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Thursday, August 9, 2007

On Building: Keeping your house cool.

Many people in Baja will rely on their air-conditioning during the summer months to keep their homes cool. While there are times when air conditioning is a good choice for cooling, there are other less energy intensive ways to regulate the household temperature.

We will start with some basic techniques for controlling heat gain that can be applied to any home, then move on to more advanced construction methods and materials.

During the hottest parts of the year in San Felipe, we focus on lowering the indoor air temperature. To do this, start by looking at the house from the exterior. Our goal is to minimize the amount of heat entering the home from outside. The roof, south and west facing walls and windows of a building receive the maximum amount of solar exposure and are responsible for the majority of heat transfer to the interior. Shading these surfaces will greatly reduce the heat gain in the house. Shade for the walls can be created with structures such as porches or palapas, or by planting trees or vines. Windows are best shaded with exterior shutters, overhangs or interior curtains. Flat roofs can also be shaded with plants, or with shade cloth or a double roof system with an airspace. White or other light colored paints reflect the suns rays and also help keep the interior cool. Another possibility for an existing home is to apply rigid insulation to the exterior of the building, then re-stucco over the insulation.

These tips will reduce the load on your a/c system and therefore reduce your monthly electric bill.

But what if you want an even lower electric bill or to eliminate it totally? In these cases, we look back to the time proven tactics employed by our ancestors, people who lived in similar climates to ours before mechanical refrigeration was available. Today we call these techniques ‘passive cooling.’ Our ancestors called it ‘common sense.’ Centuries of trial and error have provided us with some extremely effective cooling strategies.

In fact, the first ice creams were produced centuries before mechanical freezers, in the hot arid environment we now know of as Iran!

Keeping cool using passive cooling requires an understanding of human comfort levels. The illustration below is a graph showing temperature vs. relative humidity. At the center, the comfort zone represents the temperature and humidity levels that most people find acceptable. (solid black line in this graph)

GRAPHIC:The dashed lines on the graph show four passive (non-mechanical) cooling strategies:

Natural ventilation - using only air movement to cool the home and occupants - refreshing sea breezes can be incorporated into the building design through cross ventilation.

High thermal mass - the ability of a material to absorb heat during the day and release it at night. To be effective the thermal mass material must be exposed to the interior. A house is considered to have average thermal mass when the exposed mass area is equal to the floor area. (a concrete slab floor provides average thermal mass). High thermal mass is achieved when exposed mass area is 3 or more times the area of the floor.

Evaporative cooling - cooling the interior by evaporating water. During dry weather, this is often accomplished with terra cotta water pots, fountains and plants.

High thermal mass with night cooling - relies on the thermal mass’s ability to slowly absorb heat from the day, then by opening the home at night, cooler night air is allowed to recharge the mass. Windows and doors are open at night and closed during the day.

Mediterranean cultures have used these concepts for centuries. The stone and adobe houses of Greece and Egypt are excellent examples of low-tech ways to maintain comfortable indoor temperatures. The whitewashed exteriors of the houses reflect the sun’s rays and thick adobe walls provide the necessary high thermal mass. Courtyards and narrow streets provide shaded areas for evaporative cooling through fountains and foliage, while creating stack effect natural ventilation during day and cool air pooling at night.

Incorporating these time-tested strategies into a building design can greatly reduce and often eliminate the need for air conditioning. Notice how much larger the comfort zone can become with passive cooling!

About the author, Jonathon E. Spinner is President of Adobe Block Works, Inc. a San Felipe based, design-build firm specializing in adobe construction and dedicated to Old World ingenuity combined with Modern technology. He can be reached by email at jon@adobeblockworks.com