June 6, 2011

Naturally Cool

By Naturalist Eric D'Aleo

The hot, humid weather of summer is almost here. On such muggy nights, it can be difficult to get a pleasant night’s sleep and I am sometimes tempted to go out and cool off in the shallow “frog pond” in my backyard. Many homes throughout the United States constructed over the last fifty years were not designed for the climate in which they are located, making the summer an uncomfortable season without air conditioning. Cooling and heating a home’s interior space can be expensive, accounting for up to 45% of its total energy use, but by utilizing natural ventilation and landscape strategies that were historically used, cooling costs can be reduced without sacrificing comfort.

The first step of this process is to observe your property and consider the following questions. Where are the existing trees and shrubs in relation to your home? Are they conserving energy by providing shade? What is the local wind direction and wind pattern during the seasons? How does the air flow through your house? Are there dead zones in the house where air does not move even with open windows? Do you have a one or a multiple story building? What types of windows do you have (casements, double-hung, or fixed) and how many are present on each side of the building? Are there operable windows on opposite sides of the house? Are there high or low windows on the house that can be opened to provide good airflow? Are there any ventilation openings including vents, exhaust fans, turbine ventilators, or a cupola? Are there cracks or gaps in the house that admit outside air? Does the house have high ceilings? Although this may seem like a daunting task, by taking the time to answer these questions you will have a good idea of which natural ventilation techniques will work best to cool your home.

The most important priority to maintaining a cool home is to keep unwanted heat out of the house. Close your windows during the hot part of the day and open them in the evening when the air is cooler. Although this may sound simple, many people often open windows on a hot, humid day to cool off, but instead may decrease the comfort level inside by bringing in outside air that is 10 ºF warmer than the air already in the house. Decrease temperature gain from heat-generating appliances inside a home by using energy efficient light bulbs, reducing hot water use, using a clothesline rather than a clothes dryer, and rescheduling heat producing tasks for cooler hours of the day. A homeowner can limit exterior air and heat penetration to a house by caulking and sealing seams, cracks, and openings as well adding additional insulation and reflective heat barriers. Additional ideas to limit the amount of unwanted heat in your house are to block sunlight from reaching the windows by using window shades or retractable awnings. Exterior roller blinds, shade sails, and sun screens (also called solar screens) can block sunlight or provide a shade area next to the house as well. These steps will make a house easier to cool on hot days and keep your home warmer in the winter.

Another natural ventilation practice is to increase airflow through a building to make it feel cool and comfortable. The use of vents, open windows, cupolas, and other structures to create a chimney effect of rising warm air will enhance cooling through air movement. The greater the vertical distance between a low inlet, bringing in cool air, and a high outlet, venting warm air, the better the resulting air movement. Cross ventilation on opposite sides of a room or house also helps to move air through the building, improving the comfort level inside. Try enhancing airflow in your home by experimenting with existing windows before considering renovation options. Casement windows can be opened to catch and direct breezes, while double–hung windows can be opened part way to let cooler air in on the bottom and warmer air exit on the top. If you have operable transoms above doors and windows, use them to exhaust hot air near the ceiling. If there are multiple levels to your house try opening high and low windows to pull air vertically through the building.

One of the best means of lowering the temperature in your house and property is through careful landscape planting of trees and shrubs. It has been estimated that as much as 30% of cooling and heating costs can be reduced this way. You may have noticed the change in temperature between a hot, sunlit meadow and a cool forest or an exposed downtown and a tree-lined residential area. This temperature difference is because the vegetation acts as an air conditioner. Studies conducted by the Lawrence Berkeley Laboratory found that summer daytime air temperatures were as much as 6 ºF cooler in tree-shaded neighborhoods than in treeless areas. The shade provided by one sufficiently high tree, strategically placed several meters away from a home, can reduce temperatures of the wall and roof between 20º - 40ºF.

In temperate climates, deciduous trees on the southeast, south, and west sides of a building offer the most protection during the summer and block less winter sun than evergreen trees. A deciduous tree with a high spreading crown is best for shading a roof, although trees with lower crowns may work well on the west side of a home. Trees that grow moderately or slowly are the best choice for shading a building. Their slow to moderate growth will make them sturdier and their branches stronger when they mature making them less prone to damage from heavy wind or snow than fast growing trees. These trees are also more resistant to drought, insects and disease.

Arbors, pergolas and other vegetative structures covered with vines are another way to use vegetation to cool your home. Traditionally used to shade areas near houses, vines reduce the air temperature through evaporation of water from the plant’s leaves. Be aware that vines are fast growing and some stick easily to the walls that they grow on and may cause damage, while other types need a trellis or other structure to support them.

Shrubs and other foundation plantings can create dead air spaces between the plant and the foundation, helping to insulate the house from the summer sun and winter wind. Evergreen shrubs work best and should be placed far enough away from the foundation so that when mature their closest branches are five feet away from the building. This distance will provide shade and good airflow avoiding moisture buildup near the foundation. Large shrubs can also be planted to shade an operating air conditioning unit in addition to acting as a visible screen to hide it.

Even low growing ground cover and grass growing around a building can help cool it by 10 – 15 ºF compared with asphalt or gravel. Groundcover and turf have a cooling effect due to evapotranspiration (water loss) from their leaves while also preventing reflection of sunlight toward the building. Additionally setting your lawn mower to a higher cut height (2 ½ - 3 inches) in summer will cool the surrounding air better than if it is cut close to the ground.

Inside a house using ceiling fans and portable fans to direct airflow where you need it, makes warm temperatures feel more comfortable due to increasing the evaporation rate on your skin. The fans themselves do not actually cool the room but raise your comfort window, so that you can tolerate a temperature 4 ºF warmer with a fan on. An important reminder is to turn off a fan when you leave a room, because the air movement provided by the fan cools people, not the actual room and the fan itself will generate heat into the room over time.

As you can see, there are many ways to cool your home during the summer using little or no energy. Even wearing loose, light-colored clothing has an impact. So consider your options over a glass of lemonade and start cooling down – naturally.

1 comment:

Alex Novikau said...

That's really useful. Here, in Belarus extremely hot summers are becoming common as well.