by Doug Korty
INTRODUCTION Passive solar energy is greatly underutilized in this country. Depending on climate, a well designed house can receive 30% to 100% of its space heating from passive solar. New homes could easily be designed to get a large proportion of their heat from passive solar; but very few are and each year the number of new homes is less than 2% of existing homes. At least 80% of existing homes do not have the orientation or room arrangement appropriate for passive solar. The costs to retrofit them would range from reasonable to prohibitively expensive, most would need new windows and added interior masonry and many would require the rearranging of rooms. Even if a large part of the cost effective retrofitting were done, passive solar in existing homes is not likely to save more than 2.5% of our total energy use (Residential space heating accounts for 16% of total energy use, 40% of homes obtaining an average of 40% of their heat from passive solar = (0.16 x 0.4 x 0.4 = 0.026). Passive solar design of new homes would have a larger effect in the long run. Both conservation and energy efficient landscaping can also contribute significantly to home energy savings since a large percentage of homes could use some measures cost-effectively.
PASSIVE SOLAR A tiny fraction of architects and home builders incorporate solar energy into their designs. The common-sense rural solar designs were lost with increased urbanization in the 19th century. There was some interest in passive solar before and during WW II. But after the war growing prosperity and cheap fossil fuels led people away from it. [1]
Knowing the amount of available sunlight and seasonal patterns is important for solar design. Heating and cooling degree days are available from state maps on solar availability (state energy offices). State solar centers or energy offices can also provide expert guidance. Solar Pathfinder, analyzes solar suitability of any site (www.solarpathfinder.com).The Sun Angle program (www.sundesign.com/sunangle) calculates bearing angles for overhang length.
A number of software programs are available for solar designing. HEED (free from University of California
Orientation is important for passive solar. The building should be elongated on an east-west axis. In most locations true north and south and magnetic north and south do not coincide. In order to calculate solar noon in your area, take the midway point between local sunrise and sunset. The sun will be due south at this time. The building's south face should receive sunlight between the hours of 9:00 A.M. and 3:00 P.M. (sun time) during the heating season.
When south facing glass does not exceed 7 percent of the total floor area, buildings are called “Sun tempered”; they need not have added thermal mass (see below). Solar heat should provide 20-30% of the total. If solar glazing is above 7%, extra thermal mass is required. Non solar glazing should be less than 4% of floor area for both north and east, and less than 2% on the west. Shading, special glazings and other measures can be used to compensate if more glazing is used. Various types of shading can be used against the summer sun. Not even deciduous trees should be planted on the south side; in winter the trunk and branches can block 40% of sunshine.
Open floor plans can help distribute heat via convection. In a retrofit it may be advisable to remove interior walls. Rooms that are frequently occupied such as living rooms, bedrooms, and home offices should be located along the south side of the home. Spaces that require less heat, including corridors, stairs, halls, utility rooms, laundry rooms, pantries, workshops, storage areas can be situated along the north side. Garages and porches should be on the north and west sides of the home.
THERMAL MASS Thermal masses receive solar energy during the day and release it during the night. Good thermal masses conduct heat slowly. Dark masonry floors and interior walls which are directly heated by the sun are best. Solar radiated mass stores four times more energy than shaded mass. Each sq. ft. of solar glazing over 7% of total floor area requires 5.5 square feet of uncovered and sunlit floor mass.[3] Concrete floors can be covered with paving bricks, slate, quarry tile, or dark ceramic tiles. Adding masonry walls or floors can be expensive and may require reinforcement under the floor.
WINDOWS The Solar Heat Gain Coefficient, SHGC, measures direct solar radiant heat that gets through a window. For solar heat, south facing windows should have a high SHGC (above 0.50). In hot climates, the SHGC should be below 0.4. The U factor measures thermal conductivity, R-value is the inverse. Look for high performance, well-made windows with low U-values and low rates of air infiltration. Double glazing and gas fills help reduce heat loss. Gas fills such as argon between glass layers decreases heat conduction. In mixed climates, look for both a low U-factor and a low SHGC except for south facing windows where the SHGC should be above .50. New windows should have an energy information sticker from the National Fenestration Rating Council (NFRC).
In the summer about 40% of the unwanted heat that enters our homes comes through windows. Glass loses heat up to 30 times faster than well insulated walls in winter. Low e (low emittance) windows reflect thermal radiation and keep heat out in the summer and in during the winter. A second layer of glazing reduces solar gain by 18%, but reduces heat loss by about 50%. Low e, high SHGC windows do exist but are difficult to find.[4] Thermal shades, shutters or heavy curtains can reduce heat loss through windows at night by up to 80%.[5]
SHADING Overhangs on south facing windows protect against the summer sun. (Sustainable Buildings Industry Council, www.sbicouncil.org) publishes overhang data and some of the software tools mentioned above will calculate overhangs. Spring and fall may have different temperatures, so the best solution is to compromise on overhang, and use window shades when needed. One drawback of overhangs is that they are difficult to add. Awnings will block as much as 65% of the summer sun's heat on south facing windows. However, on east or west windows awnings have to slope down over 2/3 to 3/4 of the window. Unfortunately, few homes are oriented directly north, south, east, and west, so it may be more difficult to determine what shading options are best. [6]
CONSERVATION The first energy problem with the average American home is size. The average size of new single family homes is around 2500 sq ft, almost double the size of 40 years ago in spite of 25% smaller average families. There are several methods for diagnosing energy problems in homes. One of the most commonly accepted is the Home Energy Rating System (HERS) done by home energy raters. In addition to rating the home's current efficiency, the report lists what energy-efficient improvements can be made and the effect on energy use that each will have. It details the estimated cost of each improvement, the estimated monthly savings, and the payback time. HERS ratings can be used to secure favorable mortgage loans and government tax incentives and other advantages. Blower doors, pressure gauges, nanometers, infrared cameras and smoke sticks can measure air leakages in your home.[7]
By sealing air leaks and insulating, you reduce losses due to convection and conduction. Thermal resistance, measured by R-Value, reduces heat transfer. In general what you do to reduce winter heat loss will also reduce summer heat gain. EPA Energy Star provides recommended insulation levels (www.energystar.gov). In winter, hotter air rises and passes out the roof, this draws colder air in below. This “stack effect” dominates air movement. Bath
There are many lists of free or low cost measures (e.g. air dry dishes) to reduce energy use. The savings for most of these measures are quite small but some are worth doing and doing a number of small things can add up. The Sustainable Buildings Industry Council (SBIC) notes that "occupant effects" can result in annual energy use ranging from 70% to 140% of the average; this includes things like thermostat settings and the intensity of use of appliances.
COOLING Most summer heat gain (up to 2/3) occurs through the roof of a house. Insulation, ventilation and radiant barriers can protect the living space. Opening high placed windows permits hot air from lower levels to escape. Attic and whole-house fans are useful. Ventilated attics are about 30 degrees cooler. Whole house ventilation can be integrated into forced air central heating and cooling systems. Ridge vents at the peak and soffit vents at the eaves can cut heat flow into the house by 35%. During the hot season, thermal mass inside a house acts as a heat sink, releasing the heat at night. This works well in hot arid climates with cool nighttime temperatures.
LANDSCAPING Energy efficient landscaping can cut as much as 30% off energy bills. Windbreaks make sense if you spend more on winter heat than summer air conditioning. A row of trees and shrubs can filter, divert, or obstruct up to 85% of the force of cold winds. Spruce trees make excellent windbreaks, as do white pines. In cold climate, to the east, north and west, plant shrubs and hedges to block winds. Coniferous trees close to the home are preferred on the northern side. Small-scale evergreen trees and shrubs are valuable when planted close to homes because they create dead-air spaces that insulate buildings.
Trees and other vegetation not only block the sun, they also cool the air surrounding a house by evaporation. An average full-sized deciduous tree evaporates 100 gallons of water during a sunny summer day, which uses up about 660,000 BTUs of energy and cools outside a house as well as five average (10,000 BTU) air conditioners. Plants are self regulating, in cold springs the leaves arrive later and in Indian summers they last longer. Cool breezes can be channeled into living areas. Wind funnels increase wind velocity at their narrowest point. At 70 degrees an 18 mile an hour wind makes the air temperature feel like 58 degrees. If both the summer and winter winds come from the same direction, you must choose which is the more energy efficient to control.[9] Use plants that are native to your region; this will save water.
FINANCES Increases in fuel prices should wake us up to the advantages of energy conservation investment. The savings with Energy Efficient Mortgages (EEM) could save you enough to offset your energy savings investments. HUD/FHA offers the Title 1 home improvement loan for up to $25,000. HUD also has the 203(k) loan to purchase a home in need of repair or modernization. Some energy companies give rebates for energy conservation expenditures especially for income-qualified homeowners. Free or reduced rate installations and energy audits may also be available.[10]
CONCLUSION Some combination of passive solar, conservation, and landscaping could save many homeowners as much as 75% on their energy bills. Passive solar in new homes can cut home heating costs up to 100%, and with retrofits in existing homes by as much as 50%. According to Lawrence Berkeley Laboratories (hes.lbl.gov) the most cost effective conservation measures in the upper Midwest are: programmable thermostat, CFL lights, sealing heat ducts, and air sealing. These four could cut a home’s total energy use by 20% and cost little. The next most cost effective measures are efficient furnace, wall insulation, efficient gas dryer, and attic insulation. They could save 32% with paybacks ranging from 5 to10 years. Conservation measures are more cost effective if you were going to replace the item anyway or the measure is incorporated into a general remodeling project.
Other alternative energy sources such as biofuels, windpower, and active solar have potential (the Energy Dept. estimates alternative sources will supply 9% of our energy by 2030 – and this is probably an underestimate) However, to solve our national energy problems, we need to find ways of conserving energy with changes not only in our homes but in almost every aspect of our lives including land use, transportation, agriculture, construction, industry and commercial buildings. Serious energy conservation would also help with air pollution, global climate change, and other environmental problems.
[1] Butti, Ken and John Perlin, A Golden Thread, Cheshire
[2] Chiras, Daniel, The Solar House, Passive Heating and Cooling, Chelsea Green Publishing, 2002
[3] Chiras, Daniel, The Solar House, Passive Heating and Cooling, Chelsea Green Publishing, 2002
[5] Anderson
[6] Home Energy Magazine, No Regrets Remodeling, Energy Auditor and Retrofitter Inc., 1997
[7] Home Energy Magazine, No Regrets Remodeling, Energy Auditor and Retrofitter Inc., 1997
[8] Harley, Bruce, Insulate and Weatherize, Taunton
[9] Moffatt, Anne Simon et al, Energy Efficient and Environmental Landscaping, Appropriate Solutions Press, 1994
[10] Home Energy Magazine, No Regrets Remodeling, Energy Auditor and Retrofitter Inc., 1997
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