Passive Solar Design for the Home

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To understand how passive solar design works, you first need to understand how heat moves. Heat-Movement Physics As a fundamental law, heat moves from warmer materials to cooler ones until there is no longer a temperature difference between the two. A passive solar building makes use of this law through three heat-movement mechanisms—conduction, convection, and radiation—to distribute heat throughout the living space. Conduction is the way heat moves through materials, traveling from molecule to molecule. Heat causes molecules close to the heat source to vibrate vigorously, and these vibrations spread to neighboring molecules, thus transferring heat energy. For example, a spoon placed into a hot cup of coffee conducts heat through its handle and into the hand that grasps it. Convection is the way heat circulates through liquids and gases. Lighter, warmer fluid rises, and cooler, denser fluid sinks. For instance, warm air rises because it is lighter than cold air, which sinks. This is why warmer air accumulates on the second floor of a house, while the basement stays cool. Some passive solar homes use air convection to carry solar heat from a south wall into the building's interior. Radiant heat moves through the air from warmer objects to cooler ones. There are two types of radiation important to passive solar design: solar radiation and infrared radiation. When radiation strikes an object, it is absorbed, reflected, or transmitted, depending on certain properties of that object. Opaque objects absorb 40 to 95 percent of incoming solar radiation from the sun, depending on their color—darker colors typically absorb a greater percentage than lighter colors. This is why solar-absorber surfaces tend to be dark colored. Bright-white materials or objects reflect 80 to 98 percent of incoming solar energy. Inside a home, infrared radiation occurs when warmed surfaces radiate heat towards cooler surfaces. For example, your body can radiate infrared heat to a cold surface, possibly causing you discomfort. These surfaces can include walls, windows, or ceilings in the home. Clear glass transmits 80 to 90 percent of solar radiation, absorbing or reflecting only 10 to 20 percent. After solar radiation is transmitted through the glass and absorbed by the home, it is radiated again from the interior surfaces as infrared radiation. Although glass allows solar radiation to pass through, it absorbs the infrared radiation. The glass then radiates part of that heat back to the home's interior. In this way, glass traps solar heat entering the home. The term thermal capacitance refers to the ability of materials to store heat, and thermal mass refers to the materials that store heat. Thermal mass stores heat by changing its temperature. This can be from a warm room or by converting direct solar radiation into heat. The more thermal mass, the more heat can be stored for each degree rise in temperature. Masonry materials, like concrete, stones, brick, and tile, are commonly used as thermal mass in passive solar homes. Water also has been successfully used. Basic Passive Solar Design Techniques Passive solar homes range from those heated almost entirely by the sun to those with south-facing windows that provide some fraction of the heating load. The difference between a passive solar home and a conventional home is design. And the key is designing a passive solar home to best take advantage of the local climate. Elements of design include window location and glazing type, insulation, air sealing, thermal mass, shading, and sometimes auxiliary heat. You can apply passive solar design techniques most easily to new buildings. However, existing buildings can be adapted or "retrofitted" to passively collect and store solar heat. In some ways, every home is a passive solar home because it has windows, but designing a home to work in its climate is the basis for these techniques.

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