TWO
Passive Heating
We now have the ability to provide 60 to 90 percent of the heating needs in buildings by utilizing different passive strategies tuned to the microclimate of the place.
Heating Load
Heating demands vary greatly with the “metabolism” of a building as described in chapter 1. Even in the same climate a small skin-dominated house can have a high heating demand, a moderate-density apartment a lower demand, a high-density apartment complex a much smaller heating need, and a large internal-load building no heating load at all. In fact, the large building could be dominated by cooling, ventilation, and lighting demand. These variations result from the skin-to-volume ratio of the building. For large buildings, the heat generated by the building’s occupants, equipment, computers, and lights, which are called internal loads, can play a major role in meeting heating needs.
The climatic heating load for a building is usually measured by the number of heating degree days, which is defined as the difference between the average daily temperature on the site and a base temperature of 65°F. The spectrum of heating degree days ranges from Resolute Bay, Canada, 74°N, at 15,356 HDD (SI units 8,542 HDD); to 52 HDD (SI units 29 HDD) for Key West, Florida, 24°N.
Internal Loads
The temperature in a building is affected by internal heat from occupants, cooking, computers, machinery, and lights. These heat sources can be a benefit in winter and a problem in summer. In a typical home with inefficient appliances, the largest sources of heat gain can be the refrigerator and freezer, clothes dryer, and water heater. Cooking, lights, and even people (100 watts each) also contribute to heat gain. The contribution from cooking heat depends almost entirely on use patterns; a baker and a nonbaker will have very different internal heat gains in the same house. The heat gain will be less if the water heater, clothes dryer, or washer is in an unconditioned space like the garage. Much of water heater’s energy goes down the drain with the hot water and the dryer’s heat is vented outside.
We can get a sense of relative importance of these by looking at the energy consumption for a typical 1,500-square-foot all-electric house in the Southwest of the United States, shown in table 2.1.
Computers, game players, printers, televisions, and other electronics can also add heat. Even on standby, many appliances, electronic systems, and charging systems for cell phones and PDAs add small but continuous loads. In larger buildings with more people and equipment, internal loads may dominate building metabolism.
Table 2.1. Family energy use in the southwestern United States.
Heat Sources
Internal heat loads and solar radiation are the primary heat sources for passive heating in sustainable buildings. The goal is to reduce energy needed at the site enough so that a small mechanical backup system will suffice.
Fig. 2.1. Convective heat transfer versus radiative heat transfer.
Heat distribution with a standard mechanical system is usually done by moving heated air with fans. This results in people and air being heated by convection and cooled by evaporation at the same time. In passive systems, the approach is to warm thermal mass, which in turn heats the occupants by radiation. The effects of these two methods of heat transfer are quite different. Convection heating can be uneven; it’s often noisy, and the air is often uncomfortably dry. The comfort and health advantages of radiant heating are readily apparent to anyone who has experienced both systems.
Fig. 2.2. Degree day (DD) definitions.
Radiation in Passive Heating
Radiation
Since radiative transfer of heat is one of the main elements of passive design, you need to understand some of its characteristics.
Any object at a temperature greater than absolute zero (–273°C) emits radiation, with the wavelength and intensity dependent on temperature. Radiation will always flow from a warmer object to a cooler object and will ultimately result in thermal equilibrium if no other energy is added or taken away from the objects. Radiation from the sun is in the form of short-wave radiation, while radiation from warm objects in a room is by long-wave radiation, as shown in figure 2.3a.
Overlapping figures 2.3a and 2.3b in figure 2.3c shows how the solar gain aspect of passive solar design works. Solar radiation enters through the transparent elements of the building and heats material in the interior, ideally optimized thermal mass. This material re-radiates at long wavelengths. At this long wavelength glass is opaque, therefore heat is retained within the interior space. This condition is called the greenhouse effect. When we tune a building to this condition, with optimum orientation and quantity of transparency, optimum amount of thermal mass, a method of providing control to respond to seasonal variation, and a spatial form and order that allows all of these to transfer heat to the occupants, then we have designed a good passively heated building.
Fig. 2.3. (a) Solar spectrum. (b) Transparency of glass. (c) Solar gain through glass, figs. a and b overlaid.
Responses to Radiation
When radiation strikes a surface, it can be reflected, absorbed, or transmitted. The properties of various materials may be quite different, and materials must be chosen to ensure that the desired performance is achieved. The angle at which direct solar radiation strikes a surface is known as the angle of incidence. This is particularly important when we consider the absorption, reflection, and transmission of solar energy. In general, the more acute the angle of incidence is, the greater the reflection, while absorption will be minimized.
Fig. 2.4. Reflectance, absorbtance, and emittance.
Absorptivity and Emissivity
Once the energy is absorbed by a material, it will be re-radiated or emitted. Building materials such as plaster, glass, plastic, concrete, and aluminum differ in their ability to absorb, transfer, and emit radiation, as shown in table 2.2. These properties may also vary with wavelength. The emissivity of materials is important in many designs—particularly those that rely on both short- and long-wave radiation for heating and cooling. Materials that collect energy effectively yet emit little energy are known as selective surfaces.
You can see why a selective surface is often used in collectors: The emissivity can drop from 90 percent to 10 percent. These selective surfaces may be created by direct deposition on metal (best) or deposition on a thin film of metal foil that can be stuck on the collector surface (good); or they can be applied as a paint (not as effective).
Table 2.2. Absorptvity and emissivity of common materials.
This table also shows why in hot climates it is far better to paint galvanized metal roofs than leave them bare, achieving a 90 percent emissivity, rather than just 28 percent. This is also why metal tools left out in the sun get so hot.
Elements of Passive Heating
In passive buildings we use solar radiation, internal loads, and architectural elements to provide the majority of a building’s heating requirements. Success is achieved by optimally tuning the whole building to respond to the elements (figure 2.5).
How these elements respond can be understood better by examining some extreme architectural types, what we’ve called tin boxes, thermos bottles, pyramids, and chicken coops. Response to the environment is visualized using the concept of sol-air temperature (To). The sol-air temperature is a fictitious exterior temperature that would have the same effect on the building as the combined affects of the on-site sources and sinks of its immediate setting, such as air temperatures, solar radiation, and radiation to the night sky.
Fig. 2.5. Basic passive arrangements.
Tin Boxes
For a climate with hot days and cold nights, typical of temperate zones in fall and spring, the worst building we could build thermally would be a tin box with no insulation. During the day, its shaded walls would quickly warm to the temperature of the exterior air, and walls and roof exposed to the sun would become even hotter. Since the inside temperature (Ti) is a combination of these, our building would be hotter than the outside temperature. After dark, the walls would radiate heat to the clear night sky and soon would drop below air temperature. The roof would drop to an even lower temperature because it has better exposure to the whole dome of the night sky. Thus, the interior temperature of our tin box would cycle from warmer than the outside in the day to colder than the outside at night. The tin building closely follows the sol-air temperature of its environment. This building accentuates exterior temperature swings, which results in it being more uncomfortable inside than outside.
Fig. 2.6. Tin box.
Thermos Bottles
Here we still have lightweight construction, but now our building is extremely well insulated. This reduces the heat gain and loss but does not affect gain and loss through windows or gain from internal loads. The result would be the same as wearing a bulky overcoat no matter what our activity. At times we would be comfortable, but at other times not, unless we could bring in external energy. The extensive insulation can increase the efficiency of our energy use but does not in itself optimize comfort. The thermos bottle can gradually get cooler and cooler if it is not augmented with heat. This is why energy efficiency, while a necessary prerequisite to passive conditioning, is related to but different from passive energy production, as described in chapter 1.
Fig. 2.7. Thermos bottle.
Pyramids
Consider what happens with a thick building made of very heavy materials. In contrast with the tin box or thermos bottle, the temperature variation inside a heavily massed building will be less severe. The indoor temperature will still oscillate with the daily sol-air temperature, but the oscillation is dampened. This is because the heavy material will slowly store heat during the day that is released to warm the building at night. In an extreme architectural version of this type of building, such as an Egyptian pyramid, the interior temperature remains almost constant. This is why heavy buildings are an indigenous response to climates where the mean sol-air temperature is about 75°F.
Fig. 2.8. Pyramid.
Chicken Coops
High-mass buildings are not appropriate for all situations. In hot humid climates, for example, the heat sink of the night sky is much reduced by the water vapor of the surrounding air, so the high sol-air temperature stays relatively constant. In this case, the only cooling mechanism available passively is evaporation from a wet surface. The most readily available and effective surface is the occupant’s skin if we can provide the necessary airflow with building design or form. The appropriate building form is a building with a roof designed for maximum shade and no walls to allow optimum air passage—essentially a chicken coop.
Fig. 2.9. Chicken coop.
“Classic” Passive Types
In the period following the first oil crisis in 1973, six approaches to passive conditioning were formalized into classic passive types. These consisted of three generic categories, each of which has two distinct approaches. The generic category was based on the relationship between the thermal collector—or, in the case of cooling, thermal dissipater—and the interior space of the building.
In direct systems, thermal transfer occurs inside the interior of the building. In indirect systems, thermal transfer occurs at the building envelope, and in isolated gain systems the thermal transfers occur outside the building envelope via a separate architectural element.
More specific classifications, of which there are two under each generic approach, are based on particular types of thermal mass for direct systems or the particular architectural elements in which thermal functions occur for indirect and isolated systems. Although in practical application passive buildings are usually a mixture of these approaches, there is much to be learned from looking at them separately, as discussed here.
Fig. 2.10. Six classic passive types.
Passive Heating—A Long History
Passive heating was not invented in the 1970s—it has a long history. Before HVAC systems became so pervasive in the 1950s, most buildings were passive. Although comfort standards were different, many of these buildings were amazingly effective for their time and culture. There is a historical overview at the bottom of the following pages that illustrates the evolution of passive heating over the last several thousand years.
It is interesting to consider some historical examples that are extreme in that performance often made the difference between life and death. These examples use internal loads to heat in extreme climatic conditions.
Fig. 2.11. (a) Polar region of North America: The use of ice for structure and insulation allowed survival in emergencies with only body heat as the internal heating source. (b) High steppes of Central Asia: A highly mobile yurt with a collapsible frame and felt covering allowed survival using only internal loads and a small dung fire, provided by the animals in this herding culture. (c) High plains of North America: A light frame and buffalo hide tepee could work under blizzard conditions using a trapped layer of smoke from a very small fire and an enhanced internal load (a “three-dog night”).
1970
History is important because it gives context and inspiration, but can be misused. Most modern yurts illustrate the problem. Yurts have a romantic attraction due to their roundness, structural integrity, and low cost for the volume achieved. But sadly, most modern yurts are thermal disasters.
Instead of thick felt walls and roof, which provided insulation in the historic yurts, they are often made of canvas that provides no insulation. Instead of the steady central fire fueled by dung from the animals of a herding culture vented by a central aperture they have a plastic dome in the center. This accentuates high summer sun for overheating and loses heat during the winter without admitting the low winter sun—just the reverse of what is needed.
The interior temperature in the modern yurt may be higher than the exterior during summer and colder than the exterior in winter like the tin box (see here). The result is the devolution of an elegant passive shelter for a harsh steppe environment to a design that is “hip” and economical but not efficient or comfortable.
Prerequisites for Passive Heating
A passive building must first be an energy-conserving building. This prerequisite allows passive design to take the next step to on-site energy production. Energy conservation begins with the building envelope. Three aspects of an energy-conserving envelope are insulation, reduction of air infiltration, and energy-efficient construction.
The major consideration of an energy-conserving envelope is the conductive value of the skin of the building.
An Energy-Efficient Building
The flow of heat from molecule to molecule is called conduction and can be a very important design consideration in sustainable buildings. If you hold a poker in the fire, it will gradually heat up until you can no longer hold it; the handle is transferring energy by conduction. Insulation has low conductance and slows down heat gain or loss. Several terms are used to describe various aspects of conduction in a building or clothing system. The major terms are:
U: Conductivity, per thickness
Ut: Conductivity, total value for unit
R: Resistance to heat flow per thickness
Rt: Total resistance for unit
A: Area
ΔT: Difference between temperature inside and outside
If we take each component of the building’s weather skin and add up the thermal conductivity of the area of each element, we can calculate the building’s overall heat gain/loss coefficient, usually signified by the letter Q. Thus Q = ∑UA∆T, described in the United States by BTU/hr°F and everywhere else by Watts/°C. BTU stands for “British Thermal Unit,” the energy required to raise one pound of water 1°F, about the energy of a wooden match when burned.
2010
It is possible to design and construct a modern yurt that is thermally acceptable if we apply basic passive design principles for our time and capability, as the originators of this design did for their time and situation. We could use for the walls and roof lightweight materials that provide more insulation. Modern material technology offers us this opportunity with very lightweight layers of highly reflective fabric or insulating fiberfill (as used in sleeping bags). Both would be more appropriate than one layer of canvas. In addition, the central aperture could be geometrically modified to optimize the solar radiation situation based on the site and season. Side windows or a glass door could be added for additional solar gain if needed.
Table 2.3. R-values of common envelope materials.
It is unfortunate that the United States uses different units than the rest of the world and that energy units are more complex than simpler measurements like length, area, and volume, but fortunately online conversions are available.
This book uses US units, and sometimes we also give the metric or SI units.
Reduce Heat Loss Through the Building’s Skin
The thermal resistance of materials (R) is defined as 1/conductance. R-value is described in BTU/Hr/ft2/°F. The thermal resistance of a material also depends on the airflow across the surface. Air films at the surface can increase the R-value as much at 0.07 on a vertical surface when the air is still. Moving air will drop this as low as 0.02. All wall sections should be calculated together to consider all the materials and airspaces.
Reducing Heat Loss Through the Building’s Glazing
Windows are a primary concern because their total resistance to heat flow may be only 0.9 for a single-glazed window. Dual-pane windows may achieve 1.8—still only one-tenth the value of a well-insulated wall and one twenty-fifth that of a super-insulated wall. Due to the dominance of windows in the conductive heat transfer situation, considerable work has been done to increase the efficiency of windows. The first step in this process was the use of dual-glazed windows, perhaps initially in ancient Rome. At first, modern double-pane windows had a propensity for the seal that joined the two panes to fail, especially if installed on a slope, causing more innovative passive designers to use a variation on the old storm window. These are two separate windows with a 2- to 3-inch airspace between them. Window technology has advanced enough for dual-glazed windows to become the standard application, although seal failure will still occur in many windows after ten to twenty years. It has now become hard to find new single-glazed widows without paying extra for a custom configuration.
Triple-glazed windows with argon gas between panes have become available with an R-value of up to 4.35. These are used in areas with high heating loads such as northern North America and Northern Europe.
The advent of low-emissivity (low-E) glazing led to another leap in window efficiency with increased insulation capability, but this also complicated design by requiring specification of different glasses for different orientations. Manufacturers have responded by reducing the availability of higher solar heat gain glass.
Low-E window technology uses a metallic film that reduces radiant heat flow through the glazing by reflecting radiation back in the direction from which it came. This may be applied to the glass or a plastic film between glass layers. The plastic film can be used to create a triple glazing. These windows can reflect much of the long-wave infrared part of solar radiation (good for cooling) while still transmitting the visible component, and can also reflect back the radiation from the warm interior surface of the glass (good for heating). This approach is good enough to essentially make low-E double glazing as good as triple glazing without the low-E coating. There are several types of low-E glazing that do different things—which is the tricky part for the passive designer interested in heating, because these are not equally available. For now, let us say if we want to passively heat, but we choose the most commonly available low-E glazing, we could do everything else correctly but still fail miserably, as illustrated in figure 2.14. In addition to the R-value of the glass, we must also consider its solar heat gain expressed in its solar heat gain coefficient (SHGC). The SHGC is a measure of how much heat flows through the glass to the interior of the building compared with amount that strikes it, usually ranging from 0.2 to 0.9. For passive solar heating, a higher SHGC is desirable but becoming harder to find; efforts have focused on conservation rather than energy capture and production with passive solar systems.
Fig. 2.14. A section through two identical passive buildings except for the type of low-E glazing. The building that doesn’t capture the winter sun will be uncomfortable and require more costly heating.
This problem with glazing illustrates a basic rule in passive design, which is that single-purpose functions and reductionist thinking are the biggest enemy to accomplishing our goals. If we look at glazing facing the equator as only good for energy conservation and do not take into account energy production at the same time (as described here), we can design an energy-efficient building but not achieve the next step, a passive solar energy-producing building.
Optimized Air Infiltration
Infiltration losses can be just as great as conductive losses and may easily account for half of the winter heat loss in a well-insulated but poorly weatherized building. Weatherstripping doors and windows and caulking and sealing building joints, access holes, and other areas will reduce unwanted infiltration.
Recent improvements in insulation and glazing techniques have made air infiltration the prime contributor to winter heat loss; however, complications arise in buildings that are so tight that the flow of fresh air to maintain the health of its occupants is compromised.
1961
St. George’s School near Wallasey, England. This highly glazed, highly massed, and highly insulated school provided 100 percent of the heating without a furnace. Designed by Emslie A. Morgan, an unsung passive hero, the design provides 50 percent of the heating by solar insolation and 50 percent from internal loads (34 percent lighting, 16 percent from occupants).
A fresh-air intake should be provided for any fireplace, stove, or furnace. Chemical properties of interior materials and their behavior will affect the fresh-air needs of the occupants. Nontoxic materials, finishes, and furnishings are preferable. Proximity to smoking areas, perfumes, and other odors contribute to air-quality and ventilation needs as well.
Fig. 2.15. Air-to-air heat exchanger.
Fresh-Air Needs
Advances in materials technology allow for a much more energy-efficient envelope with regard to both convection and air infiltration. When these are correctly understood and applied, buildings can heat themselves even in difficult climates with a small amount of solar insolation and heat internally generated by people, light, and electrical equipment.
This was first demonstrated at St. George’s School in Wallasey, England (latitude 52.5°N). This building operated successfully without a mechanical heating system for many years, relying on heat from the students and the sun. Eventually changing use patterns (fewer students and less internal gain) and codes led to retrofitting with a mechanical system. In the 1980s, residences with high levels of insulation and weatherization in Saskatchewan were also heated almost completely by solar energy and internal loads. These techniques of minimizing heat loss enough to allow very large solar heating fractions are most recently being applied at a much larger scale in the Passivhaus movement in Northern Europe, particularly Germany, as described here.
1988
Fig. 2.17. An example of second-wave straw bale construction. Demonstration blueprint farm Laredo, Texas, 1988, by the Center for Maximum Potential Building Systems (CMPBS).
The first residential building to demonstrate the value of super-insulation in conjunction with air-to-air heat exchangers for air supply was the Saskatchewan house built in Regina in 1977 by the National Research Council of Canada. It provided 100 percent of the heating needs in this severe climate without an installed furnace. Because of its success, similar buildings were built in Massachusetts and Montana in the 1980s, but further activity in the United States was stopped by the oil glut that occurred in the mid-1980s and the influence of the fossil-fuel industry on national policy.
Straw bale construction, first used in the upper Great Plains of North America in the 1890s, was rediscovered at this time because it was an inexpensive way of providing R-30 insulation using what was considered a waste product.
Table 2.4. Approaches to air infiltration and fresh-air needs.
Utilizing Energy-Efficient Construction
It is generally not appreciated how recently energy-efficient construction practices were adopted. However, if you get involved with many remodeling projects, it becomes clear. Insulation may not be found in many buildings built before the 1960s. Even as late as the 1970s, insulation was pretty minimal in California—only R-11 fiberglass in the roof cavity and no insulation in the walls. Even in very cold places such as Anchorage, Alaska (10,500 HDD base 65°F), the common wall was often only 2-by-4 studs with fiberglass insulation with a nominal value of R-11, but actually much less considering conduction through the wall framing.
Good insulation and weatherization did not really become a serious concern in the United States until after the energy crisis during the 1973 oil embargo. Efforts to create more energy-efficient buildings at the time often created their own problems. Increasing insulation in the roof beyond R-19 and making tighter buildings created the need for venting air cavities above roof insulation, but this was not common practice in residential construction. The improved insulation created more potential for condensation on the cold inner surface of the roof and could result in mold and rot with inappropriate detailing.
The health problems associated with the growth of mold were important. Tightening buildings to reduce air infiltration coincided with the explosion of newer artificial materials used for furnishings, fixtures, and household products with increased off-gassing of toxic materials. The result was the identification of sick building syndrome (SBS), which became a large problem in the 1980s and continues today, accentuated by sealed buildings, poorly maintained HVAC systems, and interior materials susceptible to the growth of molds or emitting harmful gases. As a result of the high costs associated with SBS, new green building standards are concerned about indoor air quality (IAQ).
With modeling and optimization studies from the 1980s to the present, a better understanding of optimal insulation levels and rules of when to use enhanced ventilation utilizing air-to-air heat exchangers become clearer for passive solar buildings. These developments are summarized in table 2.5.
Nominal insulation value stated on insulation material is typically much higher than the actual value of a wall because of conduction through framing and less-than-perfect installation. Added framing for openings, joints, fire breaks, or seismic resistance can reduce insulation values 10 to 25 percent. Insulation performance can also be degraded by moisture buildup, so placement of a vapor barrier is important and will vary depending on climate. Venting the cold surface of insulated walls or simplifying moisture control with closed-cell-foam-type insulation can minimize these losses. However, the production process, useful life, disposal, and the potential for hazardous off-gassing in a fire are life-cycle considerations for insulation. Certain insulation materials such as straw bales should be allowed to breathe water vapor, rather than seal in vapor with a barrier. In this case, providing weep screens at the bottom of the wall is important.
Table 2.5. The range of insulation values in RSI (R in the United States) for a passive skin-dominated building.
Proper Orientation
One of the earliest design decisions essential for passive heating is the orientation of the building to the sun. As shown here, orientation to the equator is best. This has been known, exploited, forgotten, ignored, rediscovered, and exploited again many times throughout history. Knowing this and really utilizing it in the design process are two different things. It’s easy to get distracted with other aspects of site planning such as views, property lines, circulation, access, and regulatory requirements. However, a really good designer should be able to take all these factors into account and not lose proper solar orientation in the process. Over-abstraction of the design process makes it too easy to ignore basics like orientation. We’ve consulted with architects who are into working drawings and still do not know which way south is, much less the nuances of magnetic declination and sun angles as shown here. We recommend showing the equatorial direction on plans to continually remind ourselves of this important aspect. Therefore, in North America and Europe use south arrows to orient drawings and plans. This may sound trivial, but it serves as a constant reminder of the primacy of proper orientation as the design progresses and multiple designers become involved.
500 BC
(a)
(b)
(c)
Fig. 2.18. (a, b) Section and isometric of the solar city of Priene in Asia Minor. (c) Many Hellenistic cities at this period were enlarged using the solar planning principles of Hippodamus. This example on the right is the city plan of Olynthus in Northern Greece.
Calculating Insolation Patterns
There are many ways of determining insolation patterns on and in buildings. You can draw multiple sections using descriptive geometry or use computer programs that can quickly show sun patterns—if you ask the right questions.
Since passive design is essentially holistic design with an emphasis on integration, we have found that it’s best to investigate insolation patterns in the most direct and easily visualized way in relation to the building’s overall form. For this reason, we are very partial to models. Three-dimensional models allow better integration, because the user can visualize the whole building and site without the tediousness of drawing multiple sections or relying on the often hidden black-box assumptions of the computer program. These are better used later in the design process when more accuracy is required. It is very easy to change a 3-D model into a 4-D model (the addition of time) with the use of the sun-peg diagrams shown in figures 2.19 and 2.20. These can be used to essentially turn your model into a personal heliodon. The steps in using sun-peg diagrams are:
1. Find the chart nearest your latitude.
2. Make a copy of the chart (expanding it on a copier or computer if desired).
3. Make a peg whose height above the chart’s surface equals that shown on the chart. Mount this peg where noted. It must stand perfectly vertical relative to the model.
4. Mount your copy of the chart with attached peg on the model. The chart must be perfectly horizontal over its entire surface, and the true south arrow must be the same as true south on the model.
5. Choose a test time and date. Take your model out into direct sunlight and tilt the model until the shadow of the peg points to the chosen time and date. When the end of the peg’s shadow touches this point, your model will show the correct sun and shadow patterns for this date and time.
Solar access is also important. Solar access was first given legal standing in Greek and Roman times. It was rediscovered in the 1970s. Some cities and many states now provide some type of protection for solar access (www.solarabcs.org/solaraccess). It is important to determine your solar access rights to avoid building a lovely passive solar building that is shaded by a neighboring home, shopping center, or line of trees.
How much leeway do we have with regard to orientation? Up to 15° off south is usually not too much of a problem. Even slipping orientation 20 to 25° off may not be disastrous. However, beyond this the building’s relationship to solar radiation gets increasingly difficult, not just for heating but more critically in the control of unwanted radiation from the east and west and during fall and summer.
South–north sections are often the most important drawing in visualizing passive design, as shown in figure 2.21. Visualization is made more difficult when the building is not oriented to the equator. This makes layout of sunlight and sun patterns in the section drawings a geometry problem because the rays of sunlight at the most important times are not seen at true angles as they would be with better orientation. A designer has to do more calculations to use profile angles to visualize interior light and insolation patterns. A profile angle is the angle a line appears in a view that is not perpendicular to the line in question—in this case the ray of sunlight penetrating the building.
Fig. 2.21. True angle and profile angle.
A device that is very helpful in dealing with site orientation questions is a solar site selector (figure 2.22). This simple device allows you to determine the effect of barriers like adjacent buildings, trees, and the like on available solar radiation.
Fig. 2.22. Using a solar site selector.
Classic Approaches to Passive Heating
Direct-Gain Systems
The six classic approaches to passive heating (see page 45) are so pure as to be caricatures. Their application in the 1980s showed that they are usually best considered as components that can be used in various combinations rather than as pure systems. However, it is helpful to look at them independently to be able to explore some of the principles involved before looking at combinations.
1300
[a] Sainte Chapelle, Paris
1500
[b] Blue Mosque, Istanbul
1900
[c] Casa Batlló, Barcelona north elevation
[d] Casa Batlló, Barcelona south elevation
[e] Casa Batlló, Barcelona interior light
1850
[f] Crystal Palace, World’s Fair, London
Fig. 2.23. (a, b) Gothic and later Ottoman architecture incorporated glass as a major architectural element. (c, d, e, f) Improvements in glass manufacturing and mass production of glazing systems allowed large public buildings to be made of glass. (c, d, e) Optimizing orientation and control, this famous building by Antoni Gaudí differentiates north and south by the type and amount of glass to provide optimum orientation and seasonal solar control. As a high-mass building of stone, masonry, and tile, it has all the elements of a good direct-gain building except for good insulation, which was not available at the time.
Each approach must provide three functions: (1) thermal collection; (2) thermal storage; (3) thermal transfer and control. In direct systems, these three functions are accomplished in the interior space of the building. Collection is accomplished by apertures that allow sunlight directly into the interior space, and thermal storage is accomplished by elements within this space. Thermal transfer is accomplished by radiation from this storage, and control is also accomplished spatially. Because of this intimate relationship to interior space, direct-gain systems are also natural lighting systems (covered in depth in chapter 4). However, some aspects must be discussed here as well since direct gain and natural lighting are so intimately interconnected.
Looking at seven hundred years of architectural history, we can see the sequential developments that were necessary to be able to achieve direct-gain passive solar heating. These are (1) the development of glazing systems that allowed natural light to become a major architectural consideration; (2) the rediscovery of orientation and control aspects that allowed better use of this light; (3) the development and availability of better-quality insulation; and (4) the conscious use of thermal mass. Most important, however, once these things were available, was the correct configuration and sizing of all these features into an integrated whole that could optimize the building’s response to the thermal environment. This became easier to achieve with the development of computer-aided simulation models in the 1980s.
1920 Nebühl, Switzerland, Solar Housing Estate.
In Northern Europe, there was much emphasis on improving health by large solar developments. Germany led in this effort, which was cut short by World War II. After some false starts, south was rediscovered to be the optimum orientation.
1940 Solar buildings by George Fred Keck, Chicago, Illinois.
Libby Owens Ford developed the first commercially available double glazing, which stimulated solar designs in the United States. Insulation, however, was minimal, thermal mass unknown, and most designs overglazed, so performance was not up to its potential.
1946 Prefabricated solar house, Camden, New Jersey.
1946 Prefabricated solar house, Camden, New Jersey.
1952 Frank Lloyd Wright’s Solar Hemisphere, Madison, Wisconsin.
As usual, Frank Lloyd Wright led in innovation. This house has good orientation, some mass on the north wall, is bermed on the north, and has good solar control. Insulation was still inadequate and the house is undermassed and overglazed, but it was a step up from conventional housing.
1961 St. George’s School, Wallasey, England.
An obscure architect, Emslie A. Morgan, builds a very close to optimum passive building in a challenging climate (see also here). Overglazing and overheating could be a problem, but the building is not occupied in the summer during school recess.
1972 Sunscoop House, Santa Fe, New Mexico, by David Wright (no relation to FLW).
This was probably the first close to optimized direct-gain building besides St. George’s School. It is highly massed by adobe walls, insulated on the exterior with polystyrene foam, masonry floors, and water bancos. Sizing, while improving, is still a guess; in hindsight, it is probably overglazed.
1980 Development of simulation models.
Phil Niles in California and Doug Balcomb in New Mexico develop the first performance-based passive models allowing optimized sizing of components. Many simulation programs exist, and a list can be found at www.eere.energy.gov.
System 1: Direct Gain/Distributed Mass
In the simplest direct-gain system, thermal mass is spread around in relatively thin layers throughout the interior space. Transfer of heat from sunlight to the mass is by radiation if directly in sunlight, or by re-radiation or convection from warmer areas. Radiation coupling for all thermal mass is desirable, but it is not possible without overglazing a building; convectively coupled mass works, however, and can be used to good effect. It’s usually best for this distributed mass to be a light color. While this reduces direct absorption, it helps reflect light onto a greater surface of the distributed mass. In addition, light colors are also much better for natural lighting, reducing the glare that comes with dark interiors contrasting too greatly with large bright openings.
Distributed mass is best for balanced heating and cooling conditions. Night ventilation cooling over distributed mass allows greater surface area for heat dissipation by convection to the cool night air vented over the mass during the cooling mode.
System 2: Direct Gain/Concentrated Mass
In this approach, the thermal mass is more concentrated, perhaps as a water tank or masonry wall. The concentrated mass allows greater efficiency for heating but diminished effectiveness for night ventilation cooling. Water in tanks, tubes, or containers is one of the most effective thermal-mass materials since it is 2.7 times as effective at storing energy as concrete by volume and 6 times more effective by weight. Thick masonry elements such as heavy walls, floor slabs, or structural elements made of stone, rammed earth, adobe, or concrete can also be used as concentrated mass.
Concentrated mass is usually smaller in surface area and can be darker and therefore more absorptive without creating a dark interior. Re-radiation at long wavelengths (see here), however, is less dependent upon color, so surfaces not radiatively coupled can be a light color to help with natural lighting.
Details for System 1: Direct Gain/Distributed Mass
Aperture
Ideally, each room should have an equator-facing solar aperture of the size shown. This is rarely practical, and thought should be given to which rooms should have aperture priority. Usually, rooms used the most face the equator, with less used spaces behind.
Fig. 2.25a. Solar dormer, exterior. A dormer or roof monitor has the advantage of putting sunlight high in the middle of the room, helping reduce glare, fading of fabric, and so on. It can also add to the roof structure with fins to better reflect light around the space.
Fig. 2.25b. Interior. An optimized dormer prevents unwanted sun in summer by utilizing a horizontal overhang to shade high sun and small wing walls to shade early-morning and late-afternoon horizontal sun.
Fig. 2.25c. Section. Control is easy for the seasonal extremes, but more complex in spring and fall. Early spring usually requires heating and early fall requires cooling, yet the sun angles are equal. Thus it is desirable to adjust the horizontal overhang as shown here.
Thermal Mass
More distributed mass is needed than is usually imagined. It must be spread over such a large part of the building’s interior; while some areas can be radiatively coupled to available insolation, others have to be convectively coupled.
The most common materials used are shown here. Their heat capacity in bulk and in their most common usage is shown in table 2.6.
Table 2.6. Heat capacity of common building materials in bulk and by common application.
Fig. 2.26a. An advantage of direct-gain/distributed-mass systems is that the mass is equally helpful for heating and night ventilation cooling, as shown in the sensitivity charts for Fresno, California (latitude 37°N).
Fig. 2.26b. In passive systems, often the thermal mass is the only added expense beyond a standard well-insulated building. Therefore it’s helpful to be careful and efficient in regard to the mass provided. Free mass is usually available in the floor slab if it is not insulated from the interior space by carpet and is insulated from the exterior.
Fig. 2.26c. The slab plus additional mass on interior wall surfaces is usually enough to meet the mass requirements.
Sizing
Rules of thumb are used to size these components to develop a preliminary design allowing 50 to 60 percent performance. Design refinement using a performance simulation model should then allow an additional 20 to 25 percent improvement in heating performance.
Table 2.7. Rules of thumb for areas of solar aperture and thermal mass for direct-gain/distributed-mass systems.
As can be seen from the sensitivity charts above, on a diurnal basis concrete and masonry aren’t much more effective beyond 2 inches in thickness. Gypsum plaster or wallboard can be used as cheap distributed mass in two 5/8-inch layers but has less heat capacity, so divide wallboard area by 4.
More Details
These details are typical for residential and light commercial buildings at a mid-latitude temperate climate with balanced heating and cooling loads.
Fig. 2.27. Example of smart fascia for seasonal adjustment.
Fig. 2.28. Skylights accentuate high summer sun gain and overheating and increase nighttime heat losses in winter and are not very effective in gathering low winter sun. If they are steep enough with operable louvers, however, they can be effective and have the advantage of being able to adjust to spring and fall seasonal variations.
Fig. 2.29. For higher spaces, the equatorial wall becomes difficult to shade in the summer and still receive sunlight in the winter. A common approach is to use two horizontal overhangs. The lower one provides control to the lower part of the wall and can also act as a light shelf to reflect light deeper into the space. This has additional advantages for improving natural lighting.
Fig. 2.30. The rediscovery of straw bale construction as mentioned on page 18 was a boon for the direct-gain/distributed-mass approach. Bale walls provide greater insulation than most wall systems, and the stucco or plaster applied to the wall surfaces is the right thickness to act as optimized distributed mass, as shown here. The result is a composite wall system that is an excellent fit for the needs of a direct-gain/distributed-mass passive system. It is a prime example of achieving synergy in design, where the whole is greater than a sum of the parts.
Fig. 2.31. A sample rule-of-thumb calculation for balanced heating and cooling of a direct-gain/distributed-mass system in the Northern Hemisphere.
New developments promise to increase the capability of direct-gain/distributed-mass systems while also reducing their cost. These include transparent insulation (see figure 1.22) and phase-change thermal mass (see Thermal Mass). Transparent insulation can allow more aperture with less conductive heat loss and hence greater spread of sunlight to distributed mass without adding to conductive heat loss, which has been the limit on glazing to date. Phase-change thermal mass can used as an additive to masonry or gypsum wallboard to appreciably increase distributed thermal storage capability. With these recent developments, the direct-gain/distributed-mass approach to passive heating and cooling becomes a much more viable option for many situations than ever before.
Details for System 2: Direct Gain/Concentrated Mass
The difference with concentrated-mass designs is not so much about aperture or spatial relationships but with thermal-mass placement. Here mass is more concentrated, allowing greater efficiency especially for heating. The most efficient thermal mass short of phase-change material is water. Therefore direct-gain/concentrated-mass designs have traditionally been approaches that use large amounts of water in architectural configurations.
Early applications, because of a concern with leaks, used off-the-shelf items that had proven watertight characteristics. However, these components often were not optimally shaped in relation to desired mass and aperture relationships. With performance modeling, later applications were able to better optimize this relationship, often as custom steel water tanks used below the standard windowsill on the south wall.
Fig. 2.32a. First modern waterwall by Steve Baer. Corrales, New Mexico, 1970.
Fig. 2.32b. Corrugated culverts as waterwalls 1975, Living Systems.
Fig. 2.32c. Early large-scale waterwalls 1976.
Fig. 2.32d. Since 1976, Kalwall Corporation (now Solar Components) has manufactured fiberglass tubes that offer many advantages as translucent waterwalls.
Fig. 2.33. Sensitivity analysis of concentrated -thermal mass for heating and cooling.
Table 2.8. Common concentrated-mass materials.
The advantages of water can be seen in these sensitivity charts for both heating and cooling for Fresno, California (latitude 37°N). For heating, water has a higher heat capacity and is also able to redistribute heat within the container by convection currents. For cooling via night ventilation, larger surface areas needed for conductive transfer from cool air to the mass diminish somewhat the advantage of concentrated mass for this function. The corrugated surface on culvert-type containers helps by exposing more surface area to cooling air currents.
Table 2.9. Rules of thumb for thermal mass for direct-gain/concentrated-mass systems.
Sizing
We’ve only begun to explore water as an architectural material. This effort is important because water has wonderful heat capacity and amazing aesthetic potential, and it’s potentially the cheapest and most available material for thermal-mass purposes. We live on a watery planet. If we are to really develop an earth-expressive architecture, water as architectural material will be a part of this evolution.
Fig. 2.34a. 2006 application of a waterwall to a social hall.
Fig. 2.34b. Waterwall tank: concentrated-mass detail.
1800
Professor Edward Morse of Salem, Massachusetts, patents a solar heating system made of a glazed wall and air vents. Unfortunately the uninsulated buildings of this era were not efficient enough to make very good use of such a system.
1947
From 1938 to 1958, MIT developed and tested four solar houses funded by solar enthusiast Godfrey Lowell Cabot. Most of these were active systems using flat-plate collectors and isolated storage. However the second study done in 1947 was a passive direct-gain system. Great effort was made to make the test rooms the same as a standard “well-insulated” house of the time, which was about R-12, neglecting the prerequisite to passive design discussed on pages 47–49. As a result, performance was disappointing and this approach was prematurely abandoned.
1967
Félix Trombe, a French scientist, creates a wall that uses the thermal mass of 12 inches of masonry as a solar heater, allowing a lag of heat flow so that the wall warms the space at night. Heat radiating from the storage wall is supplemented by air vents that allow faster heat transfer to the conditioned space by convection. In passive circles, this design became known a Trombe wall and found widespread use in the 1970s.
1981
Beginning in the 1950s, modern architecture became plagued by the all-glass curtain wall that—although visually exciting—was thermally disastrous. Using a concept developed by Richard Levine, the Hooker Building in Buffalo, New York, created a thermal wall utilizing curtain-wall technology, an air plenum, and insulated louvers. Since then, this double-skin thermal wall approach has been utilized in many larger buildings, particularly in Northern Europe.
Fig. 2.35. System 3: History of thermal walls.
Indirect-Gain Systems
In indirect-gain passive systems, some part of the building’s weather skin is evolved to provide the thermal collection, storage, transfer, and control function. For indirect cooling, we would add thermal dissipation, described in the next chapter. Although there are many variations for indirect gain, there are two major approaches. In the first, the exterior wall becomes the passive system, hence the name thermal wall. The second approach uses the roof as the passive system This has traditionally been called a roof-pond system since most approaches have used water-pond roofs; however, high-mass solid materials or phase-change materials can also be used. Therefore, the most generic term would be thermal roof system. Although to date these have been separate systems, combinations of the two are possible.
[a] A six- to eight-hour time lag through 12 to 14 inches of masonry provides heating after sunset. Vents create a thermal siphon air loop, providing early-morning heating while keeping the south surface of the wall cooler and thus a more efficient solar collector.
[b] If the climate is mild, the air loop may not be needed, simplifying the wall’s construction. A rule of thumb is that an unvented thermal wall can be used if the average January temperature is 50°F or over with good insolation.
[c] A waterwall similar to that in figure 2.34b but full height can be three-quarters the thickness of a masonry wall and has no need for the air loop, since water mixes itself by stratification.
[d] Thermal walls of this type can be enhanced by various techniques shown below.
[e] Larger buildings have less need for heat (as described in figure 1.14) and greater need for cooling and lighting, therefore a double thermal wall can be used that can control heat gain, enhance natural ventilation, and provide better lighting for high buildings.
Fig. 2.36. Variations in the evolution of thermal walls.
System 3: Indirect Gain/Thermal Wall
The history of indirect systems shown on pages 66–67 provides the basis of the definition of system 3 (and sometimes system 4) in the classic passive nomenclature. However, progress in Europe with Foster’s evolution of the thermal wall into a thermal envelope suggests that a more up-to-date term for system 3 should be thermal envelope.
Details for System 3: Indirect Gain/Thermal Walls and Thermal Envelopes
In recent years, innovation in passive design has moved from the United States to Europe. There the evolution of the thermal wall into thermal envelopes with expanded functions of daylighting and natural ventilation have become more common.
Fig. 2.37. The evolution of thermal walls to thermal envelopes in buildings in Germany by Norman Foster and engineers Battle and McCarty.
Floor plan
Section
Passive cooling and heating capability to achieve thermal comfort.
Fig. 2.38. An example of the waterwall approach. The size of this assembly space (140-foot span) made it very economical to use a pre-engineered rigid steel frame. However, it is difficult to get the amount of solar aperture and mass needed for passive conditioning within this structure. Therefore the passive component was decoupled from the structure by creating a 128-foot-long waterwall on the entire south face above the lobby space. Heating is done by this 1,365-square-foot waterwall, leaving only 112 square feet of direct gain in the roof. The performance model for this building uses the Energy 10 performance prediction model. Cooling is done by night ventilation over high-mass floors and the waterwall. Each type of thermal wall has its unique character that can be used for other functions in an integrated design. Illustrated here is a situation where the massive barrier-like qualities of a large waterwall are used as sound protection and space definition, and for movement of cool night air when needed.
1600
Yakh-chal (ice wall) ice-making structure in Iran.
1969
Harold Hay, the inventor of this system, with roof pond test cell in Phoenix, Arizona.
1973
Atascadero Roof Pond Prototype. Insulation retracted exposing the ponds. Insulation extended covering the ponds. This system provided 100 percent of the residential heating and cooling requirements.
Details for System 4: Indirect-Gain/Roof Pond and Thermal Roof Details
Roof ponds are very simple in concept but can be tricky in application. A large amount of thermal mass on a radiative roof (usually an inexpensive steel deck) is shaded by movable insulation during the night in winter and exposed during the day, allowing indirect-gain solar heating. By shading the mass during the day in the summer and exposing it at night, the same system achieves effective indirect loss cooling to the night sky. Although roof ponds are the top performers of the six classic passive types, they have been the least used. There are several reasons for this situation. Until recently this system was stalled by patent and licensing issues, and industrial design development of the system components has not been done. People are also afraid of water overhead, but with new materials the roof pond might be replaced with a phase-change material. Of the classic approaches, this requires the most integration with architecture, which goes against the grain of a reductionist-thinking culture that emphasizes parts over wholes.
Fig. 2.40. Roof pond applications, stepped ponds, lifting and sliding insulated covers.
Fig. 2.41. The adaptation of roof ponds in various climates can be accomplished by changing the roof configuration and the type of movable insulation.
Fig. 2.42. Adaptation of roof ponds to a multistory urban situation. This design was predicted to provide 100 percent heating and cooling in Sacramento, California.
Fig. 2.43. Evolution toward thermal roofs employing no moving parts.
More holistic design will lead to widespread use of this approach. New developments in insulation and thermal mass can allow for the reduction of redundancies and greater simplification of this proven and highly efficient approach to passive heating and cooling.
Isolated-Gain Passive Heating and Sunspaces
Isolated-Gain Heating
Isolated or indirect solar heating systems can also be installed on new or, more commonly, existing buildings. These may be called a sunspace, solarium, sunroom, solar greenhouse, or thermosiphon air system. We consider four general types: sunspace, seasonal sunspace, solar greenhouse, and thermosiphon.
Sunspace
A sunspace, solarium, or conservatory typically is on the equator-facing side of the building. It adds window area for added solar gain, but buffers loss because it can commonly be closed off from the house at night. The mostly vertical glass can be shaded in summer to reduce overheating. Thermal mass can be added to reduce temperature swings to protect plants in the sunspace, but for maximum heating less mass may be preferable.
The essential details for an effective sunspace are sufficient vent areas for effective air exchange with the conditioned building space. This means large vents up high to move hot air into the house and low vents for cooler return air. In a suspended-floor building this may be by an underfloor plenum (channel for airflow) from the polar side. The sunspace floor can be set below building-floor level to reduce backflow of cold air into the building. A photovoltaic-panel-driven fan or normal fan can be used to increase heat flow into the building. High-efficiency low-voltage motors and propellers from electric model airplanes may prove very useful for moving the air.
In summer, vents, roll-down shades, or landscaping is needed to reduce overheating. Again, vents should be high for venting and low for intake. The intake air may be pulled from outside or from the house, to provide ventilation flow.
Thermal mass is optional. In many cases, it will be desirable to temper the microclimate in the sunspace throughout the year. This is particularly important if it will be used to grow plants, or will be open to the house for much of the year. Sunspaces are often very pleasant, and people and pets tend to spend more time in them than might be expected.
Seasonal Sunspace
A seasonal sunspace may be very effective for retrofitting existing buildings. It can add the considerable solar energy needed to warm an under-insulated building. One of the best strategies is adding seasonal glazing to an existing porch, wall, or patio area. This can be as simple or sophisticated as design and budget allow. Twin- or triple-wall polycarbonate panels offer a very strong, light method of glazing. You might consider such panels as large storm windows for the porch.
Fig. 2.44. This open-porch-to-sunspace retrofit in Berkeley, California, dramatically improved thermal performance and created additional living space.
The most effective orientation is equator-facing, but east- or west-facing porches can also be very useful heaters in winter, early spring, and late fall. An east-facing porch can pop the building temperature early in the morning, and a west-facing porch can give a nice boost in the afternoon, but both will be heaters in the summer unless shaded. In very cold climates, the sunspace may be used in the warmer months of the year but insulated in the cold dark months. It will still act as a buffer space and wall-insulation upgrade. The keys are glazing, vent area and placement, and supplemental fans as needed. Thermal mass may be desirable as well, and isolating it is usually important.
Fig. 2.46. A solar greenhouse in central California.
Solar Greenhouse
A solar greenhouse typically has much more glazing, including sloped glazing overhead. This adds more light and heat, but also adds risk of leaks and increases heat loss at night. Solar greenhouses are also known for summer overheating.
Solar greenhouses were once considered a good way to add the large glazing areas needed to provide sufficient heat for an under-insulated building. It usually makes more sense to improve the building shell—particularly for a new building. For existing buildings, a solar greenhouse can make sense. Or if the building owner plans to grow food or use plants as part of a biofilter or treatment system, a greenhouse may be worthwhile.
The biggest challenge is to avoid overheating in summer. This typically requires a roll-down shade cloth or awning, large vent areas to the outside (low and high), and often a supplemental PV fan. A very simple integral solar water heater can be placed in the solar greenhouse.
Making good use of winter solar heat demands a good vent system to the house (low and high), perhaps a supplemental fan, and in some cases thermal mass. Insulated curtains or shades can help reduce night heat loss. Insulated doors or shutters for vent openings are desirable.
Thermosiphon
Air and water density changes with temperature, and this makes it possible to create a circulating system without fans or pumps. These have been used on a wide range of buildings. Air systems are easier to work with, but are most favorable on hillsides or two-story homes where the collector can be placed below the living space. A wall-mounted unit can also be made, but the warm air is delivered at the ceiling instead of near the floor, and a small fan may be needed to help circulate the warm air.
Fig. 2.47a. U-tube air collector.
Fig. 2.47b. Back-venting air collector.
Air collectors can use mesh or solid collectors (often black-painted metal roofing is used under double glazing), and the heated air can pass through the mesh or in front of or behind the collector plate. Air gaps should be generous, ½ inch to 2 inches or more to allow free flow. Large vent areas, smooth duct runs, and free-flow collector spaces can improve performance. Back-flow preventer flaps can prevent reverse action at night. Seal joints and connections carefully to avoid cold-air leaks. Insulated shutters can also be helpful.
Thermal-mass storage has been used with some thermosiphon systems. Rockbeds appeared promising for air systems, but problems with mold and pests have occurred in some cases. Using high-mass floors or distributed phase-change mass may be safer and more effective. You might also use pavers over a sand floor with concrete-block air channels (as used by New Mexico solar pioneer Peter van Dresser).
A small window-box air heater can be added to almost any window if you need a heat boost or would like to take a baby solar step before doing a major retrofit. A larger wall-mounted unit can provide heated air that circulates across the building under a high-mass floor, rises to the second story, and then is returned as cool air to the collector.
Example of Integrated Design for Passive Heating
Integrated Holistic Design
You may have noticed in the last several pages the difficulty of keeping heating separate from other functions such as cooling, lighting, and ventilation. This is because the most characteristic aspect of passive design is that it is integral to architecture—not only to other thermal functions but also to aesthetic, social, and cultural functions and expressions. The material in this book is organized to emphasize particular functions but should always be considered in the complete context of passive design for heating, cooling, lighting, and ventilation.
Integration is important because it is a change society needs to make if we are to critically respond to the problems and opportunities of this unique period of history, when we must make the transition from reductionist to holistic thinking.
One way to clarify this issue is to review the literature about how we should respond to the massive threat of global climate change. In this debate, what’s striking is the size, complexity, and cost of many climate correction proposals, such as:
1. Injecting SO2 into the upper atmosphere to reduce solar insolation.
2. Seeding the ocean with massive amounts of iron to increase plankton blooms to take up CO2.
3. Sequestering CO2 from burning coal by pumping it into underground geological strata to contain it.
4. Creating huge satellite photovoltaic arrays in space where sunlight is more intense, then microwaving this energy to earth to replace fossil fuels.
Green planning and passive solar architecture are rarely mentioned, being the proverbial elephant in the room that our myopic society can’t see. Since buildings and communities are the primary source of global warming gases, it might be expected they would be a large part of this debate. The reason they are not is related to the conceptual problem of reductionist versus holistic thinking. Reductionist thinking breaks a problem into discrete parts, and solutions are sought at that level; synergetic thinking strives for solutions at the level of the whole. Because a synergetic whole is far more efficient than the sum of its parts, integral thinking is the most effective approach. Our industrial culture values reductionist thinking and often fails to understand or value synergy.
The geo-engineering approaches described above are examples of reductionist thinking. They are single-purpose, linear processes that exist in isolation with unknown side effects. They are emergency responses that treat the symptoms, not the disease. In contrast, green environmental design and construction with passive design address the causes, are multipurpose, provide multiple benefits, and are connected intimately to our settlement patterns, which are causing much of the problem.
In the integrated-design section of each chapter, we will offer examples of passive systems that are integrated enough into the environmental, community, and architectural context to become synergetic in character.
State Office Building for an Era of Transitions
Historical and Urban Patterns
The site is downtown Sacramento, California. This inland city has much of the character of the Middle West. It was platted with large square city blocks that have wide streets with large trees. In contrast with late modern state office buildings, the goal for this project is to become an integral part of the existing city layout and character.
The program required that the building contribute to urban street life and provide clear circulation to the public. This low-rise building achieves this by creating a series of storefront offices fronted by ample sidewalks and street landscaping. Government offices have the traditional street address. From the street, the building is structured along a “publicness gradient” to open into a series of interior-block open spaces more related to employees than to the general public.
Historical Architectural Patterns
An analysis of the state buildings of the capital area revealed a variety of building styles from eclectic 1920s to art deco 1930s, monumental 1950s, Le Corbusier–inspired slabs from the 1960s, and corporate towers of the 1970s. Despite this variation, there was a consistent progression of buildings becoming increasingly large in scale, disconnected from the outside, and unpopular to work in; and each was accompanied by a large increase in energy use per square foot. The architectural goal for new state office building was to recover the scale of the earliest buildings in the area, to reverse this trend of resource intensity, and to utilize natural energy through passive design. The plan reduces the building’s imported energy use while providing a healthier and more pleasant working environment.
Figure 2.48. Historical patterns of land and energy use.
The new state office building is designed to serve work groups of six to twelve and work clusters of approximately one hundred people. The result is a series of modular pavilions and mezzanines that interlock at split levels along the publicness gradient spine. Each module provides space for one work group on each level. The thermal roof provides thermal control, natural lighting, and a sound-diffusing shape in the central work space. The scale achieved is closer to that of a large residence than that of the newer state office buildings. On the interior, the scale is more like the earliest state office building. This scale would better serve the six-to-twelve-person work groups and more likely stimulate user control and responsibility mentioned in the program. Groups of three and four modules combine to form the work clusters that serve about a hundred people each.
Structural System
The structural system for the roof evolved from the thermal and acoustical factors plus the desire to use thin steel as efficiently as possible. The result is a shape generated by translating one circular arc polygon over another, creating equal-sided parallelograms that also act as shear panels. The result in an extremely stable structure using 16-gauge sheet steel to support 54 tons of water for each module.
Natural Lighting
The lighting design was based on using artificial light only in a supplementary capacity to daylight. Daylight was calculated to provide 75 to 100 percent of the background illumination and up to 50 percent of task illumination in all interior spaces.
Fig. 2.49a. Detail section, day and night.
Fig. 2.49b, c. Exterior perspective sketches.
Fig. 2.49d. Interior perspective sketch.
Thermal Patterns and Architectural Form
The modular form of the pavilions lend themselves to the roof pond system of heating and cooling, which is by definition modular, while achieving 240,000 square feet of office space on a 152,000-square-foot site without exceeding three stories in height. Thermal control is provided by roof ponds and movable insulation coupled with ceiling coils that provide heat and coolth from the ponds to the perimeter of the modules and to lower floors not so affected by direct radiation from the ponds. Night ventilation cooling of the ponds can be accentuated by allowing air to flow over both the top and the exposed conductive bottom surfaces of the ponds while venting the warmer air from the interior space.
Fig. 2.49e. Overall section in relation to the urban context.
The ratio of interior floor area served by the roof pond is slightly less than 4:1. Dynamic-response modeling was done by the admittance method, and predictions for 100 percent heating in December (except during extended fog conditions) and 100 percent cooling in July were obtained. Since evaporation was not needed for cooling, this thermal control was achieved without adversely modifying the exterior environment. This is an important urban design consideration because in a high-density urban setting, small modifications to the exterior environment by many buildings add up rapidly to create environment modifications such as heat islands.
In this application, resulting savings due to passive solar heating and night sky cooling were predicted to be 4,292 therms/year for an average module or 188,900 therms/year for the whole building for a reduction of 1,134 tons of CO2 per year.
It’s interesting to note that only in the year 1976 did the energy produced by nuclear power in the US as a whole equal that produced by firewood.
—Peter van Dresser, 1977
Although we’ve described and illustrated some 100 percent passive systems, generally the passive approach, unless super-insulated and with heat-exchange ventilation, will not be able to ride through more than two to three days of cloudy weather, and backup heating will be required. The question is how big this backup system must be. A good passive building in most temperate climates should be able to provide 60 to 90 percent of its heating needs, and the backup need is far less than the heating equipment of a standard building. This is an important point, for it is here that first cost savings can be achieved. It is important to recognize that the backup is just that, a smaller supplementary system.
Too often clients we’ve had can’t bring themselves to trust the passive approach enough to not demand a full-on supplementary conditioning system because they have no experience with living in a passive building. Most often such a large system is not well suited to the backup needs of a passive building.
A popular choice in residences is to ask for a radiant hydronic system in the floor slab. Such a system is the ultimate in comfort because, like the passive approach, a radiant slab is the most uniform and comfortable approach to mechanical heating. However, the radiant floor slab charged by a natural gas or fuel-oil boiler is not an ideal backup system for a passive building because it’s a slow-response system when what is needed is a fast-response system just for intermittent extreme events or cloudy periods. It’s also an expensive large system when a small-scale inexpensive system is all that is needed.
We’ve found that small natural-gas- or propane-fueled heaters do the trick even for commercial-scaled buildings. Usually, the smallest commercially available heater is all that’s needed, 30,000 to 50,000 BTUs/hour.
Other systems worth considering include installing a small solar hydronic heating system using the solar hot-water heater and its backup natural gas water heater as supply for radiators or a fan coil. With very low heating demand in super-insulated passive buildings, this type of system can work well. They are becoming more common in Europe. These systems also work well with district heating.
Another option is a hot-air system modeled on the Roman hypocaust. Hot air from a heater or solar system is used to heat internal walls and thermal mass. There is no risk of leaks (as there is in the hydronic system) in this approach.
Wood pellets and woodstoves are still a good option if they burn clean enough, but air pollution can be a problem if everyone uses wood or yard wastes for heat. Efficient wood fire heaters will allow the remaining demand to be met by a local small woodlot. This woodlot should be composed of a mix of trees, with preference given to nitrogen-fixing edible tree crops. With a sustained yield of 0.4 ton/acre/year, the need for heating can be met from less than ½ acre. Compost heat can also be used for supplemental heating.
The high-mass masonry stoves used in Russia, Finland, Korea, and other cold areas are excellent for winter comfort and are becoming more common in the United States and Canada. The fire in these high-mass systems can be burned hot and clean for a relatively short time; then the thermal mass will provide warmth for many hours. The drawback is that they are slow response and they are best used in a passive house in the far north, in very cold climates, or in situations with long cloudy periods. For further information, see Masonry Heaters: Designing, Building, and Living with a Piece of the Sun by Ken Matesz.
The primary focus in passive heating is hoarding internal heat gain and capturing the sun’s energy. The challenges include keeping this heat for use at night and on cloudy days and avoiding overheating on sunny days. These are not difficult, but take care and attention. Many of the solar homes built in the 1970s, flush with solar excitement, weren’t very good at controlling overheating or saving energy for use at night. They were overglazed, under-insulated, and often had insufficient mass. We know better today.
Although we have shown the spectrum of passive systems for heating for residential application in temperate climates, there is much to be said for simple, easy-to-operate passive designs. For most situations, we’ve found the solution is a very well-insulated building with direct gain and both distributed and concentrated mass. Many other options have been tried, including rock and gravel beds, massive underground water tanks, and many other strange and wonderful creations. These may be useful for some extreme sites, but they have proved more challenging to make foolproof and long-lived.
Larger buildings are a different animal; rather than heating, the need for lighting and cooling is dominant, so we refer you to chapter 3 on passive cooling and chapter 4 on natural lighting.