Passive Solar Designs for Northwest Homes

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1 Passive Solar Designs for Northwest Homes Written by Jack Brautigam EB1859e Solar heating systems are generally divided into two categories, active and passive. Active systems use collectors, pumps, or fans and require electricity to operate. Passive designs use windows and the building features to collect and store heat since passive designs minimize or eliminate moving mechanical parts, they are generally less expensive and easier to maintain than active systems. For this reason, passive designs are preferred by homeowners and designers. The purpose of this factsheet is to acquaint you with the basic principles of passive solar design. These can apply whether you are building a new home or retrofitting an existing home. Passive Solar Design: How it Works The Greeks knew that in winter the sun travels a low arc across the southern sky and in summer it passes high overhead. They built their homes with openings (they had no glass) on the south side for winter sunlight to enter. The main rooms were on the warm south side and sheltered from the north to keep out cold winter wind. In summer, overhanging eaves shaded rooms from the sun. These simple design principles served as the basis of solar architecture in ancient Greece. Today, passive solar design can be used to reduce the home's heating system requirements and save energy. It can be used in conjunction with other energy-saving approaches, such as earth sheltering or using higher than average levels of insulation, or it can be used alone. In Washington State, where winters tend to be cold and cloudy, combining higher than average insulation levels with passive solar design has proven especially effective. A home using this combined approach is sometimes called a "sun-tempered" home. WSU/CE Energy Program Library 925 Plum St. SE, Bldg. #4 P.O. Box Olympia, WA Tele:(360) Fax: (360) Spokane Office 1212 N. Washington #106 Spokane, WA Tele: (509) Fax: (509)

2 Energy Efficient Construction When thinking of building a new solar home or retrofitting an existing home, you should be concerned with the overall energy efficiency of the home. Regardless of the energy sources for your home, solar energy can make an effective contribution only when the home has been well insulated and air leakage is minimal. Energy conserving construction practices from the foundation to the roof are the keys to effective passive solar design. At a minimum, the home should follow statewide energy and ventilation codes. The Basics The primary objective in passive solar heating is to allow sunlight to heat the home interior while minimizing heat losses from the home. But the priorities you set will affect the way you achieve that objective in your home. In any passive solar design, you need to consider: Siting on the Property Energy-Efficient Construction Home Orientation Floor Plan and Space Use Window Area and Direction Window Treatment Heat Storage and Balance Siting on the Property Before designing passive solar features into your home, you should determine whether or not you have significant sun on your property during the heating season to warrant the effort. The mildest winter conditions are found on southern slopes, avoiding windy hilltops and low cold spots. After selecting a likely building site, note any existing or potential obstructions to the south, southeast, and southwest (such as trees, hills, and other buildings). If you see obstructions in those directions (or think they may exist in the future), a solar site survey can help you determine how each of these obstructions will affect the solar design. The survey can help you see where the sun strikes your property throughout the year. Home Orientation The overall orientation of the home should be due south or slightly southwesterly and elongated, if possible, along east/west axis. Most of the window area should be on the south side of the home. This serves two purposes. First, it gives a larger possible window area exposure on the south side, and second, it reduces the potential overheating exposure from the west in summer months.

3 A solar site survey can help you assess the amount of solar energy available to your home at different points of the year. Protect the south windows access to the winter sun by keeping evergreen trees and tall fences away from the south side of the building. Seasonal Sunpaths Homes are generally located on a site to maximize view and access roads and utilities. This is not always optimal for solar benefits. With a solar site survey in hand, you can assess trade-offs of solar benefits versus other property features. There are other site considerations that affect the comfort and efficiency of homes. While obstructions in the southerly directions reduce solar performance, obstructions to the north and west are an advantage. Hills or trees to the north shelter a home from winter winds. Obstructions to the west shade a home from the hot summer afternoon sun. Floor Plan and Space Use Consider space use and sun location when positioning rooms in the design. If you want to wake up or eat breakfast to morning sun, the bedroom or breakfast table must have east-facing windows. Major daytime living spaces will be warmest in winter on the south side. Closed space, stairwells, a garage or shop can buffer the home from cold northern and hot western exposures. Open floor plans with fewer doors and walls allow heat to circulate more readily through the home. It is also more spacious and less expensive to build. Open floor plans may sacrifice some privacy, however. Review your priorities when conflicts arise. Home Orientation The overall orientation of the home should be due south or slightly

4 southwesterly and elongated, if possible, along the east/west axis. Most of the window area should be on the south side of the home. This serves two purposes. First, it gives a larger possible window area exposure on the south side, and second, it reduces the potential overheating exposure from the west in summer months. Commit Your Resources Wisely As in any construction project, make sure you commit your resources wisely. You can probably get most % - - of the energy savings or other benefits you want by spending only 1/5 of the time, energy or money that it would take to get 100%. On the other hand if you insist on achieving 100% of the energy savings or comfort levels possible, you may end up spending a lot of your resources to make some very small gains. So, it's important to determine your priorities. For example, is comfort more important to you than the resale value of the home? This will affect the kinds of decisions you make when incorporating passive solar design into your new or existing home. Take a moment to rank the following: Comfort Aesthetics Resale Value Cost Efficiency Savings Window Orientation, Tilt, and Area Orientation While due south is the optimum direction, the southeast and southwest directions are 86% to 90% as effective. A small amount of glass on the east side is good for light and warmth on chilly mornings. Glass on the west should be avoided to reduce overheating from the summer afternoon sun. Minimize north-facing glass to avoid winter heat loss, but remember that north windows are best for diffused natural lighting and operable north windows provide the coolest breezes on a hot summer day. Tilt Horizontal skylights should be minimized because they lose heat in winter and contribute to overheating the home in summer. Avoid skylights on a south-sloping roof, as they may also be a source of overheating. Vertical clerestory windows are thermally superior (and more expensive) than skylights, although both provide good quality natural lighting for the home. Area The table below shows acceptable ranges for "glazing area," depending on the average degree days per year in your location.

5 (The glazing area refers to the ratio of glazing area to total floor area. "Glazing" refers to transparent materials, such as glass and plastics, used in window systems.) In a typical light frame home, with no additional means of heat storage, you can include as much glazing as shown in the low end of the ranges, without risking overheating. In a home with some additional means of heat storage, you can include more glazing, as much as is shown at the high end of the ranges. But increasing the glazing area above these maximums won't improve solar heating performance. In fact, performance will decrease with higher nighttime losses and increased overheating. Degree Days Per Year Average January Temperature % Glazing Area (a) Acceptable Range 4, % -10% 5, % - 12% 6, % - 12% 7, % - 14% (a) Refers to the ratio of glazing area to total floor area. Window Treatment Night Insulation In Washington State, double-pane vertical south-facing windows can have an average net gain of heat to the home throughout the heating season. The addition of an insulated window covering at night or during cloudy weather greatly improves the performance of double glass. In fact, it is essential for good performance from passive heating systems in the coldest months. The difficulty is the window insulation must be operated by the occupant if it is to be effective. Automatic, motor driven shade and shutter systems are expensive and have not proven to be reliable. High-performance Windows Recently, high-performance glazing systems have been introduced that give performance equivalent to, or better than, double glass with night insulation. Use insulating shades, drapes, or shutters (interior or exterior) to reduce night heat losses through glass. Some window systems use plastic film instead of glass for the inner layers of three- or four-pane units, resulting in lower weight than all-glass units, and improved solar heat gain. Other systems use a special "low-emissivity" coating (Low-e) or special gasses between the panes to reduce heat loss from the room. High-performance glazings hold great promise for improving the performance of passive solar systems. They are more expensive and lack the proven long-term reliability of regular glass, but offer an alternative to the problem of operating a night insulation system. Shading Overheating in the summer has been one of the most frequent Use double pane windows to add insulation.

6 problems with passive solar structures. Shading devices offer one solution. Shading from the exterior keeps sun out of the structure altogether and is preferable to interior shades such as venetian blinds. Horizontal overhangs over vertical south windows reduce unwanted summer heat gain. Adjustable overhangs or awnings are preferable to allow fine-tuning during the year. Slanted south glass is difficult to shade with overhangs and will require roller shades on the exterior or other methods of control. Horizontal or vertical louvers are required on east and west windows to block the low morning and afternoon sun. Deciduous plants and trees, or vines on a trellis in front of windows can be a part of your shading system. They leaf out in summer to block sun and lose leaves when temperatures drop-allowing light and warmth into the home when you need it. Heat Storage and Balance Additional thermal mass can prevent overheating and store heat for use at night. Masonry, such as concrete, brick, tile, and adobe, and water are the most commonly used forms of thermal mass. These materials, when dark in color and exposed to sun, act like a thermal battery. Heat is absorbed during the day and released at night to the home. This reduces temperature fluctuations. Every home contains some thermal mass in walls, floors, ceilings, furniture, etc. For this reason, adding thermal mass for heat storage is not required in well insulated, light frame houses when the south-facing window area is at the low end of the percentage range shown in the preceding table. If the window area is at the high end of these percentages, additional thermal mass should be added to increase comfort and energy performance. The goal is to add thermal mass until only mild overheating occurs on an average sunny winter day. As a general rule, for each 1 square foot of south glass beyond the percentage shown, add 5 square feet of dark colored masonry, 4" thick, where it will be exposed to direct sun as long as possible during the day. Plant deciduous trees which allow the winter sun to reach the house walls and windows. Increasing the thickness beyond 4" does not improve performance substantially. As the amount of south glass increases, it gets more difficult to find enough room for the area of mass required to store the heat gained. Also, with the large glass areas, night insulation or improved glazing systems may become necessary to reduce heat loss. Design Options Direct Gain The figure on this page illustrates a direct gain home, which is the most straightforward approach to passive solar design. Any home

7 with a south-facing window is a direct gain space. Window area is concentrated on the south side to allow sunlight into the interior of the home (note glass area limitations and mass requirements above). This sunlight becomes heat as it is absorbed by the walls and floor. Using the existing walls and floors of the home for heat storage, rather than providing additional thermal mass, lowers construction costs relative to other passive designs. Direct gain is simple, maximizes view potential, and provides excellent natural lighting. Some disadvantages of direct gain include fading of fabrics and possible damage to finishings from the ultraviolet rays in sunlight, potential lack of privacy, and greater indoor temperature fluctuations than in other passive designs. Glare can also be a problem where glass is installed predominantly on one wall. Glare can be minimized by balancing the natural light with skylights, clerestories, or windows on other walls. Build tight window and door openings to permit the ventilation necessary in summer but reduce winter infiltration. Solar Design Adapts to the Pacific Northwest Books on solar design written before 1985 are likely to emphasize the "high glass, high mass" approach that has been so successful in warmer southern climates. This factsheet attempts to emphasize higher insulation levels with more moderate glass areas. Often called the "sun-tempered" approach, it has emerged as the more cost-effective form of energy-efficient construction in the Pacific Northwest. Build tight window and door openings to permit the ventilation necessary in summer but reduce winter infiltration. Summer Shading Design overhangs or adjustable louvers that provide summer shade but admit the winter sun. Indirect Gain The indirect gain design places a wall of water or masonry directly behind the home's south-facing window area. This is not a recommended approach for residences in most areas of Washington State because they tend to be less efficient in climates with sustained cloudy conditions. They perform best in very sunny climates. Design overhangs or adjustable louvers that provide summer shade but

8 As with other passive designs, either high performance glass or insulated curtains are important for most efficient performance. The mass wall absorbs solar radiation during the day and slowly conducts that heat through the wall to the living space at night. If the thermal storage wall is composed of masonry it is often referred to as a "Trombe Wall," after its inventor Felix Trombe. With this system, large areas of glass can be used -- on the order of 1 square foot of glass (and associated mass wall) per 1 square foot of floor area to be heated. In Washington State, Trombe walls are usually 6" to 10" thick. Water walls have been made from steel drums, culvert pipe, 5-gallon cans and other leakproof materials. In some cases, small vent areas at the top and bottom of the wall are constructed to allow natural convection of warm air to the room and a quicker warm-up in the morning. admit the winter sun. The thermal storage wall is attractive to some homeowners because it results in less indoor temperature fluctuations than direct gain does, and solves the problems of privacy and sunlight fading of materials. However, as noted above, they perform best in sunny climates. They can also cost more than direct gain, may restrict views to the south (though thermal walls can have windows) and limit the depth of living space (if effective radiant heating from the wall is to be achieved). Window insulation systems can be difficult to operate and maintain behind a thermal wall. Isolated Gain Greenhouses and sunspaces are often described as "isolated gain" structures because the collection area can be closed off (isolated) from the rest of the house. This prevents the collection area from becoming a source of unwanted heat gain to the home in summer, or a source of heat loss during cold and cloudy weather. When integrated into the south wall of a home, the greenhouse or sunspace becomes a solar collector absorbing the sun's energy. Heat transfer to the home can be achieved by natural convection, or with a fan to draw heated air through vents, doors, or windows. Greenhouses and sunspaces can be designed into new home construction or added on to the south wall of existing homes. Greenhouses and sunspaces designed with only a vertical south wall of glazing can perform well in Washington State. A vertical design is easier to build, waterproof, and insulate, and offers better headroom inside the structure than a sloped design. Sunspaces are primarily designed to provide a pleasant living space. Greenhouses are primarily designed to produce plants or food and to maximize solar energy gains. Because of these distinctions, design elements, such as glazing, insulation, venting, and light and temperature requirements, are dealt with differently depending on the purpose of the space. Suggested Reading

9 Passive Solar Energy, Anderson, Bruce and M. Wells, Brick House Publishing, Andover, Massachusetts, An excellent introduction to passive solar energy use. Solarizing Your Present Home, Carter, Joe, Rodale Press, Emmaus, Pennsylvania, A complete homeowners guide to energy conservation including details of building solar collectors and greenhouses. The Passive Solar Energy Book, Mazria, Edward, Rodale Press, Emmaus, Pennsylvania, A primer on passive solar energy with rules of thumb for construction and sizing glass/mass. Fine illustrations. Passive Solar Design Handbook, Balcomb, Douglas, U.S. Government Printing Office, # /25. The bible of passive design. Technical and thorough with climatic data for 219 U.S. and Canadian locations. Superinsulated Design and Construction: A Guide to Building Energy Efficient Homes, Lenchek, Mattock, and Raabe, Van Nostrand Reinhold,1986. An up-to-date guide to low energy construction techniques. Washington State University Cooperative Extension publications contain material written and produced for public distribution. You may reprint material, provided you do not use it to endorse a commercial product. Please reference by title and credit Washington State University Cooperative Extension. Alternate formats of our educational materials are available upon request for persons with disabilities. Please contact the Information Department, College of Agriculture and Home Economics. Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Replaces EB0950. Published June Subject Code 669