Introduction
What is a Passive Solar Home?
Elements of Integrative Passive Solar Design
Benefits of Passive Solar Design

Passive Solar Design
By Anne Jakle, Rocky Mountain Institute
Originally published in Smart Energy Living magazine, Spring/Summer 2005

Introduction

Some of the first homebuilders in the West, the Ancestral Puebloan Indians (Anasazi), constructed high-mass adobe-type dwellings in southern-facing caves. Their design harnessed the direct heating energy of the sun on its lower path in the winter, while rock overhangs blocked heat gain and the sun’s harsh rays on its higher path in the summer. Today, we have a technical term for using the free and abundant energy of the sun to light, cool, heat, and ventilate the home: passive solar design.

What is a Passive Solar Home?

A passive solar home is one that optimizes window, wall, and floor areas to collect, store and distribute solar energy in the form of heat in the winter, while rejecting unwanted solar heat gain in the summer. A passive solar home can range from one heated almost entirely by the sun, to one that simply offsets a portion of the heating and electric load by utilizing south-facing windows. Ideally your home is oriented within 15 degrees of true south (in the northern hemisphere) by having the south and north sides typically being twice as long as the east and west sides, and incoporates design features which allow the warmth collected on the south-side to flow to other areas of the house. These features can cut energy use 30 to 40 percent. New construction that utilizes an integrated passive solar design can lead to a radical downsizing of heating and cooling systems, generating savings that can be pocketed or reinvested in other efficiency measures.

Elements of Integrative Passive Solar Design

Proper building orientation on an east-west axis, with the home’s longest walls and windows facing south, is an essential component of effective passive solar design. Living and working spaces that require lighting or heating should be arranged along the south side of the building; workshops, closets, garages, or utility rooms that require less light and are used less frequently should be located on the north side.
Thermal mass refers to the property that allows building materials to store heat or “coolth.” High mass materials such as stone, concrete, water, and adobe all have considerable amounts of thermal mass. Incorporating these materials into floors, walls, and fireplaces will help absorb and then later reemit thermal energy to maintain comfortable indoor temperatures without the use of active heating or cooling. As a rule-of-thumb, passive solar homes should have approximately 150 ft of 4” thick mass for every 1000 ft of conditioned floor area to optimize performance. Much of Colorado has an ideal climate for summertime passive cooling because of its high daytime and low nighttime temperatures. Night venting allows cool, dry air to flow into the house, transferring coolth to the structure’s thermal mass. This coolth is then reemitted throughout the day, and if circulated properly, may eliminate the need for air conditioning.

Low-e windows are a critical component of effective passive solar design. Low-e windows are windows that have been glazed with a special coating to allow light and heat to enter your home, but they trap 40 to 70 percent more heat inside than a conventional, clear glass, window. Window performance is further enhanced when a low-e coated film is suspended between two panes of glass, effectively making the window perform like a triple-pane window. The two vacant spaces can then be injected with a highly-insulating, inert gas—such as krypton or argon—to maximize the insulative properties of the window. Double-paned, insulated windows with a suspended low-e film retain up to 90 percent more heat than a single-paned window. There are many efficient window technologies available, and to find the best fit for your home, consult an experienced energy rater, green builder or Environmental Building News’ GreenSpec® Directory.
Daylighting features allow you to move natural light deeper into your home. Some daylighting options include skylights, light monitors, clerestories, lightshelves, light tubes, atria, courtyards, and glass or glass-block partitions. Furthermore, you can install venetian blinds with reflective upper surfaces that bounce light off ceilings, while the use of light colored paints and finishings reduce the need for artificial lighting. These features will both reduce your electricity bill and make your home a more pleasing space bathed in a full spectrum of natural light.
Proper overhangs and venting are also crucial for successful passive solar design. Despite all the benefits of bringing natural light and heat into your home, passive solar structures that are poorly designed can produce glare and result in overheating. It is important to consciously place overhangs, such as awnings, shutters, trellises, or deciduous trees to block the sun’s summer path. Proper ventilation is also essential for preventing excessive heat gain. An experienced solar builder or energy rating consulting firm should be consulted for overhang measurements and ventilation design.

The Benefits of Passive Solar Design

Building a passive solar home produces significant savings in heating and electricity bills, and in some cases eliminates the need for heating and cooling systems. Aesthetically, passive solar homes are filled with natural light and maintain pleasant temperatures. Passive solar homes also help offset peak electricity loads during the summertime, when energy needs are high, and keep carbon emissions out of the atmosphere through decreased reliance on fossil fuel-generated power.

The energy-efficiency experts at Rocky Mountain Institute’s headquarters building know these benefits well. The 4,000 square foot building located at 7,100 feet in Old Snowmass, Colorado, which doubles as CEO Amory Lovins’ personal residence, has no central heating or cooling system to regulate temperature throughout the seasons. Whole system passive solar design integrates southern orientation, thermal mass, highly insulated walls, and energy efficient windows to maintain comfortable temperatures year round, with only the help of two small wood stoves. The home’s greenhouse, a central component to the passive design, has produced 27 banana crops, sometimes in the dead of winter. The structure saves 99 percent on space-heating energy and 90 percent of household electricity compared to a conventional structure of the same size.

For More Information

Rocky Mountain Institute is an entrepreneurial nonprofit organization that works to show businesses, communities, individuals, and governments how to meet their goals in an integrative way that creates true wealth and protects the natural and human capital. To learn more about passive solar design, whole system design, and energy efficiency in the home, go to Rocky Mountain Institute’s series of nine Home Energy Briefs, free and available for download from the website http://www.rmi.org/.

For additional information, view the EERE’s “Passive Solar Heating, Cooling and Daylighting”, www.eere.energy.gov/RE/solar_passive.html; Edward Mazria’s Passive Solar Energy Book; or call a professional at an Energy Rating company to evaluate how you can integrate passive solar design into your existing home. 

Anne Jakle works in the Public Information Department at the Rocky Mountain Institute (outreach@rmi.org).

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