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Dug: There are a number of books out there on this topic. I will see if I can post some text I started writing a few yrs ago but never finished. I have a summary for solar home design, but it is on another computer. Let me know if you are interested. Doug Stockman
Penfield, NY Building a Passive Solar Home Introduction
During the oil crisis of the 1970’s many Americans built homes that relied less on non-renewable energy products like heating oil and more on free renewable energy sources like solar. At that time many solar designs were tried. Most of them did not stand the test of time. Active solar systems that collected solar energy in one place and then moved it to another place for use proved less than reliable. By the late 1990’s and the early twenty first century, passive solar home designs became the norm. Passive solar homes do not require moving energy by any mechanical means. By using a few simple rules when designing and building a house, reliance on fossil fuels can be reduced by 10-100%. This text introduces the reader to the design principles for creation of a passive solar home. The home shown throughout this book is our own home that we built during 1999-2000. Although our style choices may not be for others, the passive solar principles remain the same. Whether you are building a two room log cabin or a 5,000 square foot mansion, the ideas remain applicable. Our goals in building this home were to create a pleasant living environment that used less energy for heating and cooling than a similarly sized typical builder’s special. In much of America, subdivisions bursting with new homes are created every day that ignore simple design considerations that would greatly reduce energy consumption. In many of these tract homes, the sun is seen as an enemy to be kept at bay. The result is higher energy costs in a less comfortable environment. Although having a fair amount of land is helpful when collecting solar energy, it is not required. Even tract homes could be built to take advantage of the sun’s energy. This would require builders planning ahead. Environmental purists might balk at the idea of new home construction. In many ways, building a new home requires huge amounts of energy. Maybe even more energy to build than can be saved by the energy-saving techniques over the life of the new home. The creation and transportation of raw materials is huge. Just think of all the fossil fuels required to harvest timber, process the timber into lumber, dry the wood, then transport the processed lumber to your site. The same use of non-renewable energy occurs for almost every building material. Therefore, the purists can make a case for everyone adapting older homes or living in apartments in cities. This approach would definitely reduce the use of resources. Fortunately, or unfortunately depending on your perspective, America offers a great deal of freedom to choose how you want to live. In the real world people will build homes where ever and however they want. Many people, me included, enjoy open spaces away from neighbors. Therefore, since many people will choose to live outside cities, it makes sense to build an energy efficient house in place of an inefficient house. The use of building materials will remain about the same for both houses.
Basic Design Principles
Many methods exist to build a comfortable home that reduces reliance on exhaustible energy. This book mainly addresses methods applicable to temperate climates in North America. Our home was built near Rochester, NY. Most of the design ideas will work in areas that experience a true winter season. Some of these concepts are less useful for warmer climates. For those who live in the southern hemisphere, all references to North and South must be reversed.
Below is a list of solar principles. Although the principles are straight forward, their implementation can vary greatly. This text cannot provide a step-by-step guide to passive solar home design. Rather, many concepts and suggestions are put forth. The home owner will pick and choose from the examples to create a structure that meets their passive solar needs. It is definitely desirable to create novel solutions that meet the basic solar collection principles. For example, I had never heard of any passive solar home using SpanCrete for a flooring material. I knew I needed thermal mass to store solar energy. The SpanCrete was my unique answer for a ready-made floor with high heat storage capacity. How aggressively solar energy is collected is up to the individual. The more aggressively solar energy is collected and stored, the less energy from non-renewable sources will be required. The trade off is in the cost of construction and the day to day work the home owner must do to collect and store that energy. There is a lower threshold to this relationship. Applying very basic solar principles will cost no more than “normal” home construction. But for those who want a larger percentage of their home heating energy to come from the sun, additional cost may be incurred, or changes in home design must be used. In general, the larger the contribution of home heating that comes from the sun, the more the construction costs and the more work the home owner must perform after the initial threshold is passed. This concept will be addressed throughout the text as we discuss solar design options. That being said, passive solar homes do not have to cost much more than non-solar homes and the extra cost can definitely be recovered by reduced heating bills over the life of the house. In most situations, it is not possible to get all home heating energy from only the sun. All renewable sources of energy are intermittent in nature. There will be cloudy periods when minimal solar energy is available for collection. There is also a practical limit to how much heat energy can be stored during sunny periods for use during non-sunny periods. It is not unusual in Rochester, NY to have thick cloud cover for more than a week at a time. To store enough heat energy to last more than a week of clouds is unreasonable. Therefore, an alternate heat source must be available. Our goal in Rochester is to reduce our reliance on non-renewable energy by 50%. To reduce that amount further would be difficult. Other more sunny areas may be capable of reducing non-solar energy demands by even 80-90%. To reach full 100% reliance on solar heating is quite difficult. This 100% reliance on solar heating was one of the downfalls of many of the solar home designs of the 1970’s. The cost to build and operate these 100% solar homes was high. In addition, during cloudy periods these 100% solar homes became cold and uncomfortable. They also often looked different than “normal” homes. These facts created the belief among the general population that all solar homes are inherently unreliable and often cold. By not chasing the magical 100% solar heat goal, a normal appearing house that is comfortable to live in and requires minimal home-owner effort is a reasonable goal. For example, our house appears like a typical suburban home from the Northeastern United States. The casual observer would not know our house is a solar home. In addition to solar heating, we use a high efficiency wood stove and a natural-gas fired boiler that powers a radiant floor heater. The radiant floor system allows the house to never get colder than a pre-set minimum. On days when the sun does not warm the house enough for our comfort, we can either fire up the wood stove or raise the thermostat on the radiant floor system. Collecting solar energy reduces our dependence on firewood and natural gas heat, but we have the luxury of always having a warm home.
Solar Principle #1
Orient the house properly with respect to the sun. The long axis of the house should face true South. A variation of about 15 degrees either way from geographic South is an acceptable compromise. Going much beyond that will reduce solar heat gain and may cause overheating. One problem many people face is when a desirable vista is North or West of the house. Hard choices need to be made regarding window placement. A compromise is possible, but both the view and the solar heat gain may be less than ideal.
South
Figure 1
The long axis of the house runs in an East-West direction so that the widest part of the house faces true South
Magnetic South and geographic or true South are in most cases not the same thing. For example, Rochester’s magnetic declination is approximately 8 degrees West of true North. When facing South, take a magnetic South reading, then swing 8 degrees to the West to find true South. Another method to achieve the same result is to place two sticks in the ground along the shadow line at the exact middle of the day. Those two sticks define the line to true South.
Magnetic North Geographic North .
Solar Principle #2
Design on a 12 month basis. Build with appropriate window overhangs so sun penetrates deeply into the house in winter, and not at all in the summer. The fact that the sun is low in the Southern sky during the Northern hemisphere’s winter and directly overhead in summer is a very important point. When appropriately designed overhangs are used, almost the entire inside of the house in bathed in direct sunlight in December, and no direct sunlight enters the house in June. Having large windows on the East or West sides of the house can defeat this feature. The result is usually an overheated house in the summer. One adaptation to allow large windows on the East and West sides of the house is to use deciduous trees on the East and West to block morning and evening summer sun. In the winter when the added solar insolation is welcome, the deciduous trees have dropped their leaves to allow sunlight into the house. Use evergreen trees, outbuildings, hills, or other natural barriers on the North to block cold winter winds.
Figure 2
Elevation of the sun in the Southern sky at various times of the year
Figure 3
An ideal passive solar orientation with open space to the South, Large evergreens to the North, and deciduous trees to the East and West.
Solar Principle #3
Provide effective thermal mass to store free solar heat in the daytime for nighttime use. As sunlight enters the home through South-facing windows, the sunlight will warm whatever it falls on. If the sunlight falls on surfaces with minimal thermal mass, there is no place for the excess heat to be stored. The temperature of the surface with the low thermal mass rapidly rises. This high temperature surface then transfers the heat quickly to the surrounding room. The result is an overheated house on sunny days.
If the sunlight falls on a surface that can absorb the heat and has adequate thermal mass to store that heat, the surface of the high thermal mass structure stays fairly cool. Excess heat is transferred to the excess thermal mass. This means the surface of the thermal mass does not get too hot because all the heat is getting stored in the thermal mass. Once the sunlight stops hitting the high thermal mass surface and the room temperature begins to drop, the stored heat in the high thermal mass surface will gradually return the stored energy back to the room. The laws of nature, i.e. thermodynamics, guarantee that all heat stored in the thermal mass cannot just disappear. It must be released sooner or later back into the room. Water is about twice as efficient as concrete at storing heat. The attraction of concrete is that it can be part of the house’s normal structure (floor, walls, etc.). A balance between window size/solar insolation and thermal mass storage must be achieved. If too much thermal mass is used, money is wasted on building materials that can serve no purpose. If not enough thermal mass is used, the house will overheat. Thermal mass can also be used to store nighttime coolness for release during the heat of the day in the summer (see Solar Principle #8). The PSIC BuildersGuide computer program can calculate the thermal mass size once the dimensions, including window areas, are known. Solar Principle #4
Insulate thoroughly and use well-installed vapor barriers. Keep whatever heat enters the house inside the house by super-insulating. The house should be sealed tightly to avoid air infiltration. The goal is to have the house built so well and tightly that an air heat exchanger is required. Use air lock entrances on at least the most commonly used entrance, if not all entrances. The goal for house “tightness” is less than 0.35 total air exchanges per hour. At this level you will need a heat exchanger. Sidebar
A basic principle of thermodynamics is that heat will attempt to distribute itself evenly. If a metal bar that is 110 degrees is placed on another metal bar that is 50 degrees, the warmer bar will transfer heat to the cooler metal bar until both bars achieve the same intermediate temperature. If the inside of a house is heated to 70 degrees while the outside temperature is 0 degrees, the natural tendency is for the heat to flow from the high temperature inside to the low temperature outside. When talking about house design, two main factors determine how quickly the heat moves from the warm inside to the cold outside. The first factor is the amount of insulation between the warm and the cold. How well insulation (or any building material) resists the movement of warm to cold is called the R-value. The higher the R-value the better an insulator is at slowing the movement of heat from the warm inside to the cold outside. Modern homes built in cold climates generally have walls containing insulation of at least R-19 and roofs of at least R-40. The second factor that determines how quickly heat is transferred from the warm inside to the cold outside is based on how leaky the house is. If doors and windows do not fit well, gaps will exist where warm air leaks out and cold air leaks in. The same leaky situation can exist where any two surfaces meet. For example, between the walls and the roof gaps may exist that allow the free passage of air. The degree to which a house leaks in measured in the number of air exchanges per hour. The total volume of air that can be contained in the house is measured. Then a measure is taken of how much air can escape over an hour. A “tight” house will allow less than 35% of its total volume of air to escape every hour. This value of leakiness would be stated as “0.35 air exchanges per hour” and is calculated by a blower door test. When a house is this tight, an air heat exchanger is usually required. This will be discussed more fully in the insulation chapter. Solar Principle #5
Use windows as solar collectors in the winter and cooling devices in the summer. When combined with effective thermal mass, the South facing windows can be up to 12% surface area of the floor’s surface area. This means that if the main floor is 1,000 square feet, the southern exposure windows on the main floor can equal up to 120 square feet. South facing windows let in solar energy. If the south-facing windows let in more energy than can be stored by the available thermal mass, the house overheats. This means reduced comfort for the homeowners. Many times the sweating homeowner will then be forced to open a window in the middle of the winter. Sitting near an open window in winter is often not a pleasant situation. The North-facing windows should be less than about 4% of the floor surface area. In the Northern hemisphere, North-facing windows allow no solar energy into a home. They allow heat to escape. The R value of a typical Low-E window is around 4. The typical insulated wall has an R value of around 20. Therefore, any North facing window allows heat to escape easily compared to the surrounding walls. To make matters worse, in the Northeast USA the predominant winter winds come from the North and West. This increases heat loss through the North-facing windows. North-facing windows should be kept small and only used where interior lighting or code requirements demand them. East and West facing windows can potentially cause more comfort problems than North-facing windows. This is because morning and evening sun comes in through East and West-facing windows respectively. In the winter this is not a bad thing. But in the summer, the result is generally overheating. Image a house in August which is 78 degrees inside. Then imagine a person standing in front of a West-facing window at five PM. The strong summer sun is beating down on the person making them think it is 100 degrees inside. To avoid these large swings in temperature, the East and West facing windows can be in the 4% range. This 4% figure can be increased if deciduous trees are used to block morning and evening summer sun. The trees lose their leaves in the autumn and allow East and West windows to collect winter sun, but block the collection of solar energy at other times of the year. When large trees cannot be planted, minimize East and West window surface area to 4% or less. In this situation, the East and West-facing windows two main purposes are to illuminate the home’s interior and to allow cross ventilation during the warmer months. As stated previously, windows have a low R-value. Therefore they lose heat quickly. During sunny periods, the large window surface area may allow more heat energy to be collected than is lost during the night. But during cloudy periods, large window surface areas may allow more heat to escape than is collected during overcast days. To avoid this excessive heat loss during nighttime and extended overcast periods, window coverings that have insulating properties should be used. Unfortunately, commercially available window treatments that are attractive and have high R-values remain elusive. This topic is discussed more fully in a later chapter. The type of window you choose can make a big difference in energy efficiency. Consider using low-E windows to reduce heat loss. These newer windows are double paned, gas filled, and often have a special membrane between the glass panes that reduce heat transfer. A Low-E window has an R-value of around four. Although this does not seem like much, it is double the R-value of a normal double pane window. One problem with these specially treated windows is they can reduce the transmission of solar radiation. Placing Low-E windows on the South side can significantly reduce the amount of solar energy collected. Using low-E windows only on the North, East, and West facing windows may be the most intelligent decision. Solar Principle #6
Do not over-glaze. This cannot be overstated. Remember that the ultimate goal is to have a comfortable home that reduces reliance on non-renewable energy sources. Over-glazing generally results in large swings in temperature. Stick closely to the rules stated in Solar Principle #5. Avoid skylights whenever possible. South-facing skylights increase solar collection in both the winter and the summer. They lead to summer overheating unless a method is available to stop all solar collection during the warmer months. North-facing skylight have no benefits with regards to reducing energy consumption. They do not collect solar energy during the winter, but they allow excess heat loss when compared with an R 40 roof. They do allow heat collection during the summer when excess heat is not wanted. The result is the exact opposite of the stated goal. Solar Principle #7
Do not over or undersize your active heating and cooling systems. Take into account the super-insulated characteristics of the house. An oversized furnace or air conditioner will cycle on and off frequently wasting energy. Given the intermittent nature of solar energy sources, size the furnace to provide all the required heat in the event the solar heating addition is negligible. Solar Principle #8
Provide fresh air to the home without compromising thermal integrity. The house should be well constructed so that unwanted air infiltration is minimal. Then design systems such as air heat exchangers which precisely control the number of air exchanges per hour. At least 2/3’s of a home’s total air volume should be exchanged every hour. Excessive air moisture should be quickly vented to the outside. Therefore, bathrooms require fans that move steamy air directly outside. During summer months in temperate climates, ceiling/attic fans can effectively cool a house. During the daylight hours, a small attic fan vents hot attic air to the outside and replaces it with cooler outside air. During the night, a larger fan vents air from the inside of the house into the attic. This brings cool outside air in through open windows and vents the warmer house air into the attic which vents the hot attic air outside. The cool night air being drawn in through open windows cools the thermal mass. During the next day, the thermal mass slowly releases the coolness gained during the night into the home’s interior. Solar Principle #9
Passive solar homes do not require different or unusual building materials. Common standard building materials are used in slightly different ways to achieve energy efficiency and solar thermal gain. Solar Principle #10
Passive solar design principles can be used in diverse architectural styles. Complete independence from fossil fuel-based heating and cooling methods is often not the goal. By carefully applying various passive solar techniques where architecturally appropriate, a reduction in heating/cooling bills can vary from 10-100%. Some reduction in dependence on fossil fuel heating/cooling is better than none. Land Selection and Orientation Thermal Mass
Solar Glazing
Insulating and Wall/Roof Construction
Heating and Cooling
Example Home
This Chapter discusses some of the choices made on our own house.
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