Natural Cooling Strategies

Who needs air conditioning? These design strategies can help you keep your home cool without it.
By Nathan Kipnis
August/September 2011
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Two elements of natural cooling are shady porches and deep window overhangs that block the sun in summer.
PHOTO: BARBARA BOURNE
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Most people have limited tolerance for hot weather. As the thermostat rises, we quickly become uncomfortable, and if it becomes too hot inside our homes, it’s even dangerous. Our modern response to this problem is simple: “Turn up the air!” However, air conditioning consumes a lot of electricity, and most of it comes from polluting fossil fuels. Electricity is also a limited resource: On the hottest days of the year, some cities don’t have enough electricity to meet demand, leading to brownouts or rolling blackouts.

Fortunately, many old-fashioned design strategies can keep a house cool naturally, which conserves energy and saves money. Although home builders largely have stopped using these techniques over the past 100 years, there’s no reason we can’t rediscover them and use them in our homes. This article explains how to use a few basic natural cooling strategies, whether you’re building a new house or making improvements to an existing home.

Natural Ventilation

Before society embraced air conditioning, we all found simple ways to beat the heat. One was to sit on a shaded porch, sipping a cold drink. If the porch was positioned correctly, gentle breezes would blow past. Breezes help moisture evaporate from your skin — one of the body’s main methods for cooling off. In fact, many natural cooling techniques boil down to one basic principle: Keep the air moving. So how do you improve airflow within your home?

Ideally, when you’re building, you choose the site and orient your house to take advantage of naturally occurring wind patterns. You can also direct summer breezes into and through the house by carefully choosing the types and locations of windows and doors to funnel air through a building.

Many historic architectural styles, especially those used in hot climates, relied on tall windows on opposite sides of a room for cross ventilation. If you’re building a home or planning an addition, you can try this technique yourself. If you cannot place windows on opposite walls, try to place windows on adjacent walls, which still produces some airflow.

Another way to encourage natural ventilation is through a design technique called the stack effect. The general idea is to allow the warmer air in a home to rise up and out of the living space. To create the stack effect, you have to provide an opening toward the top of the space in conjunction with an opening toward the bottom, so that the hot indoor air will be naturally drawn up and out. The greater the height of the space and difference in temperature, the greater the natural draw. This essentially provides a naturally ventilating, unpowered fan system. You can take advantage of this effect by building a home with a ventilation tower. An option for retrofitting a home might include installing an operable skylight.

A solar chimney is an enhanced version of a ventilation tower. Like the stack effect, a solar chimney takes advantage of height and temperature differences to move air. But a solar chimney uses solar gain to increase the temperature difference at the top of the stack. The way it works is that the sun shines into the high section of the chimney. The surface where the sun hits the high wall should be a darker color to increase the temperature difference. This causes the vertical draft to work even more effectively, using the hotter air at the top as the “thermal engine” to drive the conductive airflow.

Once any of these elements are in place, control airflow by opening or closing windows, doors and skylights. For example, if you’re using a stairwell to allow hot air to rise, you could control airflow by opening and closing a door at the bottom of the stairs. Or, you might use operable transom windows above bedroom doors to let a breeze into these rooms while still maintaining privacy.

Limiting Solar Gain

Another way to cool your home is to reduce its solar heat gain by providing some strategic summer shade. It’s easiest to block sunlight entering the house through southern exposures. Designing the home with properly sized overhangs or awnings, specific to the home’s latitude, will keep the high summer sun out, while letting the lower winter sun in for passive heating. It’s harder to block the sun from east and west windows, so consider placing fewer windows on those sides of your house.

If you already have windows where you’d like to block sunlight, white or light-colored shades are a good option. Also consider planting trees where they will shade your windows in summer. Deciduous trees are best, because they shed their leaves in the winter to allow the sun in, which helps with passive heating. Deciduous vines on the building or a trellis outside your window can have a similar effect. Not only does this vegetation provide shade, it also has a natural cooling effect as plants release water vapor through transpiration.

Using Thermal Mass

In certain climates — primarily hot, dry climates that have wide daily temperature variations — appropriately sized thermal mass within the home can help moderate interior temperature swings. Thermal mass includes dense, heavy materials, such as concrete or stone floors, plaster or adobe walls, or a large masonry fireplace. These materials are good at slowly absorbing heat during the day and releasing it at night.

Because the effectiveness of thermal mass is dependent on so many factors — including the specific materials used, the amount of direct sun that strikes this thermal mass, the geometry and insulation values of the home, and the building’s climate — a design professional should be consulted to advise with this.

Using Fans and Evaporative Coolers

Once you’ve exhausted the natural cooling options, consider some other eco-friendly alternatives to air conditioning. Electric fans are a great way to create air movement within your home, especially when there are few breezes outside. Ceiling fans are an excellent choice: Whenever you’re under the fan, the air movement makes you feel cooler, and it requires minimal electricity usage.

Whole-house fans are another easy retrofit, and they work well as long as you understand the basic principles involved. These units consist of a large, centrally located, ceiling-mounted exhaust fan. Simply open a few lower-level or north-facing windows, and turn on the fan to suck large quantities of air up and out through the attic. The time you want to run the fan is when the outside air is cooler than the interior air, which is often true at night. Modern versions of the whole-house fan from companies such as Tamarack Technologies have variable speed control, incorporate insulated shutter doors to prevent heat loss during the winter, and are quieter than their predecessors.

Another alternative to air conditioning is to use an evaporative cooling system, which operates with much less energy than compressor-driven air conditioning. These devices, which are often called swamp coolers, work by removing heat by evaporating water. Many older homes in drier climates successfully use this system for cooling, although they are not recommended for humid climates.

Altogether, you have many options for keeping your home cooler by tapping into natural and low-tech cooling techniques. And whenever you take advantage of these basic strategies, you can feel great knowing that you’re saving money, saving energy, and living more in tune with the natural rhythms of your environment.


Natural Cooling Resources

The Solar House: Passive Heating and Cooling by Dan Chiras

Passive Solar Architecture: Heating, Cooling, Ventilation, Daylighting and More Using Natural Flows by David A. Bainbridge and Ken Haggard

Passive Solar House: The Complete Guide to Heating and Cooling Your Home by James Kachadorian

BuildItSolar.com 


Nathan Kipnis is principal of a Chicago-based architectural firm that specializes in sustainable design. Check out the solar home plan he designed for MOTHER EARTH NEWS. 


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Post a comment below.

 

jay.lloyd.319
5/28/2013 1:19:23 PM

I agree with Baloobearmak, there is no way these designs can make life livable without an AC in the deep south, like where I live in Mississippi. It's just too hot for passive cooling to be the main cooling method. That being said, shade trees over windows, wind corridors, and these other methods are present in all the older house designs here, and I am glad to see some of these methods are making a resurgence to lower energy costs.


baloobearmak
5/24/2013 12:09:11 PM

Bah! Humbug!! Yeah right this design might lower Mississippi daytime inside temps from from 100+ to maybe 95+. This is not a design for the deep south. This "solar" slab can store the nightime 85+ temp to moderate the 100+ daytime temps(actually that 100+ is outdoors. No telling how hot this house would heat up to...cars in the sun can get up to 130+ degrees. Every one of these so called passive house designs are designed for somewhere other than the deep south.


Gary Reysa
8/21/2011 3:21:40 PM
Hi -- One of the studies on cooling says that for the homes they studied 47% of the heat gain was from solar radiation through windows, so the advice on shading windows is (I think) very good. Window shading on the outside is much more effective than inside blinds or the like. Shading with outside shades, trees, ... is (I think)more effective than overhangs because it blocks both the direct sunlight (like overhangs) and also the diffuse and ground reflected light -- the diffuse and ground reflected light are a surprising large part of the incident radiation on windows. Thanks -- Gary

K RHOME
8/19/2011 11:03:31 AM
Why aren't earth tubes mentioned in this article? Aren't they a good option also?








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