Desert Botanical Garden: THE DESERT HOUSE: Passive Solar (continued)

Builders use R-value to describe heat resistance of building material. The higher the R-value, the more resistant the material is to the transfer of heat. A building constructed with high R-value materials is easier and more cost effective to maintain comfortable temperatures. Cooling and heating systems do not have to work as hard or as long, thereby reducing the amount of energy needed to operate the system.

The following table provides the R-values of Common Construction Materials:

[ US Department of Energy: R-value Recommendations for Eight Insulation Zones ]

Montezuma's Castle effectively uses the principles of thermal mass and insulation in the following ways:

location: inside a shallow south-facing limestone rock shelter
winter sun: when low on the horizon provides daily warming
heat storage: stored in the rock and cliff walls of shelter - dissipates slowly
summer sun: cliff overhang provides shade during daytime
walls: thick adobe walls provide thermal mass and insulation
windows and doors: small to minimize heat loss and easily covered
ventilation: allow rising warm air to flow away while pulling cool air through structure

Throughout the arid southwest ingenious developments in architectural technology can be idenfified in the homes of historic and modern inhabitants. The O'Odham ki, or round house provides protection from heat, cold, and wind.

During the winter the ki can be sealed tight with adobe to keep the cold out, but during the summer the brush walls can be left open to allow air to freely blow through the structure and cool the occupants through evaporation of perspiration of their body surface. The brush walls of the ki can be moistened to provide a greater surface area for water to evaporate. This evaporation will reduce the air temperature inside the house.

Other passive cooling characteristics include vertically oriented ventilation shafts, such as stairwells, high ceilings, chimneys or ducts. These shafts allow lighter hot air to rise and escape during the day and dense cool air to enter and descend during the evening. Wind capturing devices, known as malqafs in Arabic, resemble a chimney and funnel air into buildings. Cooling towers are a modern version of this old technique. These towers use moistened pads to further inhance the cooling effect caused when dense cool air drops. Fountains and ponds can also contribute both psychological benefits with pleasing sensory experience and some cooling effect.