Using Trees to Save Energy

The facts and figures of the following article are adapted from the text Urban Forestry - 2nd ed. by G.Grey & F. Deneke, pub. Wiley & Sons 1986.

We have all appreciated the cool shade of a large tree on a hot summer day, but most people don't realize the extent of the energy involved, and the potential savings to be gained from properly sited trees. It may also not occur to everyone that properly sited trees can have a substantial effect on the indoor environment as well as the outdoors, both in the summer and winter months.

The most obvious cooling effects from trees come from their ability to block sunlight with their leaves. Less obvious, but equally important, is their ability to act as evaporative coolers. Evaporative coolers (similar to air conditioners), as well as trees, work on the principle of evaporative cooling. As water evaporates from a surface, it absorbs heat energy which is used to convert the liquid water to water vapor. A single tree can evaporate up to 88 gallons of water a day. This is the equivalent cooling power of 5 air conditioners running 20 hours per day (@ 2500 kcal/hr). Imagine the overall effect of a group of trees, or an entire forest of trees, on the immediate environment. It is much more than just shading of sunlight that contriubutes to the relatively cooler and moister environment found under trees. The forest understory can easily be 25 degrees cooler on a still day than a sunny site. Even a few trees in an urban area can lower the understory temperature by 10 to 15 degrees (Federer. 1970).

Another important, but underappreciated effect of trees and shrubs is their ability to channel or block windflow through and around buildings. Optimal placement of trees and shrubs has been noted (Parker, 1983) to decrease the temperature of a west facing wall by 28 degrees. Using only a few small trees to shade an air-conditioning unit was shown to increase its effeciency by 10 percent. Moving a mobile home from a sunny site to a forested site was shown to reduce its summer cooling energy needs by 75%. We can start to see how trees and shrubs can have a real and visible effect on our summer cooling bills. The great thing about deciduous trees, of course, is that they loose their leaves in the winter, letting in all the solar energy that they were blocking in the summer.

Trees can also have positive effects on the winter heating bills. In the summer, the prevailing winds are from the south-west, while in winter the prevaling and coldest winds come from the north-west - a fact of nature that can be used to advantage. Evergreens, especially, make an effective wind-break, and by placing a mass of evergreens to the north-west of a house, winter winds can be blocked without blocking the desirable cool summer breezes. Again, the potential savings are not insignificant. Creating a "dead air" space around the northwest walls by using evergreens is esentially equivalent to adding another layer of insulation, and has been shown to reduce heating costs by up to 23 percent for an average home (Rosbinette, 1972).

Other Important Uses of Trees and Shrubs

WINDBREAK: In open areas, where gusty winds are common and can be uncomfortable, windbreaks can also create a more comfortable outdoor space. Because of the way wind flows, a dense windbreak of trees can create a protected space for a distance of up to 10 times the height of the tallest trees, where windflow is reduced by 50 percent. Lesser reductions in windflow will also be seen at a distance of up to 40 time the height of the tallest trees. It is actually derirable to allow some wind to filter through the windbreak, as this will extend the "lift" or boundry-layer.

SNOWFENCE: Windbreaks of trees and shrubs can also be used to create an effective snow-fence, eliminating driftin in areas (driveways/roads/sidewalks/parking lots) prone to drifting. Snow will drift to the leeward (downwind) side of a windbreak, so trees should be planted some distance (30 to 40 ft) to the west or northwest away from a road or driveway to achieve the desired result. This principle could also be used to gather snow in areas like a ski-run or sledding hill.

RUNOFF and ERROSION CONTROL: Trees will slow rainfall and increase infiltration rates into the soil, resulting in less errosion and runoff of nutrients and pollutants. Especially useful in stablalizing streamside areas and keeping nutrients and polluntants out of the watershed.

NOISE REDUCTION: Wide belts or hedgerows of mixed trees, shrubs, and grasses are the most effective type of planting for reducing noise from nearby roads or neighbors. The plants both absorb and scatter sound waves, and reductions of 8 to 12 dB - equivalent to doubling or quadrupeling the distance to the noise source - are commonly observed (Cook & VanHaverbeke, 1971). Rustling of leaves will also help mask other types of noise, making them less noticible.

POLLUTION CONTROL: leaves will trap dust to an extent, and capable of absorbing some common pollutants to a degree.

PEDESTRIAN TRAFFIC CONTROL: trees and shrubs can be used to limit and direct pedestrian traffic. Useful near sidewalks, on college campuses, or in other high-traffic areas.

TREES ARE BEAUTIFUL!: The joy of having trees just can't be measured. Make the world a better place and plant some trees!

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