1 Barely noticeable but streams of dye are added to the water comfortable supply, it quickly becomes apparent 50 0.6 0.25 1.0 2.4 1.3 Design velocity for air outlets to what extent water flows through that are near occupants the model, thereby indicating how the 80 1 0.4 1.6 3.5 1.9 Noticeable and comfortable wind will flow through the building. 160 2 0.8 3.2 5 2.8 Very noticeable but accept- See Appendix G for a full description able in certain high-activity of the water table and for detailed con- areas if air is warm struction drawings. 200 2.3 1.0 3.7 6 3.3 Upper limit for air-condi- tioned spaces Good air velocity for natural ventilation in hot and dry 10.8 COMFORT VENTILATION climates 400 4.5 2.0 7.2 7 3.9 Good air velocity for comfort Air passing over the skin creates a ventilation in hot and humid physiological cooling effect by evapo- climates rating moisture from the surface of the 900 10 4.5 16 9 5.0 Considered a gentle breeze skin. The term comfort ventilation when felt outdoors is used for this technique of using air *The values in this column are the number of degrees that the temperature would have to drop to create the motion across the skin to promote same cooling effect as the given air velocity. 308â ‡ â ‡ Passive Cooling if air-conditioning will not be used, that when the air temperature above roof overhangs to shade walls made lightweight construction is appropri- unshaded asphalt was 110°F (43°C), entirely of glass doors and windows ate. In such climates, any thermal it was only 90°F (32°C) over an that can be opened for ventilation mass will only store up the heat of the adjacent shaded lawn. The lower the (Fig. 10.8b). Since Chicago has very day to make the nights less comfort- incoming air temperature, the more hot and humid summers, plentiful able (Fig 10.8a). In the United States, effective comfort ventilation will be. ventilation and full shade were the only southern Florida and Hawaii For comfort ventilation, the oper- major cooling strategies before air- (climate region 16) fit in this cat- able window area should be at least conditioning became available. egory. In these climates, a moderate 20 percent of the floor area, with the Large overhangs are also needed amount of insulation is still required openings split about equally between to keep the rain out. Besides the dif- to keep the indoor surfaces from get- windward and leeward walls. The win- ficulty of closing windows just before ting too hot due to the action of the dows should also be well shaded on a rain, the relative humidity increases sun on the roof and walls. The insu- the exterior, as explained in Chapter 9. with rain; consequently, windows lation keeps the mean radiant tem- One of the examples presented there need to remain open during the rain. perature (MRT) from rising far above was Frank Lloyd Wright’s Robie Comfort ventilation is most effec- the indoor air temperature, since that House (Fig. 9.1i). It has very large tive when the indoor temperature would decrease thermal comfort. However, if the walls are fully shaded, less insulation will be needed. Insulation is also required when the building is air-conditioned. Thermal mass is helpful for buildings that are Figure 10.8a The mostly air-conditioned even in humid Mayan Indians of the climates. It allows the air-condition- hot and humid Yucatan Peninsula build ing to be turned off during times of lightweight, porous peak electrical demand, because the buildings for maximum mass will prevent quick temperature comfort. Note that changes. The building can also be pre- although mud and rocks are available, cooled during the night. experience has led the Some control is also possible Mayans to the most over the temperature of the incom- comfortable construc- ing air. For example, tests have shown tion method. Figure 10.8b Frank Lloyd Wright’s Robie House (1909) in Chicago has whole walls of doors and windows that open for cross ventilation. It also has large roof overhangs to keep out the sun and rain.  10.9 Night-Flush Coolingâ ‡ â ‡ 309 and humidity are above the outdoor passive technique is called night-flush typical diurnal temperature range of level. This is often the case because of cooling. a climate. internal heat sources and the heating This cooling strategy works best Night-flush cooling works in two effect of the sun. Thus, in such cases in hot and dry climates because of stages. At night, natural ventilation or comfort ventilation not only brings the large diurnal (daily) tempera- fans bring cool outdoor air into the in cooler air but also produces ther- ture ranges found there—above building to make contact with and mal comfort from the resultant air 30°F (17°C). A large temperature cool the indoor mass (Fig. 10.9b). motion. However, when it is much range implies cool nighttime tem- The next morning, the windows are hotter and/or more humid outdoors peratures of about 70°F (21°C) even closed to prevent heating the build- than indoors, the windows should though daytime temperatures are ing with hot outdoor air (Fig. 10.9c). be closed to avoid excessive heating quite high—about 100°F (38°C). The mass now acts as a heat sink and or humidification of the building. However, good results are also pos- thus keeps the indoor air temperature Ceiling or table fans can then be used sible in somewhat humid climates, from rising as fast and far as it would to move air across people. which have only modest diurnal otherwise. However, when the indoor temperature ranges—about 20°F air temperature has risen above the Rules for Comfort Ventilation in Hot