Passive Solar
Passive solar space heating is the logical first choice of energy conscious buyers and builders. Passively solar heated homes and commercial buildings often reduce total building occupancy costs, by tapping the great flow of solar energy using simple, proven technology.
Passive solar heating systems function extremely reliably, because, as part of the structure, they operate with no moving parts. Passive solar buildings benefit from the seasonally varying solar path. In winter, south-facing windows collect the sun's rays; in summer, overhangs, awnings, or shutters prevent heat gain through the same windows.
Passive solar is not a research project
The ancient Greeks, Romans, Anasazi, and Chinese mastered passive solar heating and climate control without modern technology. Modern passive solar construction developed over the past century, with buildings of varying success guiding designers and builders toward reliable performance. New passive solar homes insure performance by using well-known guidelines for windows, heat storage, and overhangs for summer shade. In Iowa, passive solar systems can easily provide most of a building's heating requirements and daytime lighting. Improvements over the past half-century in insulation, sealing against infiltration, glazing, designs, and building methods have led to passive solar construction becoming the major additional fuel saver in new construction. It offers savings on heating and cooling costs with minimal or, even, no additional construction cost.
Three key elements of passive solar design fit into any building style:
- Heat gain through south-facing glass - To maximize heat gain, the long axis of the building and the majority of windows should face within 15 degrees of south. The sunlight must strike interior (inside the insulating envelope) energy absorbing surfaces of the storage mass.
- Heat storage in sunlit mass - High thermal mass materials, such as concrete, brick, or stone, used in sunlit floors and walls, store heat during sunny days and release it slowly at night. Dedicated heat storage media, such as water tanks or rock-filled chambers, can also provide thermal mass. Conversely, cooling these materials with ventilation on summer nights allows the thermal mass to help keep the house cool throughout the day.
- Containment of building heat - requires minimizing air infiltration and installing sufficient insulation in roofs and walls, including basement and crawl space walls or under floors. Tight, well-insulated construction increases the benefits of southfacing window area and storage mass. Windows must have low infiltration and high R values. Insulated shutters or shades also help to limit heat loss, but require daily opening and closing. For retrofits, effective sealing against infiltration and good insulation are essential and almost always require upgrading, since few older buildings meet current standards. In Iowa, extremely energy efficient construction is more cost-effective for reducing fuel use than installing huge collector area and storage mass.
Adequate ventilation of any well-sealed house is vital to maintain indoor air quality. In winter, a heat recovery ventilator (air-to-air heat exchanger) best provides for interior air quality. It brings in fresh air from outside, while recapturing most of the heat from the exhausted air. In summer, properly planned vents or windows placed high in rooms allow warm air to escape. If necessary, high efficiency fans can improve ventilation. Good air movement alone will help to make a space feel cooler in the summer.
Preventing overheating: Sufficient heat storage capacity for the south-facing glass area will prevent overheating on sunny winter days. Large thermal mass lengthens heating-cooling cycles, helping to improve mechanical heating equipment efficiency and reduce maintenance. Oversized storage mass does not harm performance, but has a diminishing benefits to costs ratio. Reasonably, storage might provide heat through one very cold night after a sunny day, with backup heat (required by most codes) for series of cloudy days. As a rough guideline, south-facing glass area should be 7 to 15% of the house's total heated floor area. Storage capacity should include about one cubic foot of masonry or one-half cubic foot of water for each square foot of south-facing glass. For direct gain, the sunlit area on the heat storage mass should be at least three times the area of the south-facing glass.
A passive solar home begins with an acceptable site or, for retrofits, existing site assessment. A site on a south-facing slope is advantageous. Solar energy comes without charge in most locations, but on steep north slopes or in the shadow of tall buildings or evergreen trees, direct sun may be unavailable in winter. On any lot, one must determine accurate cardinal directions and latitude. For the intended south wall location, calculate the solar path to insure that shadows from structures or trees (taking into account future growth of trees) on neighboring properties will not occlude winter sun from windows or collectors. Ideally, neither trees nor other buildings should block the sun's rays in winter, especially between 9:00 am and 3:00 pm solar time. A less than perfect site that dictates an other than ideal design should not deter the homeowner from using passive solar heat.
Successful passive solar homes exist in all regions of the United States. Homes in cloudier areas will tend to have greater south-facing glass area and greater heat storage capacity. Designers may adjust upward the glazing or collector area to offset partial shading by leafless deciduous trees; enough heat can still get through to provide substantial benefit, and shading of windows and collectors in summer may justify acceptance of some degradation of heating performance.
Municipalities, zoning boards, or developers can require good solar access for the vast majority of lots in new developments. Normal plat review then would include verification of compliance with solar access requirements or guidelines. In urban developments, streets running east-west are usually necessary for unobstructed south sun to reach windows or collectors. Passive solar construction on north-south streets (unless the lots are quite wide) has potential problem of shading of the south wall by the neighboring house and trees. Providing solar access for solar space and domestic water heating in a housing development's original plat entails little extra effort, but if a plat leaves many houses shaded by their neighbors, retrofitting for solar heat becomes difficult, expensive or, even, impossible.
Ideally, original construction should provide passive solar features in the structure.
Masonry and concrete within the insulation envelope become beneficial thermal mass, if they receive direct (or reflected) sunlight during the heating season. Placing that beautiful masonry wall in the interior of a house may mean that it does not impress passers by, but the homeowner can enjoy feeling the heat it stores on many a cold night. Concrete, stone, or brick floor and wall construction methods are the same as in sun-ignoring homes, although thicknesses may be altered to provide for more storage or adjust the heat conduction rate through interior walls. Most windows should be on the south side, with only a few others for balanced daylight and to avoid excessive summer heat gain.
Careful planning of passive solar retrofits includes balancing heat collection and storage to gain the most benefits. While thermosiphoning air panels can provide daytime heat without interior remodeling, other solar retrofits are more difficult. If adding south-facing windows beyond the "sun-tempering" level, comfort requires sufficient thermal mass to prevent daytime overheating. Increasing interior thermal mass may require structural modification or reinforcement. Added south windows increase winter solar gain, but avoiding summer heat gain requires appropriate shading. Simple solar features and orientation for new construction demand major remodeling once a sun-ignoring house is built. For this reason, many solar retrofits are new solar additions, with passive solar features in the new construction.
Passive solar homes and buildings save money
Owners benefit immediately when saving on fuel cost is greater than any increase in mortgage payments. Compared to well-built "conventional" homes, passive solar construction substitutes careful design and, often, some additional construction cost for a reduction in heating fuel cost. Construction cost difference is small, because passive solar construction uses normal building elements slightly rearranged. Added window area and additional masonry or concrete walls or floors will increase costs. But extending the insulating envelope to contain concrete or masonry that would be used anyway is inexpensive. Reducing north and west window area partly offsets the increase in south window area. Weather sealing and insulation are the same as tightly sealed, energy efficient "conventional" construction. Thermal drapes or shutters to prevent heat loss at night may be an extra cost. "Right-sizing" heating (and cooling) equipment to reflect smaller demands of a passive solar home offers some savings.
Passive solar homes, as investments, have a partial buffer against future fuel price fluctuations and should outperform sun-ignoring homes. While resale value depends on the local market and the tastes of individual buyers, the passive solar home represents the safer investment. When modern builders and development planners refuse to plan for solar access and to use solar heat in new construction, they signal their clear rejection of fiduciary responsibility to the customers. Only easy availability of inexpensive fuels allowed 20th century builders to ignore the sun in city planning and construction.
Passive solar homes help the environment, and the local economy. Using solar energy creates no pollution and reduces combustion of heating fuel, thereby decreasing greenhouse gas production. With future affordability of fossil fuels doubtful, long-term economic security demands use of indigenous renewable fuel resources. Because the United States imports large amounts of fossil fuel (particularly oil), much from politically unstable or unfriendly regions, passive solar homes help the national balance of payments, reduce involvement in foreign conflicts, and leave more money in the local economy.
Many reputable architects and builders have experience in passive solar construction. One may find them by contacting one's local builders association or the AIA Committee on the Environment.