Monday, March 7, 2011

Passive Solar Design- Living in Harmony with Your Climate

Solar photovoltaic panels while extremely cool to have are a very expensive way to make electricity. Passive solar design on the other hand can be cost effective, but few of us build homes. Most people buy a pre-existing house, but passive solar and energy efficiency should be considered in selecting a home. Passive solar design is not new, it was used by ancient civilizations to make their living more comfortable in whatever climate they lived in. Passive solar heating techniques are based on three methods of heat transport: direct gain, indirect gain, and isolated gain. Direct gain is solar radiation that directly penetrates and warms the living space. Indirect gain collects, stores, and distributes solar radiation using some thermal storage material. Conduction, radiation, or convection transfers the energy indoors. Isolated gain systems collect solar radiation in an area that can be selectively closed off or opened to the rest of the house.

A passive solar home begins with the orientation of the house. The long axis of the house should run east/west. This means that the house should either face north or south. Because I wanted a roof that would be suitable for solar panels (without neighbor complaints) and I work at home and wanted my everyday living spaces to utilize daylight rather than electric lightening (kitchen, bath, family room, my office and my bedroom should all face south with adequate windows), my house is oriented so that the long axis to the south is the back of the house, the rooms we truly live in are all facing dead south to our garden and the watershed woodland beyond. It is a lovely setting and just the right orientation for passive solar design (as well as solar photovoltaic panels) . It would be difficult at this point to pick up and turn the house (especially since the drainage is also perfect for keeping my basement dry). It is my goal to maximize the passive solar design of my existing home over time.

The next step is to make sure that the house is well sealed and insulated.
Heating and cooling account for 50% to 70% of the energy used in the average American home. Inadequate insulation and air leakage through ducts, walls and roofs are the major sources of wasted energy in most homes (despite the popular notion that is the phantom loads of chargers and cable boxes). Though, my house was built in 2004 the insulation and thermal properties were not optimal. After inspecting the attic and accessible areas of the basement and crawl spaces for adequate insulation, I turned to the Building Envelop Research of the Oak Ridge National Laboratory for guidance. The Oak Ridge National Laboratory performs their Building Envelop Research for the US Department of Energy, DOE, and publishes their guidance in their “Insulation Fact Sheet,” which is available on the blog home page and through this link. Following the recommendations by the Oak Ridge National Laboratory the attic, crawl spaces, eves, ductwork, underside of a large portion of the main level floor were insulated. The pipes, wall end caps, knee walls, sump pumps and all identified areas were sealed, ceiling fixtures were capped and insulated, the garage ceiling was insulated and an insulated garage door installed. I was actually surprised at the winter energy savings (25% on propane and 6% on electricity despite adding two refrigeration units) and pleased with the improved comfort in the master bedroom and bath.

Both traditional and modern passive solar design strategies vary by location and climate, but the basic goal remains constant- to maximize solar heat gain in winter and minimize the solar gain in summer. Many passive solar techniques are either or, either they are intended for cooler climates that are primarily concerned with minimizing heating costs or they are intended for hotter climates where minimizing cooling costs is the main goal. The simplest example is roof color, in Phoenix or Palm Springs a white roof would reduce cooling costs significantly, in Boston or Bangor a black roof to increase heat gain and reduce heating costs would be chosen. The problem in the moderate zones like Virginia is which strategy would produce the most benefit. Rather than count the heating days and cooling days in Virginia to determine which roof color would be optimal, I’ll stick with the dark grey that still has another 18 or so years of life in it, and the benefits of sealing the heating and cooling envelop of the house.

According to DOE, window selection, sizing, and orientation is also important. The natural properties of glass let the sun through and can eliminate the need for daytime interior lighting, but trap long-wave heat radiation which will heat the house. Trapping the sun’s heat is the greenhouse effect. The difficulty is properly sizing the south-facing glass to balance heat gain and loss especially in a four season humid environment like Virginia. In the winter the ideal would be small windows on the north side of the house (rectangular shaped homes tend to have fewer windows on the short wall) to orient most of the windows to the south and have additional thermal mass to store heat. In the summer or climates where air-conditioning is used most of the year, larger north facing windows and shading of south facing windows is ideal.

Most locations in the United States have a four season climate that needs to balance heating and cooling, so that passive solar strategies that can change with the seasons are best. In terms of energy efficiency, soaring cathedral ceilings with a massive wall of two story windows are never a good idea, because increasing the glass area can increase the heat loss in winter and heat gain in summer, and the two story rooms are difficult to heat and cool. In the variable climates, deciduous tress shading the southern side of the house allows the warmth of the winter sun while protecting the home from direct summer sun and the buildup of the greenhouse effect inside. It does take years for a tree to grow, though. Exterior solar screens will trap the heat outside the house in summer, but should be removed in winter or angled to allow only the winter sun which is lower in the sky to warm the interior. Retractable awnings (like the summer awnings we had when I was a child) can be put up or opened in summer to reduce the summer heat gain while allowing the winter sun to penetrate. Interior blinds can be used to warm a room in winter by absorbing sunlight and providing greenhouse warming in the air space between window and shade. In the summer shades continue to increase the heat is stored inside in the gap between the window and the blind due to the greenhouse effect.

Draperies that are light colored and lined can serve to allow daylight (and eliminate the need for artificial lighting) while still providing an additional layer of insulation. The old fashioned and classic combination of sheers and drapes allows you to adjust the window coverings based on season and time of day. (Grandma is looking smarter by the minute). Finally, the type of window and selective window coatings like the 3M window films can also serve to reduce infrared radiation and solar energy. I installed the 3M Prestige films on all my windows the first month I owned the home to protect my husband and his books from UV radiation in our south facing home, I have no personal data what the impact is on heating and cooling costs. The main concern these films addressed was skin cancer and fading of furniture and books; however, the product technical specifications state that the films reduce UV radiation by 99.9%, reject 97% of infrared light and improve the solar energy rejected by a double pane window by 20%.

Windows with lower solar heat gain coefficient, SHGC, values reduce heat buildup in summer, but they also reduce the free winter solar heat gain. Proper window design really is climate specific. In moderate portion of the county a mixed window strategy is best. The insulating properties of window glass are described by the U factor. Really, even the best window are poor insulators. A single pane window has a U factor of about 0.99 which is equivalent to an R-factor of about 1. A triple pane window typically has an R-value of 4 for the glass surface. Low U-factors are most important in heating dominated climates, although they are also beneficial in cooling dominated climates. In a mixed climate, maximizing the insulating properties all year and minimizing the SHGC values in summer are optimal.

Wall design and materials are an important element in the thermal performance of homes. There is little we can do to change our walls without rebuilding, but the type of wall framing, sheathing and insulation can significantly impact the heat loss in winter and heat gain in summer. Wall insulation is important and should be appropriately addressed when building a house and when replacing siding. Using traditional construction materials to provide a thermal mass, such as concrete, stone and brick (not facing) can absorb heat during days and slowly release the heat at night. This can reduce the effect of outside air temperatures and indoor temperatures. However, work at ORNL found that exterior insulation finish systems, EIFS’s, are superior on thermal performance because the insulation is applied around the outside of the framing, avoiding thermal bridging common in traditional wood framing. The EIFS’s are among the most cost-effective building technologies for improving the insulating value of walls and the energy efficiency of buildings. However, system designs for vapor control without recognition of their ramifications for humid climates, along with window and joint water leakage due to poor installation unrelated to the EIFS walls, lead some to believe that EIFS walls were causing moisture-related envelope failures. Testing by ORNL disproved this and performance of these systems should improve as building codes reflect climate specific installation needs and the building industry gains more experience with the product.

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