Windbreaks and Shade Trees save Energy, Money, and the Environment
Planting trees around homes is an ancient concept used to conserve home energy use. Windbreaks, which consist of rows of trees placed perpendicular to prevailing winds, were greatly used in the Midwest to protect exposed houses, livestock, and crops from severe winds. The use of shade trees was especially emphasized during the 1970s to combat the energy crisis caused by Arab oil embargoes. The recent concern over global warming has made tree planting and energy conservation important issues again. The global warming problem is thought to be caused by industrial and automotive emissions of greenhouse gases, such as carbon dioxide, to the atmosphere as well as from clearing of tropical forests for agriculture.
Many scientists believe that a historical buildup of greenhouse gases in the atmosphere is trapping more of the sun’s energy once it reaches the earth. The result has been a slow rise in the earth’s temperature by about 0.5°C (1°F) over the past century. Many computer models now predict a 0.3°C (0.5°F) temperature rise over the next decade. The results of such changes in the earth’s temperature could be severe. Scientists believe that changes in rainfall patterns, increases in global sea level, and a general shift in climates may be imminent. The use of planted windbreaks and shade trees can combat this problem on two fronts. First, trees consume carbon dioxide and produce oxygen, thus reducing the amount of carbon dioxide in the atmosphere. Second, the strategic location of trees can reduce home energy use and, therefore, reduce the number of greenhouse gases produced by energy utility companies. Since residential heating and cooling represents about 11 percent of total U.S. energy use, windbreaks and shade trees may offer significant energy savings. If windbreaks and shade trees could reduce heating and cooling energy needs by 10 percent at the residential level, total U.S. energy demand would be reduced by about 1 percent. Such savings seem minor, but when multiplied by millions of households, the overall reduction in utility emissions of greenhouse gases could be important. The incentive to homeowners to plant windbreaks and shade trees is based on their potential to save money from subsequent energy reductions. Winter heating bills may be reduced by 15 percent while summer cooling needs may be reduced by 75 percent in certain types of homes. This publication is meant as a guide for homeowners interested in utilizing windbreaks and shade trees for energy savings. The material includes sections on how energy savings are accomplished, how to properly build and locate windbreaks, and how much windbreaks will cost and save the typical household.
Home Heat Exchange
To take full advantage of the effects of trees, the ways in which homes gain or lose heat must be understood. Heat exchange in a home occurs through three major processes: air infiltration, heat conduction, and sunlight transmission through windows. Air infiltration is the passage of outside air through cracks around doors and windows or other home openings. Outside air is forced or drawn through these openings by the wind on the outside of the home or by temperature differences between inside and outside air. Wind on the outside of the home will replace indoor air with an equal amount of outdoor air.
Depending on the outside temperature, this process may cause undesirable temperature changes in the house. Temperature differences between inside and outside can also create a natural circulation of air in the home. Warm interior air will rise and escape the house through openings near the roof while cool air is drawn into the home through lower house openings. This process is known as the “chimney effect” (Figure 1). Air infiltration due to the chimney effect and from wind exposure often occur simultaneously. But the chimney effect often is most important in winter due to the large temperature differences between inside and outside air.
The combined effect of wind and temperature differences may cause air within a home to be completely changed several times per hour. Properly placed trees can reduce air infiltration by reducing wind velocity in the vicinity of the home. However, a large, dense forest near the home may also reduce exterior air temperatures and tend to increase air infiltration by the chimney effect in winter. Heat conduction is the transmission or loss of heat through home construction materials. Different construction materials conduct heat differently depending on their thermal conductivity, thickness, and surface area. Most walls and ceilings are composite layers of materials and are effective in reducing heat conduction by trapping air within or between the layers. Windows are less effective at stopping heat conduction unless a double-pane-style window is used with an air layer sandwiched between two panes of glass. Control of the temperature difference between inner and outer surfaces of walls, ceilings, and floors offers the best opportunity for reducing heat conduction. The inner surface temperature is largely controlled by the interior air temperature. One method of conserving energy in winter is to lower the interior temperature, reducing the difference in temperature between inside and outside surfaces. The outer surface temperature of a home is controlled by the outside air temperature, wind velocity, and solar radiation, as well as by the amount of heat being conducted through the material. Trees can reduce the amount of sunlight reaching the outer surfaces of home and thereby reduce the temperature difference between inner and outer building surfaces in summer when heat is rapidly being conducted into the home. However, in winter, solar heating of the building’s exterior surfaces can be beneficial in reducing rates of heat loss. Winter shade from trees would interfere with this beneficial heating.
Sunlight transmission through windows can also transmit heat into homes. If sunlight is received perpendicular to a single-pane glass surface, up to 90 percent will be transmitted into the home. Although sunlight can pass through glass, heat produced inside the home cannot escape. Thus, the net effect of sunlight transmission is the heating of the inside of the house. Obviously, the size, position, and type of windows in a home relative to the position of the sun in summer and winter greatly influence the role of sunlight in home heat exchange. Many homes are being designed to capture greater amounts of radiant energy from the sun. In these homes, radiant energy is absorbed and used to heat water or air. Trees around a home can be used to influence sunlight transmission by blocking sunlight from windows during midday, which is desirable in summer.
The potential role of trees in home energy conservation in Pennsylvania varies between summer and winter seasons due to shifts in the importance of heat exchange processes. In winter, air infiltration becomes the major heat exchange process and the use of trees to reduce wind velocity is most important. Shading in winter reduces the already small amount of beneficial solar heating at this time of year. In summer, air infiltration accounts for small heat gains in the home due to the relatively small temperature differences between inside and outside air. Heat conduction and transmission of sunlight predominate in summer and the use of trees to shade the exterior surfaces and windows is most important. Shading by trees in the summer reduces the amount of sunlight absorbed by the home. One observer in New Jersey noted that the exterior surface temperature of a shaded, wood-sided home was 9°C (16°F) cooler than similar unshaded surfaces in June. Such reductions in exterior surface temperatures can lower the exterior-interior temperature differences substantially and thereby reduce the rate of heat conduction into the home. Without air conditioning, differences in heat conduction will cause differences in interior air temperatures. In a mobile home shaded by trees, interior temperatures were up to 11°C (20°F) less than in an unshaded trailer during mid-day. Maximum temperatures in the shaded trailer occurred up to 3.5 hours later than at an open site. Cooling of interior air to tolerable levels also occurred sooner at the shaded site. When a home is air-conditioned, trees can save energy. In Alabama, where the need for air conditioning is greater than in Pennsylvania, results of one study indicated that shaded mobile homes had annual electricity bills ranging from $45 to $100 less than unshaded mobile homes. Differences in electric bills prevailed even when homes averaged only 20 percent of their roof shaded per day. In a Pennsylvania study, the energy required for air conditioning a mobile home was estimated to be 75 percent less in a grove of tall deciduous trees than in an open, unshaded site. Sunlight on the shaded mobile home was only one-tenth of that in the open.
Providing shade appears to be the only important effect of vegetation on home energy conservation in summer. Trees will reduce wind velocity and air infiltration rates in summer, but the effect on heat gains to a home in Pennsylvania will be small. Small, open groves of trees common around homes in urban and suburban areas will generally have little effect on air temperature. Trees that provide shade in summer may be detrimental in winter if solar heating of the home is interrupted. In a dense red pine plantation in Pennsylvania, which reduced solar radiation by 75 percent in winter, heating energy needs for a small trailer were estimated to be 12 percent greater than at an unshaded site. Here any benefit from reduced wind velocity was completely offset by reduced solar heating of the trailer. Energy consumption for trailer heating in a deciduous grove was up to 8 percent less than for a similar trailer in an open site. Even though the deciduous grove reduced wind velocities by 40 percent in winter, solar radiation was also reduced by 37 percent by the leafless tree canopy. Thus it appears that trees around a home can reduce wind velocity, but heat energy savings will not occur if the dwelling is heavily shaded at the same time. Vegetation arrangements that reduce wind velocities around homes but do not shade homes will produce the greatest energy savings in winter. Windbreaks located considerable distances from a building can reduce wind velocities without shading it. Research in Pennsylvania indicated that up to 15 percent of heat energy savings are possible using windbreaks. Most of these savings resulted from reduced wind velocity and, therefore, reduced air infiltration in homes downwind from the windbreak. Effects of windbreaks are greater at higher wind velocities. Savings will increase with the amount of reduction in wind velocity affected by the windbreak. Savings also will be greater for loosely constructed homes. It should be remembered, though, that windbreaks have almost no effect on air infiltration during calm days.
Hints for homeowners
An optimum arrangement of trees for year-round energy savings seems to include windbreaks for reducing wind velocity in winter accompanied by several large trees that shade the home in summer. The difficulty of achieving this optimum arrangement depends on vegetation already present on the property and ownership of sufficient land. Homes built on land formerly in the forest very often have enough trees on the property to achieve the desired arrangement relatively quickly. However, when homes are built on cleared agricultural land, nearly all of the necessary trees and shrubs must be planted.
Windbreak considerations Windbreaks should be located upwind from the home in the direction of the prevailing wind. In Pennsylvania, the winter wind is primarily from the west and northwest. The rows of trees should be oriented perpendicular to this direction. Local topography and structures can channel the winds so that the prevailing direction in the vicinity of the home can vary considerably. Observation of drifting snow can be used to determine the prevailing direction around the home (Figure 2).
Location of the windbreak should be upwind from the home a distance of 50 to 200 feet, with the optimum distance being 100 to 150 feet. To minimize problems presented by drifting snow, plant trees at least 50 feet away from the home or driveway. Where possible, the rows of trees should extend 50 feet beyond the ends of the area to be protected. Limited lot size often necessitates reducing both the distance from the home and the length of the windbreak. There is no need for planting additional windbreak trees if large areas of dense forest already occur upwind at the required distances. Design and composition of the windbreak depend on the space available on the property and on the species and size of planting stock that can be obtained. Local ordinances may restrict species and location. Where space is limited, a single row of spruce trees is sufficient. However, five rows of several evergreen species are much more effective. The outside rows—both upwind and downwind—should be trees with dense, low growth such as white, Colorado blue, or Serbian spruce, Douglas fir, or white fir. The inside rows should be faster, taller-growing trees such as red, white, and Austrian pine, hemlock, or Norway spruce. White pine is recommended for planting only south of the Blue Mountains and east of the Susquehanna River because the white pine weevil is present north and west of this area. Colorado blue spruce and Douglas fir also are only recommended for use in the southeast. Any of the spruces and any of the firs may be used to the north and west. Most spruces other than Colorado blue spruce is not adapted to the climate in the southeast portion of the state (figure 3).
Spacing in one-, two-, and three-row windbreaks should be 6 feet between trees. With four or more rows, the spacing should be 8 feet. Rows should be 10 to 12 feet apart, with trees planted in a staggered arrangement. If there is enough space and quicker, partial protection is desired, then one or two rows of faster-growing trees may be planted at least 15 feet upwind of the permanent windbreak. Hybrid poplar, Lombardy poplar, or Japanese larch should be spaced 4 feet apart in single rows or 6 feet in staggered arrangements if two rows are planted. These will grow to 10 or 15 feet in height in 5 years and should be regarded only as a temporary planting. They should be removed within 10 years so they do not retard the growth of the permanent planting.
Lombardy poplar should be regarded as a short-lived tree due to fungal disease problems. Sizes and choices of planting materials include seedlings, transplants, container-grown trees, or trees that are dug with a ball of soil and wrapped in burlap (B&B). Seedlings are lowest in price but slowest to become established; container plants and B&B trees are higher in price and become established and grow more rapidly. Transplants are moderate in price, establishment, and rate of growth. Windbreak tree species may be purchased from most commercial nurseries.
Property owners should determine if local ordinances restrict planting near boundary lines or restrict planting of certain species. Early spring generally is the most successful planting time. However, B&B trees may be set in fall if given proper winter care. Container plants can be set any time the soil is not frozen. Preparation and maintenance of the planting site are recommended to ensure optimum growth. Competing vegetation such as grass, weeds, or woody plants must be eliminated by cutting, cultivation, or herbicidal treatment or a combination of these. A soil test will reveal whether soil acidity and nutrient levels should be adjusted for proper establishment and growth. Recommendations for lime and fertilizer use will accompany the soil test results.
Assistance in planning and establishing a windbreak may be obtained from your local extension office and the conservation district office. Lists of commercial nurseries selling tree planting stock and soil testing kits are available at county extension offices. Most commercial nurseries offer advice on selection, planting, and care of trees. Approximate cost for planting stock by tree size and windbreak size is based on a 150-foot-long windbreak. This windbreak length would protect a home or building 50 feet long, and extend 50 feet beyond either end of the structure (Table 1). Protection and care are needed. After the windbreak trees are planted, they must be protected and kept in a healthy and vigorous growing condition. It is especially important to protect the lower branches from injury or stunted growth.
Exclusion of pets, livestock, and children may be necessary while trees are getting started. Competing vegetation such as grasses, weeds, and woody brush must be kept away from the growing trees. While mowing is partially effective, mowed plants still rob the trees of soil moisture and nutrients. Recommended chemical herbicides, when correctly used, are safe and effective on most annual weeds and grasses without harming the trees. Corrective pruning may be required as the trees grow. Multiple tops should be cut so that only a single terminal leader is growing from the top of the tree. Multiple tops allow trees to be easily damaged during ice and snow storms. Trees that die or become badly damaged should be replaced the following year so that openings do not develop in the screen. Fertilizing may be needed if the trees do not show normal to vigorous growth. One year after planting, fertilizer should be applied to the trees in March or early April. This process should be repeated at two-year intervals thereafter. Unless a soil test has been taken and specific recommendations received, these general rules may be followed:
■ Apply 5-10-10 granular fertilizer.
■ Spread in an 8-inch circular band at least 8 inches away from the tree stem for trees less than 2 feet high. Apply 0.25 pound to each tree.
■ Treat trees more than 2 feet high with a circular band 12 inches wide under the ends of the lower branches.
Apply 0.5 pounds to trees that are 4 feet high, 2 pounds to trees 6 feet high, and 4 pounds to trees 8 feet high. Larger trees should receive 2 pounds per inch of trunk diameter. Disease and insect outbreaks are usually first noticed when portions of the tree foliage turn brown, reddish, or yellowish. Closer observation will reveal insect and disease outbreaks before they become too serious. If the disease or insect causing the problem cannot be determined by the homeowner, foliage and twig specimens should be taken to the county extension office for identification. Thinning is often required when the trees grow together and threaten to become overcrowded. Weakened, injured, or badly crowded trees should be cut first. This stage may occur 12 or 15 years after planting. Additional windbreak benefits. Most windbreaks serve several other purposes besides energy conservation. Studies have demonstrated that trees are effective as noise barriers where busy highways or noisy industrial plants are nearby. Visual screening is also provided when trees become 5 to 6 feet in height. A well-planned and properly maintained windbreak is also aesthetically pleasing (Figure 4). The properly placed windbreak is an effective snow barrier. Multiple- row windbreaks trap some snow within the trees while much larger amounts are dropped on the protected lee side. Tree height and wind speed determine how close to the lee side the snow accumulates, but it is generally within twice the height of the trees. Wildlife is the most important benefit from a living windbreak. Birds and mammals are attracted to trees for protection and food. Songbirds may use a windbreak throughout the year or when they are migrating. Ringneck pheasants, rabbits, squirrels, and other animals may use the windbreak when the trees are large enough to provide cover.
Composition of the windbreak may be altered if the owner is highly interested in encouraging wildlife. The incorporation of some deciduous shrubs and trees would increase the attractiveness to wild birds and mammals but would reduce the wind barrier effect. Deciduous shrubs with proven wildlife food value could be planted in the outside rows and conifers in the core section Summer shade considerations Summer shade is best provided by several large deciduous trees strategically located along the southerly edge of the home. Location of these trees is determined by the mature height of the tree and the angle at which summer sunlight is received. In Pennsylvania, the position of the sun in the sky in summer ranges from about 45 to 75 degrees above the horizon during mid-day. Trees should extend along the southeast to the southwest edge of the home and be of sufficient height to protect the home at these angles. In winter the leafless deciduous trees should not shade the roof of the home. Typically, the sun in winter during mid-day is less than 45 degrees above the horizon and what shading of the home does occur will be largely by the tree trunks. For this reason, only trees necessary for summer shade should be maintained along the southerly edge of the home. Trees too far away from the home to provide summer shade may provide unwanted shade in winter. Prompt removal of diseased or damaged trees is also necessary to avoid future damage to the home from falling debris. Exact placement of the trees may also depend on maintaining a desirable view from windows, aesthetic appeal in landscaping a home, and avoiding overhead wires and underground pipes.
Trees for summer shade may be present on forested home sites, provided the developer can save them during construction. If the trees are to be planted, fast-growing poplars can be intermixed among slower growing, more desirable shade trees. Poplar trees can be removed later and the larger B&B trees planted for permanent shade at the southern edge of the home.
Final Effect and Costs
The final effect of vegetation arrangements on total energy needs for heating and cooling will vary with location, weather, and the characteristics of the home. Although up to 75 percent reduction in cooling energy needs can be achieved by providing summer shade, air conditioning is normally not needed in many portions of Pennsylvania. However, summer shade would make the interior living spaces more comfortable. In winter, at wind velocities typical of Pennsylvania, windbreaks may save 10 to 15 percent of the heating bill, with the greatest savings occurring in loosely constructed dwellings. The final costs of developing summer shade and windbreaks must be compared to the values of energy savings plus other wildlife benefits and environmental advantages. Where planting is required, the investment is necessarily long term. However, since most homeowners invest in some landscaping for their home, a little more time devoted to planning the best planting arrangement and species can pay off.