One essential role of a building in providing thermal comfort is controlling the flow of thermal radiation to and from the human body. Sometimes a building must protect the body from an excessive influx of radiant heat from the sun or from sun-warmed objects. At other times, protection is needed to prevent an excessive radiation of heat from the body to a cold outdoor environment. Exchanges of radiation also take place continually between the body and the surrounding interior surfaces of the building, requiring that the temperatures of these surfaces be regulated sufficiently to ensure comfort. In some situations, the active manipulation of radiation is appropriate as the major mechanism in a scheme for achieving thermal comfort for the occupants of a building.
Radiational Intensity vs. Distance from Radiant Source
We have already noted that thermal radiation consists of electromagnetic energy in the infrared range of wavelengths and that it behaves very much like visible light. A point source, such as the incandescent filament of a lightbulb, radiates heat to a nearby object at a rate that is inversely proportional to the square of the distance between the filament and the object (9.1). (The sun operates by this inverse-square law in heating the earth.) It is possible to receive a very high influx of infrared by placing one's hand close to the bulb, whereas if the hand is withdrawn to a point several feet away, the heating effect is scarcely noticeable.
If the same bulb is placed in front of a parabolic reflector, most of the diverging rays of the filament can be focused into a parallel beam of moderate radiant intensity (9.2). Within this beam, the rate of radiant flow is constant without respect to distance, except for the energy absorbed by dust particles in the air and by the air itself, and for losses caused by inaccurate focusing of the beam.
A surface of infinite extent also radiates to a point object at an intensity that is independent of distance. A bird flying 500 feet (150 m) above the middle of the Sahara Desert receives just as intense radiation from the hot sands as a traveler on foot does.
Usually we are surrounded by objects of differing surface temperatures. Suppose that you are sitting in a high-backed, upholstered chair, facing a cozy fire to your right and a large window that frames a view of a frozen lake to your left (9.3). The chair serves as a barrier to radiation, so that no significant quantity of radiation is being given or received through the back surfaces of your body. The flames and glowing coals of the fire are high-temperature objects, larger than point sources but smaller than infinite surfaces, which radiate heat rapidly to your body. The window is a large, cold object to which your body radiates heat. Above is a large ceiling that is somewhat cooler than your body that causes another net outflow of your body heat. The floor and walls exert radiant cooling effects similar to that of the ceiling. In such a situation, it is not immediately apparent whether the sum of these radiant flows on your body is, overall, a net gain or loss of heat.
In order to assess the net overall radiant flow, we measure the temperature of each surface (the fire, window, ceiling, walls, and floor) and the solid spherical angle subtended by each surface with respect to your body. The mean radiant temperature (MRT) to which you are exposed is an average of the temperatures of each of the surrounding surfaces, weighted according to the spherical angle subtended by each and the thermal emissivity of each. In our example, perhaps 40 percent of your body is shielded by the chair. Your body "sees" the intensely hot fire as about 3 percent of its spherical field of thermal "vision" (9.4). The cold window surface might constitute 15 percent, and the ceiling, walls, and floor the remainder. The fire is the only source of net radiant heat flow to your body in this example. Because it subtends an angle of only 3 percent of a sphere with respect to your body, it must be very hot if it is to overcome the cooling effect of the cooler surfaces that occupy 57 percent of your body's field of thermal "vision."
MRT is not in itself a sufficient measure of the thermal comfort that may be expected in response to the radiant conditions in an environment. Everyone is familiar with the experience of being
roasted on one side by a bonfire while being frozen on the other by radiation to a cold night sky. The MRT may be appropriate for comfort in this case, but the distribution of heated surfaces around the body is not (9.5). To prevent excessively rapid gains or losses from any one area of the body, there must be a balance among the temperatures of the surfaces to which the body is exposed. A balance is also required among heat gains and losses by convection, conduction, and evaporation. In a building with uninsulated walls or very large windows, regardless of the temperature to which the air is heated and in spite of a warm fire in the fireplace, one may be uncomfortable in cold weather because of the excessive radiant flow from the body to the large, cold surfaces of the building. Similarly, the frigid air from an air conditioner will not be sufficient to produce comfort in hot weather if the body is exposed to direct sunlight coming through a window or skylight. If the indoor MRT is high in winter, air temperatures can be somewhat lower, thus reducing heat losses through the enclosure of the building and saving heating fuel. In summer, a building of high thermal capacity is likely to have cool interior surface temperatures, allowing comfort at higher air temperatures, with attendant savings in cooling costs.
Manipulating Radiant Temperature
If we wish to raise the mean radiant temperature in a particular interior location, we can pursue any or all of the following strategies:
1. We can allow the sun to penetrate to that location. The sun is a fickle heating device, but a pleasant and economical one. We use it almost instinctively, as does a dog or cat who dozes happily in a shaft of sunlight in the winter.
2. We can help the building's heating system to warm the interior building surfaces to a higher temperature by installing better thermal insulation in the walls and ceilings and multiple layers of glass and/or insulating curtains or shutters in the windows. Good thermal insulation of a building pays off in increased radiant comfort, as it does in every other respect.
3. We can use highly reflective surfaces to reflect body heat back to the body. (This is how a reflective-foil emergency blanket works.) This strategy has almost never been used in buildings, but it has considerable potential for use if heating fuel becomes more scarce, especially if the reflective surfaces can be incorporated into furniture. This is an intriguing area for architects and furniture designers to explore.
4. We can heat very large surfaces, such as floors or ceilings, to temperatures a few degrees above the skin temperature of the body.
5. We can heat small surfaces, such as electric filaments, gas-heated ceramic tiles, metal stoves, or fireplaces, to temperatures hundreds of degrees above the skin temperature of the body.
These last two strategies merit further explanation. Schemes for heating floors or ceilings are fairly commonplace. They usually employ electric resistance wires, warm air circulating through multiple ducts, or warm water circulating through coils of plastic tubing. (Heating of walls is not usually attempted, because we tend to drive nails and screws into walls for hanging pictures and shelves.) Formerly, floors for radiant heating had to consist of copper pipes embedded in concrete slabs. More recently, systems based on plastic tubing have made it possible to turn even wooden floors into radiant heat sources. Floor systems are attractive because they warm the feet by conduction and set up convection currents that heat the air in the room quite evenly. But they have several limitations: Tables and desks cast infrared shadows that hamper the ability of the warm floor to heat hands and arms (9.6). The efficiency of such systems is reduced by rugs and carpets. Because of the considerable thermal capacity of the floor materials, they are incapable of reacting quickly to small or sudden changes in the demand for heat inside a building.
Ceiling systems have their own problems. The air warmed by a warm ceiling tends to remain at the ceiling, leading to lower overall efficiencies and a stratum of cool air at floor level, a defect made worse by the infrared shadows that tables and desks cast on people's legs and feet (9.7).
Small, high-temperature infrared heat sources with focusing reflectors are highly effective if installed so as not to cast shadows. They are especially useful where high air temperatures cannot be maintained, as in large industrial buildings or even outdoors, because they can produce heat instantaneously when it is needed and beam it precisely to where it is needed (9.8). Open fires and stoves are less efficient radiant heat sources because of their omnidirectional, inverse-square radiation and because of the relatively large amount of fuel that they convert to warm air rather than radiant energy.
Because of its similarity to solar radiation, infrared heat from either low- or high-temperature sources feels very pleasant on the bare skin. Swimming pools, shower rooms, and bathrooms are particularly appropriate locations for radiant heating systems.
Our means for lowering the mean radiant temperature of an interior location are somewhat more restricted than our means for raising it. We cannot, for example, cool our bodies by exposing them to a small surface at a very low temperature. Whereas we can easily heat an electric filament or gas flame to a temperature a thousand or more degrees above body temperature, there are only a few hundred degrees with which to work between body temperature and absolute zero, and devices for producing very low temperatures are expensive to build and operate. Furthermore, a very cold surface quickly frosts over with moisture that condenses from the air, thereby losing most of its effectiveness because of the insulating properties of the frost. Even a moderately cold surface becomes moist and unpleasant in warm summer weather, which is why we do not actively cool floors and ceilings except in rare cases when we can control the humidity at a level low enough to eliminate condensation. What we can do instead is to shade roofs, walls, and windows against the summer sun, surface them on the exterior with highly reflective coatings whenever practical, insulate them well, and provide enough thermal capacity in the interior surfaces of the building to ensure the maintenance of cool temperatures throughout the day. We can also, in some cases, open a building to the night sky, allowing our bodies and the warm surfaces of the building to radiate heat into space.
To implement many of these strategies for manipulating mean radiant temperatures upward or downward, we must utilize the building's enclosure and take into account its interactions with outside thermal forces. Among these, the most important are the inflow of solar and terrestrial radiation and the outflow of radiation at night.
The roof of a building is a barrier against excessive summertime solar radiation, especially in the tropical latitudes where the sun passes directly overhead. The roof surface itself is of extreme importance. A surface that is highly reflective of solar infrared radiation is heated very little by the sun, whereas an infrared absorptive roof surface can reach extremely high temperatures. The transmission of solar heat from the roof to the ceilings inside the building can be reduced by the use of thermal resistance, thermal capacity, and/or ventilation of spaces within the roof structure (9.9). Under inclined roofs, the convective draft of the heated air can provide the ventilating force to remove the heat, if the roof is appropriately designed (9.10).
In tropical latitudes, the sun passes so high overhead that north and south walls of buildings receive relatively little solar radiation, but east and west walls need protection of the same general sort as that for roofs described in the previous paragraph (9.11). In more
northerly locations, the roof and the east and west walls require protection against the summer sun, and also the south wall. For low buildings, deciduous shade trees and vines are a preferred method of sun control, especially because they shed their leaves and allow most sunlight to penetrate during the cold months. A roof overhang or horizontal shading devices at each floor can block the high summertime sun from south-facing walls but still admit light and heat from the low winter sun (9.12, 9.13).
In cold weather, of course, such solar radiation as can be absorbed by roofs and walls is usually a welcome addition to the heat content of the building. Some designers have gone so far as not to insulate the south-facing sides of buildings. This is a misguided effort, for cloudy weather and short hours of daylight bring relatively few hours of sunshine in winter, with the result that for about three-quarters of the winter hours in a typical northern climate, a south-facing surface is losing more heat than it gains. A more productive strategy is to insulate south-facing wall surfaces well and to utilize large south-facing windows to trap solar heat, closing them off with insulating shutters or heavy curtains or shades (preferably reflective of long-wave infrared) when the sun is not out. The well-insulated wall surfaces do not gain appreciable heat when the sun is out, but their heat losses are reduced or eliminated by the solar heating of their exterior layers.
The same highly reflective exterior surfaces that are useful in reducing solar heat gain in the summer can also help reduce building heat losses in winter because of their low rate of emission of radiant heat. Polished metals are the only common building materials that are inefficient emitters of low-temperature infrared. White paint, excellent as a barrier against solar radiation, does little to slow the emission of building heat in cold weather.
In warm weather, sunlight entering through windows can produce an undesirable heating effect on the interior of the building by warming interior surfaces that in turn warm the air of the room and radiate heat to its occupants. Where possible, it is best to intercept the sunlight outside the glass, with trees, vines, roof overhangs, louvers, or awnings (9.14). In this way the heat absorbed by the shading device is reradiated and convected largely to the outdoor environment rather than the interior. Outdoor shading devices are difficult to adjust from inside the building, however, and they are vulnerable to the destructive forces of Nature. Some architects, moreover, are unwilling to accept the appearance of exterior shading devices on their buildings. For these reasons, interior shading devices such as roller shades, Venetian blinds, and curtains are often used instead. The effect of these interior devices is largely to absorb solar radiation and convert it to convected heat in the interior air while shading occupants and furnishings inside the building. Interior shading devices are therefore relatively ineffective in reducing the solar heating of air inside a building. They are, however, effective in protecting individuals from high radiant heat gain from the sun and in reducing visual glare from direct sunlight.
Ordinary window glass transmits more than 80 percent of solar infrared radiation but absorbs the majority of the longer-wave infrared from sun-warmed interior surfaces. In cold weather the glass loses most of this absorbed heat by convection to the outside air. The glass does serve, however, to prevent the passage of heated air from the sun-warmed interior back to the outdoors, and it is primarily for this reason that greenhouses, parked automobiles, and flat-plate solar collectors are heated so dramatically by the sun.
Heat-absorbing and heat-reflecting glasses are frequently used in buildings to reduce summer air-conditioning loads. However, the heat-absorbing glasses, usually gray or brownish in color, are not as effective as one might first assume from published solar-absorption figures. A glass that absorbs 60 percent of solar heat, for example, does not transmit only 40 percent to the interior. The 60 percent that is absorbed must ultimately go somewhere, and roughly half is radiated and convected to the interior of the building and half to the outdoors, giving a net overall reduction in solar heat gain of only about 30 percent (9.15). Such a reduction may be perfectly sufficient, of course, for many applications. Of much higher efficiency are the heat-reflecting glasses, which bounce back most of the sun's heat without absorbing it. But a large wall of reflective glass can reflect enough sunlight to overheat adjacent buildings and outdoor areas quite seriously and to cause severe visual glare problems in neighboring streets and open spaces. A designer using reflective glass must exercise restraint and good judgment to avoid such problems.
If sufficient care is given to window orientations, much of the expenditure of materials, equipment, and fuel energy to deal with problems of solar heat gain through windows can be avoided. In temperate, northern hemisphere locations, north-facing windows lose radiated heat in all seasons of the year, especially in the winter. East-facing windows gain heat very rapidly in summer because the sun shines into them at a very direct angle, but this gain occurs only in the mornings, when some influx of heat is often welcome after a cool night. South-facing windows receive solar heat for most of the day in summer, but at a low intensity, because the high sun strikes the glass at a very acute angle. In winter, the sun arrives at a low angle through south-facing windows for the entire day, bringing a warmth that is usually beneficial to the occupants of the building. West windows receive heat rapidly on summer afternoons, when the building is already warm, and generally overheat the rooms in which they are located. This effect is especially unfortunate in west-facing bedrooms, which reach maximum temperature at bedtime in summer. It is further aggravated if the west walls of the house are poorly insulated, allowing the interior wall surfaces to heat up and emit radiant heat. Shade trees planted to the west, or deep awnings over the windows, can be of considerable help in correcting existing problems of western exposure. In tropical latitudes, of course, east and west windows gain huge amounts of solar heat and should be shielded by exterior shading devices.
Radiational outflow of heat from a building is at a maximum on clear, dry nights and is especially rapid from roof surfaces that "see" large portions of the night sky. Roofing materials frequently cool by nocturnal radiation to temperatures that are appreciably below surrounding air temperatures. In areas where nighttime air humidities are generally low and skies are clear in hot weather, water may be cooled by contact with the roof at night and then stored in an insulated tank for use in cooling the building during the day. Building surfaces may be cooled directly by the water if the humidity is low enough, or the water may be used to absorb heat from the coils of a mechanical air-conditioning system.
A more direct use of night radiant cooling is the construction of roofs of high thermal capacity in hot, dry climates. The roofs lose heat rapidly at night and become quite cool, which enables them to absorb a great deal of the sun's heat during the day without allowing much of it to get to the interior of the building. The most direct application of all, however, is simply to sleep outdoors at night in warm weather, as is commonly done on flat rooftops in many areas of the world, thereby overcoming the heating effect of the air
by radiating heat directly from the body to the infinite blackness of space (9.16).
Because of the opacity of glass and most plastics to long-wave thermal radiation, neither closed windows nor glass-covered or plastic-covered solar collectors are very efficient in radiating heat to the night sky. Open windows are more effective than closed ones, but a large proportion of what a window "sees" is usually warm trees, earth, and surrounding buildings, and not cold sky. The black plate of a solar collector "sees" mainly sky, but it is a very inefficient cooling device unless its glass covering is removed. Perhaps the most efficacious of all simple radiant cooling devices is water ponded on a flat roof. The water "sees" virtually nothing but sky, and the radi-ational cooling effect is enhanced by additional evaporative cooling from the surface of the pond.
J. F. van Straaten. Thermal Performance of Buildings. New York, Elsevier, 1967.
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