No. 36, Fall/Winter 1994
Desert Architecture III: Building a Sustainable Future
by Yair Etzion
Desert architecture may be characterized as "architecture of the extremes," being basically similar to "regular" architecture but differentiated from it by its obligation to address needs and problems of an extreme character.
The problem of thermal comfort in buildings is perceived as one of the more characteristic and difficult problems that desert architecture must address, even though this is not the only problem nor necessarily the most difficult one. A typical way of addressing the thermal comfort issue in buildings is by intensive use of expendable energies, but this, of course, is not an ideal approach: it leads to waste of energy, it is expensive, and not everyone is comfortable with the thermal conditions it creates (witness the number of people who do not like air-conditioning).
Various characteristics of design and construction enable the improvement of thermal comfort to be integrated into a building without the use of artificial means and expendable energy. Now, when it seems that even the drowsy Negev (the southern half of the Israeli land area, which houses only about 7 percent of the country's population) is awakening to a building surge, it is desirable to clarify these methods, and even to try to apply them in new building projects. What's more, as an ever increasing worldwide need for housing construction is evident, much of it in hot arid lands, the "right" type of building technology should be used to improve standards of living and decrease the use of purchased energies.
This article demonstrates a number of climatic and energy characteristics of building in the desert. It takes as its subject an examination of a recently completed house in the new Neve-Zin neighborhood of the Sede-Boker Campus of Ben-Gurion University of the Negev, the first real "solar" or "bio-climatic" neighborhood in Israel. The house chosen is the Etzion House, which was designed by and built for the author of this article. It was chosen for examination here because of the author's first-hand familiarity with its design considerations and characteristics.(Back to top)
It is necessary, first of all, to introduce the Negev and its environmental conditions, and also to correct several misconceptions and false "truths" concerning the climate in most of its regions. The relatively high Negev regions (300 m and above) are not areas as hot, for example, as the Arava or the Beit-Shean Valley (both along the Jordan Rift). The higher regions of the Negev may be characterized as having cold, uncomfortable winters and summers that are hot during the day but usually pleasant at night. The Sede-Boker Campus is located at 30.8' latitude north, 500 m above sea level. Average annual rainfall is 80 mm, but there is a considerable deviation from year to year.
The climate is considered hot and dry during the summer, with an average maximum temperature of 32 degrees C and an average daytime temperature of 24 degrees C. Solar radiation is very strong, and may reach 7.7 Kwh/sq m x day on a horizontal surface (during June and July). In the summer, ambient relative humidity is very low, between 20 percent and 40 percent during most of the day, but it rises considerably during the night, when the ambient temperature drops sharply, to reach 90 percent. Summer daily temperature fluctuation is about 18 degrees C, and the average temperature is within the range of thermal comfort.
Winter is cold, sometimes rainy, and uncomfortable. The average temperature in January is 10 degrees C and the average minimum daily temperature is 3 degrees C. The temperature at night often drops below freezing (0 degrees C). The intensity of solar radiation during the winter is relatively high and reaches 3.3 Kwh/sq m x day on a horizontal surface, and about 4.6 Kwh/sq m x day on a south-facing vertical surface. These conditions make Sede-Boker an almost ideal location for buildings that achieve thermal comfort in the winter by employing solar energy. Thermal comfort generated by the sun adds to the quality of life and is definitely economical.(Back to top)
The concept guiding building design in this climate is the creation of an envelope, sealed as far as possible against the passage of energy. In this envelope should be openings, allowing desirable (but controlled, both in time and in quantity) passage of natural energy from the house outwards and vice versa. The house should be massive with a relatively high thermal capacity.
The first "truth" that should be refuted regarding building design is that the directional orientation of the building is the key to achieving thermal comfort. A house built according to the concept guiding the design of the Etzion House is largely insensitive to its orientation. If the envelope is really well insulated and has a significant thermal capacity, there is not much difference in the thermal performance of a house facing south compared to houses facing other directions, because the envelope is, practically speaking, almost sealed to the passage of energy. Orienting the openings, though, is very important, as will be explained further on. Thus the popular opinion that the bio-climatic house must have a long southern exposure is not necessarily correct: a long south-facing elevation is needed only for positioning the south-facing windows, and it should be large enough to enable just that. It is possible, therefore, to say that solar or bio-climatic architecture does not have to be monotonous and boring, as many of its opponents claim.
Sealing the envelope against uncontrolled passage of energy should reduce to a minimum the possible overheating of the house in the summer and its cooling in the winter. In order to "manage" the energy economy of the house, no energy should be allowed to pass through the walls, as far as is technically and economically possible. Desirable transfer of energy, allowing for heating the house by solar energy in the winter and cooling it in the summer by nighttime ventilation, should be done only through and by the openings.
The building's significant thermal capacity should contribute to stabilizing the large daily temperature fluctuations typical of the desert and also should increase the building's thermal lag time, which is the time that passes, for instance, between the peak external temperature and the peak internal temperature. This defines the difference between a "light" building (such as a mobile home) and a "heavy" building (such as the Etzion House).
The external walls of the Etzion House were built according to the guidelines that follow. The internal layer of the wall is built of solid silicate blocks, which constitute an integral part of the storage mass of the building. On the external side of the wall an insulating layer is attached, consisting of 5-cm-thick polystyrene board. The insulating layer was glued to the blocks using a special acrylic mortar. The wall was plastered over with one layer of acrylic plaster, into which a reinforcing polyester mesh, capable of resisting an alkaline environment, was embedded before hardening. A second layer of the same acrylic plaster was then applied over the first one. In order to prevent heating of the wall as a result of exposure to solar radiation, and in order to prevent the desert sand and dust from adhering to the surface (which turns almost every building in the Negev to desert colors), the external wall surfaces were kept white and smooth.
The expected energy performance of the wall used in the Etzion House exceeds the requirements of the standard established by the Israel Standards Institute (Standard 1045). The thermal resistance (R) of the wall is 2.0 sq m(degrees C)/watt. Its thermal storage capability is 60 watts(hrs.)/sq m(degrees C), and its damping coefficient is 0.21. The thermal time constant of this wall is about 160 hours.
It is very important to note that the order of the layers of the wall is of supreme importance. If the order of the layers was different (in other words, if the thermal insulation was placed on the inside of the wall) only the thermal resistance of the wall (R) would remain constant (because it consists of an algebraic sum of the thermal resistances of all the layers). The wall's storage capacity would be reduced almost to zero because the insulating layer would cut off the storage mass from the internal space of the building and the thermal time constant of the wall would be reduced to approximately one hour. In the Etzion House, in order to increase the thermal storage capacity of the building, even the internal walls were built of solid silicate blocks.
The roof section chosen for the Etzion House is an "inverted roof" with a large dose of thermal insulation. Owing to considerations similar to those described for the walls, insulation was placed on the external side of the roof. The inverted roof section was chosen, in spite of the problems sometimes associated with installation of various devices on it and its maintenance, because of its ability to protect the sealing layer of the roof. Most sealing materials suffer in the Negev from two phenomena that significantly shorten their life spans: exposure to solar radiation, which dries them out and makes them brittle, and extreme fluctuations of their surface temperature between daytime and nighttime (up to 60 degrees C), which causes the layer to "work" and deteriorate. Placing the insulation on top of the sealing layer protects the latter from solar radiation and also reduces the temperature fluctuations to which it is exposed.
The roof structure of the Etzion House is made of structural concrete, which has a significant thermal capacity. Above it is a layer of light-weight concrete (drainage), a sealing layer, and a 10-cm-thick polystyrene board, the underside of which is perforated to permit water runoff under it. On top of the insulating layer was placed a layer of Polya gravel to a depth of 5 cm; the gravel's job is simply to keep the insulation in place. This roof has a very high R value of 3.6 sq m(degrees C)/watt, mainly to protect from solar radiation in the summer, but also to keep the heat inside during the winter; a dumping coefficient of 0.3, and a time constant of almost 300 hours.(Back to top)
The openings of a building can be a joy or a burden, depending on their design and location. The openings can either supply a significant proportion (sometimes almost all) of the heating required in the winter, or they may be a "hole" in the sealed envelope through which there is an uncontrolled passage of energy. In the summer the openings can be, on one hand, well protected, allowing for very little increase in the internal heat level, or they can be a source of significant and harmful heating; it all depends on their design, placement, and quality of performance in the building.
The design of the windows should maximize solar gain in the winter and minimize such gain in the summer. Window design also should minimize the penetration of hot air into the building during the hot hours of the summer and of cold air during the winter, while allowing massive ventilation on cool summer nights. The building's significant thermal capacity should contribute to stabilizing the large daily temperature fluctuations typical of the desert, and also should increase the building's thermal lag time. From the aspect of solar radiation, the most important factor is the orientation of the windows. The large differences in levels of radiation between winter and summer in Sede-Boker is mainly a result of the different altitude angles of the sun in each season, which causes different angles of incidence on the planes, and the different number of hours during which the wall and the roof are exposed to the sun. The different solar radiation intensities from each direction show that the most preferred direction for openings is south: in winter (January), a window in a southern wall can supply about 4 Kwh/sq m x day of heating energy (which is about 90 percent of overall radiation falling on the window surface, the transmittance coefficient of traditional commercial glass being about 0.9). The northern orientation is defined as a "loser" in the winter: in most cases in Israel, more energy is lost through a northern window than can be gained through that window from the sun (in this case, only reflected and bounced radiation). Therefore, because of winter considerations, it is desirable to reduce north-facing windows to a minimum.
For thermal comfort in the interior space, it is recommended to double-glaze northern windows (to raise their surface temperature and to reduce radiation from human bodies to them), even though these windows may not have an unequivocal economic advantage over traditional windows.
The windows facing east and west present the most difficulties. In the winter they usually will be in a kind of balance between loss and gain, and so their benefit is, at most, doubtful. In the summer (June), however, owing to the 5 Kwh/sq m x day these windows are exposed to, they are a considerable source of massive and damaging heating. The logical conclusion is that these windows should be avoided altogether, unless there is a special reason for their placement. It should be noted that in the summer there is more radiation each day per unit of area on the western and eastern walls than there is on the southern wall in the winter, and that with the radiation that falls on the southern wall during the winter it is possible to heat the house when the temperatures outside are significantly colder! The Etzion House has no windows facing west, but a few windows face east, admitting a breathtaking view of the Zin Canyon Cliffs. Blocking off this view would be a totally unpardonable "sin." Besides, one does not build a house only to save energy.
The size of the southern windows was established by the ratio of solar radiation to load (Solar Load Ratio, or SLR). In heavy and well insulated structures like the Etzion House, it is possible to calculate with a high degree of accuracy the necessary size of the windows based on average monthly external temperatures in the cold months. It is possible to calculate the expected heating load of the house, to decide which portion should be supplied by the sun (a primarily economic decision), and to design the size of the windows accordingly. An examination of a sample of the solar houses now being built in Neve-Zin shows that the area of the southern windows usually represents 15 to 20 percent of the floor area of the house. In the Etzion House there also are a number of northern rooms that benefit from the southern winter sun, which is admitted through a few clerestory windows.
Protecting the interior of the house from heating by solar radiation incident on the windows during the summer (assuming that the rest of the envelope is sealed against energy penetration) is very important for achieving thermal comfort in the house. Here, too, a number of well-known "truths" must be refuted. In the vicinity of the Neve-Zin neighborhood, it appears that in the winter a large proportion of the radiation hitting the envelope is direct. In the summer, however, the direct radiation becomes a relatively small proportion of the overall radiation hitting the structure's surface (20 percent); most of the radiation is diffused from the atmosphere or reflected from the light-colored ground surface (reflection coefficient of approximately 0.3).
The conclusion that must be drawn from this is that special overhangs and permanent shading elements over glazed openings are of very limited benefit under these conditions, and that it is not desirable to count on them, especially in the Negev. The solution is exterior blinds or other movable shading mechanisms that can absolutely, or almost absolutely, prevent solar radiation from hitting the windows. It is superfluous to point out that internal blinds, such as venetian blinds, are useless in this case, as they block only radiation that has already penetrated into the building. Since the radiation heating the structure during the summer is diffused and directionless, shutters are required in all window orientations.
The Etzion House has a number of small northern windows (area of each is about 0.65 sq m) in each of the rooms on the second floor. In the main spaces of the lower floor there are relatively large openings, double-glazed, which also point northwards. These openings are used for cooling the house in the summer and were installed in spite of their negative performance in the winter. In the winter these windows will remain closed all day. In the summer, they will be closed during the daytime, their blinds lowered, but will be wide open in the late afternoon hours, when the outside temperature already will be below that desired inside the house; they will remain open all night. Through these windows a large volume of cool desert air should flow, which will "flood" the house during the night and remove from it both the small amount of heat that managed to penetrate the "sealed" envelope and the heat that was generated in it during the day by equipment and people.
Because it is desirable to reduce the size of the windows on the one hand (winter and summer days), but on the other hand it is desirable to have large openings for ventilation (summer nights), a small window was designed whose blind opens, with the help of a special mechanism, only to be perpendicular to the wall. This blind serves as a kind of a reflection shelf, creating some higher air pressure near the window and thus increasing the quantity and velocity of air entering through the window by about 50 to 60 percent. Assisting this mechanism is the fact that the governing breezes, mainly in the afternoon hours of the summer, always come from the northwest at about a 45-degree angle to the north facade.
The house was designed to achieve, in winters, internal temperatures in the range of 18 to 20 degrees C, with the sun providing more than 90 percent of the necessary heating; summer internal temperatures average in the range of 23 to 26 degrees C. In the summer, these expected temperatures, combined with the low daytime relative humidity (20 to 50 percent during the hot hours), promise thermal comfort within the house.
It was planned to achieve these temperatures without artificial air conditioning, cooling, or heating, except for some backup, which may seldom be required throughout the year. Considering the fact that the average energy cost for heating a 70 to 100 sq m house at the Sede-Boker Campus in an average winter is more than $500, the monetary savings obtained in keeping a large house like this one thermally comfortable by using solar energy for heating and by "proper" design are considerable.
It should be pointed out that expected winter temperatures are "real," sensible temperatures, reflecting both the air temperature and surface temperatures inside the house. In contrast, in houses where heating is based on convective heating of the air within and/or radiative heating, particularly in cases where the heating is intermittent, the surface temperatures of the walls, floors, and roofs usually will be lower than that of the air, and the effective sensible temperature inside the house will also be lower.(Back to top)
Another "truth" that has been proven incorrect in the Negev Highlands is that summer days are very hot and that it is difficult to protect oneself from the heat during these hours. Observations indicate that because of the large amplitude of daily temperatures in this season, good thermal conditions actually exist outdoors during most of the day. A slightly simplistic, though quite accurate, calculation shows that assuming that the daily temperature amplitude is about 18 degrees C, average hourly change of temperature is about 1.5 degrees C. If the average daytime maximum, measured at about 3 p.m., in the hottest summer months is about 32 degrees C, then by 6 p.m. the temperature will already be about 28 degrees C and will remain below this value until about noon of the following day. The combination of 28 degrees C with 30 to 40 percent humidity is a very comfortable one, and residents of many parts of the country would welcome it. In fact, the beginning of the thermally comfortable period is usually even earlier, because, except for "Hamsin" days (characterized by very hot and dry easterly winds), the Negev is blessed with a daily northwesterly and relatively cool breeze, which starts at around 4 p.m. and continues until 10 or 11 p.m.
It is natural, then, that spending time outside, in the shade, during the hours after work, even during the hottest summer months, is comfortable and should be taken advantage of. For this reason, the Etzion House is shaped like a three-sided, U-shaped box, built around a small courtyard whose open end faces north.
All the important spaces of the house open into this courtyard: the family room, the parents' bedroom, and the kitchen. The courtyard's orientation is to the north so that it will catch the comfortable summer breezes. Facing north, the courtyard is also almost totally shaded from the sun; part of it is shaded by the second floor volume, which manages, even in June, when the sun is at its highest, to keep most of the courtyard shaded during most of the day. The shading of the courtyard reduces significantly the effective temperature in it by neutralizing the element of radiation and also by maintaining the external wall surfaces surrounding the courtyard at a relatively low temperature. The result is a courtyard, very useful during most of the year, where a large portion of the inhabitant's activities can take place.
If the courtyard were to face south, most of the positive features of the northern courtyard would be lost. It is important, therefore, to differentiate between a southern exposure of glazed openings and the location of attached external spaces, whose recommended location is on the north side of the building.(Back to top)
The performance of the house is evident in the data collected during the summer and winter of 1993-94. For technical reasons, the monitoring period has not been extended for weeks at a time, but a few spot measurements of a few days each were taken during both summer and winter. More intensive monitoring of the house will be undertaken in the near future, and those results will enable very intensive and detailed examination of various aspects of the building's performance. For the time being, however, the following results give a general idea of the house's performance.
Measurements were taken at various points in the house; those presented here show the temperature pattern in the living room. Measurements were taken at three different heights above floor level: 0.5 m, 1.6 m (head height), and 4.5 m. The results displayed in the accompanying charts are the daily averages of the three heights.
During three typical days during the summer (August 8-19) the average temperature inside the house was 24.9 degrees C, with a standard deviation of 1.61 degrees C, while temperatures outside averaged 30.8 degrees C, with a standard deviation of 8.6 degrees C. The temperature inside was, on average, cooler by about 6 degrees C than was the temperature outside, and its stability was greater: its standard deviation was less than one-fifth of the standard deviation of the temperature outside. It can also be added here that the relatively large internal amplitude stems from the intentional nighttime ventilation.
Winter measurements were taken during January 10-19, 1994, and show that the average temperature inside the house was 19.09 degrees C , with a standard deviation of 1.91 degrees C. Outside at the same time, the average was 11.11 degrees C, with a standard deviation of 2.88 degrees C.
It must be emphasized that these internal temperatures were obtained with no added heating at all, meaning that all the heating energy needed was obtained from the sun. The winter of 1993-94 recorded temperatures somewhat higher than the typical Sede-Boker winter (about 1 degree C above normal) and enabled 100-percent reliance on solar energy. It is estimated that in a typical winter the solar saving fraction will be slightly smaller.
More than anything else, the Etzion House has proved to be a pleasant and comfortable house to live in. It takes some of the biggest liabilities of the desert and uses them in a manner that turns those liabilities into assets. This, from a certain point of view, might be regarded as the essence of bio-climatic architecture.
About the Arid Lands Newsletter