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Using the sun as a heat source is nothing new. Socrates observed 2,400 years ago:
“Now in houses with a south aspect, the sun’s rays penetrate into the porticos in winter, but in the summer, the path of the sun is right over our heads and above the roof, so that there is shade, If then this is the best arrangement, we should build the south side loftier to get the winter sun and the north side lower to keep out the winter winds.”
While the house that Socrates described probably lost heat as fast as it was collected, the Romans discovered that if the south-facing windows were covered with glass, the solar energy would be trapped, causing the internal temperature to stay high well into the night. This simple phenomenon is called the “greenhouse effect.” Today we call a house that uses the greenhouse effect a “passive solar house.”
It is a common rule of thumb that, compared to a conventionally designed house of the same square footage, a well-designed passive solar house can reduce energy bills by 75 % with an added construction cost of only 5 to 10 %. In many parts of the United States, passive solar houses don't require any auxiliary energy for heating and cooling. Given current and future projected fuel costs, the additional construction cost is recovered quickly.
Like the super-insulated house, this style of building has some distinctive design features.
1. Most of its windows face south. Sunlight passes through these windows and is absorbed by materials inside the house. The heated surfaces of these materials reradiate the heat energy to warm the house.
2. Ideally, the materials that the light strikes are high-density “thermal mass,” such as concrete, brick, stone, or adobe. These materials, because of their ability to absorb energy rapidly and reradiate it slowly, can prevent the house from overheating while the sun is up and they help keep the house warm when the sun is down.
3. Architects and builders used to keep window areas on the east, west, and north sides of the house as small as possible. This is still the general rule of thumb, but the introduction of high-tech window glazings lets us relax this rule. Still, house plans with long south walls are best for passive solar houses.
4. Passive solar homes tend to be well insulated and have reduced air leakage rates, keeping the solar heat within the building envelope.
5. Since auxiliary heat requirements are greatly reduced in a passive solar home, small direct-vented units or wood stoves are often the heaters of choice.
6. Passive solar homes often have open floor plans to facilitate the movement of solar heat from the south side of the house through the rest of the house. Sometimes small fans are used to aid warm-air distribution, especially in houses having floor plans that are not fully open.
Passive Solar Techniques
The heating techniques used in various types of passive solar houses are essentially easy to comprehend. In general, all passive solar houses can be divided into two categories: those that use “direct-gain” heating and those that use “indirect-gain” heating.
ill. 4-9 (right and left): Reconstruction of Socrates’ house as described in Zenophon ‘s Memorabilia.
Direct-gain houses, considered to be the simplest type, rely on sunlight shining directly into the living spaces through south-facing windows. While some of the heat is used immediately, walls, floors, ceilings, and furniture store the excess heat, which radiates into the space throughout the day and night.
ill. 4-10: Potential for passive solar heating in the United States.
ill. 4-11: A direct-gain passive solar house. (Design by Dennis Holloway, architect.)
ill. 4-12: Internal mass storage walls serve as north-south partitions between direct-gain spaces (a) and as east-west partitions between direct-gain sunspace and north clerestory space (b).
J. Douglas Balcomb and the research team at Los Alamos National Laboratory recommend that thermal mass be spread over the largest practical area in the direct-gain space. it's preferable to locate the mass in direct sunlight: Heat storage is as much as four times as effective when the mass is located so that the sun shines directly on it. The recommended ratio of mass surface to glass area is 6 to 1 (6 square feet of thermal mass surface for each square foot of window surface).
Locating thermal mass in interior partitions is more effective than putting it in exterior partitions, because heat can radiate from both sides of an interior wall. Thin mass is more effective than thick mass. The most effective thickness in masonry materials is the first 4 inches—thickness beyond 6 inches is pointless. The most effective thickness in wood is the first inch. Thermal mass should usually be a dark color to improve its heat absorption.
In northern climates, movable insulation in the form of insulating drapes, panels, shutters, or quilts often is used to cover the inside of the windows on winter nights to reduce heat loss. These items can also help ensure privacy, which may be at a premium since passersby can easily look through the same windows that the sun shines through. During the summer, the insulation can help block unwanted solar heat, although exterior shading devices (such as canvas awnings) will work better for this purpose than interior devices. Deciduous trees and shrubs planted to cast shadows on solar-oriented glazing can shade the windows in summer. When the leaves drop, the winter sun can shine into the house.
A popular direct-gain heating strategy is the solar greenhouse or sunspace. Many homeowners claim this room becomes the favorite space in the house with its spacious outdoor feeling. The sunspace/greenhouse can, if properly designed and sited, provide as much as 50 % of the house’s heating requirements. In this situation, living spaces are better located on the south side with spaces not requiring as much heat (like bedrooms) on the north. Clerestory windows can be used in larger houses where it's important to get sunlight into the northside rooms.
If you plan to include a sunspace in your design, you’ll first need to decide on the primary function of the space. Your primary goal may be to have a space to grow plants, or it may be to gather as much heat for the house as possible, or it may be to use the space as living space. it's possible to build a sunspace that will serve all three functions, but compromises will be necessary.
• Growing plants: A sunspace intended primarily for plants will not provide much heat for the home. Much of the solar heat entering the greenhouse will be consumed by the plants themselves and the evaporation of water. One pound of evaporating water uses about 1,000 Btu of energy that would otherwise be available as heat.
Also, the sunspace will need to be ventilated. To stay healthy, plants need adequate ventilation, even in winter. There are air-handling systems such as air-to-air heat exchangers that ventilate while retaining most of the heat in the air, but these add significantly to the cost of the project.
The light requirements of a space for growing plants call for overhead glazing, which complicates construction and maintenance, and glazed end walls, which are net heat losers.
• Solar heat collector: If the purpose of the sunspace is to collect solar heat and distribute it effectively to the adjacent living space, you’re faced with a different set of design criteria. Maximum gain is achieved with sloped glazing, few plants, and insulated, unglazed end walls.
ill. 4-13a: One-story sunspace: winter sunspace cut off from the house (Section A); winter sunspace helps heat the lower story via open doors (Section B); summer sunspace helps cool the lower story by pulling in air from the north windows (Section C).
ill. 4-13b: Two-story sunspace: winter, sunspace cut off from the house (Section A); winter sunspace helps heat both stories of the house (Section B); summer sunspace helps cool both stories (Section C).
ill. 4-14: Sunspace with sloped south-wall glazing over reverse-slope vent windows (a). Sunspace with vertical south-wall glazing (sliding door), side venting windows, and sloped roof glazing (b).
ill. 4-15: Sunspace thermal storage (a). Provide 3 square feet of concrete (b) or 3 gallons of water (c) for each square foot of glazing.
You’ll get more usable heat into your living space if there aren’t plants and lots of mass soaking it up in the sunspace. Sun-warmed air can be moved into the house through doors or operable windows in the common wall, as well as blown through ductwork to more remote areas.
• Living space: If your sunspace will be used as a room, you’ll need to consider comfort and convenience in addition to energy efficiency. A room you plan to live in must stay warm in the winter and cool in the summer; it must have minimum window glare; and humidity must be moderate. It should also have carefully sized thermal mass to keep temperatures moderate both during the day and at night.
Vertical glazing may serve you best for this type of sunspace. First of all, although sloped glazing collects more heat in the winter, it also loses significantly more heat at night, which offsets the daytime gains. Sloped glazing can also cause a sunspace to overheat in warmer weather. Vertical glazing is cheaper and easier both to install and to insulate, and it's not as prone to leaking, fogging, breakage, and other glazing failures.
ill. 4-16: Tromble-wall house with attached sunspace, near Lyons, Colorado.
The second passive solar house type, indirect gain, collects and stores solar heat in one part of the house and then distributes this heat to the rest of the house. One of the more ingenious indirect-gain designs is called the Trombe wall. Named after its French inventor, the wall is constructed of high-density materials—masonry, stone, brick, adobe, or water-filled containers—placed 3 or 4 inches inside an expanse of south-facing glass. The wall is painted a dark color to more efficiently absorb solar radiation.
Heat collected and stored in the wall during the day slowly radiates into the house even up to 24 hours later. The Trombe wall allows efficient solar heating without the glare and ultraviolet light damage to fabrics and wood trim that is common in direct-gain solar houses. Trombe walls also afford privacy—passersby can’t see through the masonry.
Some designers use “selective surface” materials, chrome-anodized copper or aluminum foils with adhesive backing, which can increase the absorptive efficiency of the wall to 90 %, compared to 60 % for a painted surface. These materials drastically reduce the amount of heat that the wall loses to the outdoors at night.
In several of the earliest Trombe-wall houses, small vents were used in the top and bottom of the wall: Heated air in the wall’s air space would rise and pass through the upper vent into the house, while cooler room air would be drawn into the wall air space through the low vent. This is particularly effective in a building where heat is required quickly. However, the convective movement of air in the wall results in a significant decrease in efficiency over time due to the accumulation of dirt on the inner glass surface. Vented Trombe walls are known to be only about 5 % more efficient, overall, than non-vented Trombe walls. Therefore, we generally recommend non-vented Trombe walls.
Designing a Passive Solar House
When the term “passive solar” was introduced in the 1970s, most people thought that if they wanted to build a passive solar house, they would have to hire not only an architect but a solar engineer capable of manipulating complex mathematical equations on a computer. Today, thanks to knowledge gained from a large number of completed “pioneer” passive solar houses, we are at a stage where even a high school student can design a passive solar structure.
ill. 4.17: When designing a solar home, you must locate true (solar) south, not magnetic south. This map shows how magnetic south varies from true south in different parts of the United States.
In 1983, J. Douglas Balcomb and the research team at Los Alamos National Laboratory issued a set of design guidelines for heating passive solar houses. The five-step technique they devised gives owner-builders a solid basis for basic design decisions.
Step 1: Locate your building site on the map in figure 4-18. This will give you the conservation factor (CF) to be used in your house design. Note that for each geographic zone, the CF is expressed as a range. If your fuel costs are high, select the highest number.
Step 2: Use the following formulas to determine insulation values and recommended air-leakage rates for the house.
Wall R-values: Multiply the CF by 14. This is the R-value for the entire wall, including insulation, siding, interior sheathing, etc.
Ceiling R-values: Multiply the CF by 22. This is the R-value for the entire ceiling, including insulation, finish surface, etc.
R-value of rigid insulation placed on the perimeter of a slab foundation:
Multiply CF by 13. Subtract 5 from this number. Use the same value for the insulation of the floor above a crawl space or for the perimeter insulation outside an exposed stem wall.
R-value of rigid insulation applied to the outside of the wall of a heated basement or bermed wall: Multiply CF by 16. Subtract 8 from this number. Use this value for insulation extending to 4 feet below grade. Use half this R-value from 4 feet below grade down to the footing.
Target ACH: Divide .42 by the CE If the result is lower than 0.5 ACH, choose tight super-insulation techniques with controlled ventilation to maintain indoor air quality.
Layers of glazing on east, west, and north windows: Multiply the CF by 1.7, then choose the closest whole number. (If the number is 2.3, choose windows with two layers of glazing; if the number is 2.6, choose windows with three layers.) If the number exceeds 3, explore insulating glass and /or movable insulation.
Step 3: Next, compute a net geometry factor (GF) in table 4-1. Nearly 3,000 square feet on three load coefficient (NLC). To do this, look up your home’s For example, if the house will have a total floor area of stories, the CF will be 5.7.
ill. 4-18: Use this map to find your conservation factor (CF).
Now multiply the CF by your house’s floor area. Thus, if the floor area will be 2,900 square feet and the CF is 5.7, you multiply these two values to get 16,530. Finally, divide this result by the CE If your CF is 2.0, for example, you would divide 16,530 by 2 to get 8,265. This is your NLC.
Step 4: Locate your building site on the map shown in figure 4-19. This will give you the load collector ratio (LCR) for your home. Note that for each geographic zone, the LCR is expressed as a range. If your fuel costs are high, select the lowest number.
Step 5: To determine the area of a passive solar collector (Trombe wall, sunspace, etc.) for your home, divide the NLC (the number you got in Step 3) by the LCR (the number you got in Step 4). For example, if your NLC is 8,265 and your LCR is 20, then your passive solar collector should have 413 square feet of south-facing glazing. You can round this number up or down by 10 % (so the area could be as small as 370 square feet or as large as 450 square feet). In hot climates, the area should be adjusted downward by 20 to 30 %.ill. 4-19: Use this map to find your load collector ratio (LCR).