View Full Version : Forget AC! Cool Your Home Naturally

Wednesday, August 20th, 2008, 01:11 PM
It's summer and it's hot, so I gather this article could be of some use for folks who have no AC or wanna save some money/energy. ;)

Space cooling and heating can account for up to 45 percent of your total home energy use every year, but there are numerous strategies you can employ to reduce cooling costs. For instance, a ceiling fan used in conjunction with air conditioning lets you raise the thermostat by as much as 4 degrees while maintaining the same comfort level in a room. The U.S. Department of Energy estimates that each degree below 78 degrees on your thermostat will increase your air conditioning bill by 8 percent. You also can use natural ventilation to capture and create breezes, or to help you take advantage of nighttime drops in temperature.

Other money-saving ideas include minimizing heat gain, weather sealing, insulating, window shading and glazing, roof lightening and landscaping (see “Best Bets for Passive Cooling”). Because natural ventilation is one of the most cost-effective ways to cool your home, we’ll examine it here in greater detail.
Natural Ventilation

Natural ventilation makes the most of air motion to cool you and your home. This is the primary passive cooling strategy in all climate zones, but the nuances of its application vary by region. Understanding seasonal wind patterns will help you adjust your window openings, outdoor spaces and windbreaks to increase your comfort without relying on nonrenewable fuels.

Take some time to think about the breezes and winds around your home:

* At what time of day and year are the winds strongest?
* From which direction does your prevailing wind come (the one that blows most of the time, when there are no storms)?
* From what direction do storms come?
* Is there a noticeable breeze or wind most of the year? Does it vary much from season to season?
* Do your local breezes shift daily?
* Is local air movement influenced by geographic features or landscape elements?

There are several ways to learn about local wind direction and intensity, such as observing for yourself (at different times of the day and year), accessing weather data and asking local farmers or other people who work outdoors what they observe.

You also can hang a windsock in your yard. A friend who lives near the ocean has done this; she and her family enjoy being aware of changes in the wind’s direction and force, making them feel more like part of their natural surroundings: “Our prevailing wind comes from the northwest, so most of the time the windsock points to the southeast. But sometimes it suddenly turns and points north, and then we know there’s a storm coming in.” A weather vane on your home or garage can provide the same information.

Becoming familiar with local weather patterns can help you decide on a natural cooling strategy. In hot humid climates, for example, maximum airflow combined with shading is the dominant strategy. In hot arid climates, ventilation is welcome in the hot seasons, and night cooling of thermal mass is particularly useful due to lower nighttime temperatures. In cold climates with cool summers, there may be little need for enhanced natural ventilation. Many temperate and mixed climates will require a variety of tricks as the seasons move from one extreme to another. As you read on, think about your own climate zone and your experiences living there; focus on the approaches that feel most relevant to your situation, and see how you might improve the existing relationship between your home and the breezes.

Now look at your house to assess how air moves through it. You can make basic observations on your own, but you may want to bring in a professional for a more technical assessment. Consider the following points:

* What kind of windows does your house have: casements, double-hung or fixed?
* Does your house have other ventilation openings (that is, vents, exhaust fans or turbine ventilators, a cupola)?
* Are there operable windows or other vents on opposite ends of your house? Are some high and some low?
* Can you open enough windows to provide good ventilation in hot weather?
* Does your landscaping funnel breezes to your house in summer and protect it from cold winter winds?
* Does your house’s enclosure have cracks that admit cold air in winter or hot air in summer?
* How high are your ceilings? Do they allow warm air to collect high in the room, which can be a blessing in summer and a problem in winter?
* Do vents or fans exhaust unwanted air, such as unpleasant smells and excess moisture?
* Even with windows open, are there “dead air” zones in your house?

Natural Ventilation Techniques

Once you’ve mapped your local wind patterns and your home’s airflow characteristics, you can use a host of tricks to improve ventilation for cooling. If your home has appreciable thermal mass (dense materials that conduct heat slowly, such as stone, concrete, brick or ceramic tile), or if you choose to add some, you will want to use these ventilation techniques to cool the mass at night in the hot season, so it can absorb unwanted heat from the air and away from your body during the day.

The openings that allow air to pass through your home needn’t always be windows. We ask windows to do many things: admit light, welcome solar heat, frame views and provide ventilation. But these functions often have conflicting requirements. In hot arid climates, for example, breezes are more desirable than sunlight. Sometimes the best solution — particularly in a remodel situation — is to provide vents that are separate from windows. Adding sun protection, insect screens, louvers, insulated doors or a combination of these will allow you to fine-tune your new vents. A good example from historical buildings is a cupola with louvered vents all around that allow warm air to escape as it rises, inducing air movement through the whole house.

We can learn a lot from vernacular buildings that make the best use of nature’s cooling ventilation for their particular climate. In the South, shuttered verandas, high ceilings, operable transoms, two-story porches and dogtrot houses with open breezeways down the center maximize both cross ventilation and shade to counter the humid heat. In the Midwest, summer kitchens keep the heat of cooking out of the home, while screened sleeping porches put warm human bodies in the path of nighttime breezes. In desert regions, the thick earthen walls of adobe homes protect the interior from harsh sun during the day; at night the cool breezes sweep away any lingering warm air.

Cross ventilation. When air flows into a room from one side and out the other, you’ve got cross ventilation. As with any air movement, cross ventilation can cool your body by speeding up the evaporation of sweat, and it can cool your home by removing hot air, especially at night, if the incoming outdoor air is cool.

Any room with openings on opposite sides can be cross-ventilated if the openings are large enough. But most homes are at least two rooms deep in many places, so you also need to look at airflow through the whole house in order to ventilate the rooms effectively. Think of your house as a system of corridors and doorways that can channel air from one end to the other.

Looking at the air motion notes you’ve made, see if the combination of prevailing breezes, house orientation, room layout and openings (windows and other vents) could be improved to make the best use of those breezes. Are there air inlets on the windward side and outlets on the leeward side? If your home isn’t oriented to face the prevailing breeze, you can use landscaping or wing walls on the windward side of your house to encourage the wind to approach your inlets head-on.

As you plan to make changes in your house’s air inlets and outlets, consider these guidelines:

* Openings in opposite walls allow maximum air movement.
* Openings in adjacent walls create air turbulence, increasing the cooling effect.
* A combination of low inlets and high outlets can achieve the greatest scouring of room air. This strategy is especially useful for night cooling of thermal mass floors.
* If you install new openings, make sure the air moves around the people in the room in order to best cool them. Having either some low and some high openings or all openings at a mid-height should achieve the desired effect.

Finally, you can enhance the effectiveness of cross ventilation by naturally cooling the air before it enters your home. Shade, plantings or water (in arid climates), in the form of a pond, fountain or mister, all can remove heat from the air. When located on the windward side of your home, these features will increase your indoor comfort in hot weather.

The chimney effect. If you live where breezes are rare, or just a bit too gentle, you can use the “chimney” or “stack” effect to enhance air movement. The chimney effect is driven by the rising of warm air; when air is heated, it expands, becomes lighter and rises. If that rising warm air is allowed to escape high in a structure, it will be replaced by cooler (heavier) air entering lower in the structure.

The rate of air movement is affected by the vertical distance between inlets and outlets, the size of the openings, and the difference in air temperature from the bottom to the top of the chimney; the greater each of these features, the faster the air movement. One advantage of this strategy is that it doesn’t require any particular orientation to the prevailing breezes; it drives itself.

f you have a tall house — one with multiple levels, high ceilings or both — you may already have the necessary components for taking advantage of the chimney effect. You can use an existing stairwell, atrium, clerestory or other vertical air passage to increase airflow in your house. If you already have the necessary openings at the top and bottom of this airspace, your only task may be to open and close them as needed. Or perhaps you have the needed airspace, and the addition of vents will allow it to function as a thermal chimney.

However, if your house doesn’t have any such features, you can add a chimney or atrium to pull out hot air in summer. You can further enhance the airflow by adding south-facing glass at the top of the chimney to increase the temperature difference from top to bottom, or even by adding a fan to increase airflow when necessary. Placing the outlet on the leeward side of the building will further enhance airflow; as wind blows around the chimney, it will essentially suck air up the chimney. The rate of airflow will be greatest when the cross-sectional area of the chimney remains the same from inlet to outlet.

Wind catchers. If there are breezes in your area — but buildings, vegetation or landforms keep them from getting to your house — there’s another way to bring those breezes indoors. Wind catchers (also known as wind scoops) have been used for centuries in the Middle East, where temperatures are high and buildings are often packed close together. A wind catcher is a tower that rises from the house into the airspace above the rooflines (see illustration below). Its opening faces into the prevailing wind, scooping the breeze down into the rooms below. Capturing air higher up has additional benefits: The air is cooler, the breeze moves faster, and the air carries less dust. When the air is still, well-designed wind catchers can work in reverse, with the chimney effect drawing warm air upward and out of the house.

A wind catcher also can be an elegant solution where windows aren’t oriented well for capturing breezes. The windows may face any which way, but the wind catcher can be precisely oriented to the prevailing breeze. Based on knowledge of wind patterns in your area, you can design a wind catcher with openings in as many directions as you need. For best results, inlets should be at least 8 feet above surrounding obstructions, so take care to integrate this feature with your overall design. If your climate is not hot all year, you may want to add insulated doors to your wind catcher to avoid winter heat loss.

Vegetation and other landscaping. You can use garden plantings and landscape walls to direct, and even cool, a breeze before it enters your home. Rows of leafy trees or tall, dense shrubs can funnel air to open windows, their shade and transpiration cooling the air as it moves through.

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