What Is It and How Does It Work?
Passive solar heating includes a series of strategies to harness the warmth provided by the sun to heat your home in the winter. These strategies also account for warm weather in the summer and can help keep your house cooler then as well. Passive solar design uses your windows to collect solar energy, and your walls and floors to store and distribute solar energy to your house as heat. True passive design does not involve any electrical or mechanical components, such as an HVAC system, furnace, or fans. However, there are ways to integrate fans or pumps into a passive solar design to make it perform better. Additionally, passive solar design strategies can be used in conjunction with an HVAC system to reduce the load on your mechanical system and lower your energy bills.
Some components of passive solar design can easily be integrated into new construction with no cost implications, such as including south-facing windows that collect solar energy in the winter with overhangs that block the summer sun. Other strategies require added design elements, but can also be integrated into most homes and in some cases can be taken advantage of as living spaces. Passive solar design is also possible in existing homes as a retrofit, although your options will be somewhat limited by your home's orientation and other existing conditions.
Passive solar design works by moving heat from warm objects to cooler objects and by circulating air through your home. There are three ways that heat is transferred from warm objects in a passive solar home: conduction, convection, and radiation. Conduction is the movement of heat from one object to another through direct contact. Convection is the circulation of heat through liquids or gases (such as air); because warm air is lighter than cool air, it tends to rise to the ceiling, while cool air sinks to the floor. Radiation describes how heat moves from warm objects to cool objects that are not directly touching by passing through the air. Different objects absorb and emit warmth at different rates, depending on their color and another material property called thermal capacitance. Materials that store heat—a critical component of passive solar design—are called thermal mass. Thermally massive materials include brick, concrete, stone, tile, and even water. The reason thermal mass is so important is that by storing heat and releasing it over a long period, it moderates temperature swings and preserves some of the warmth collected during the daytime for cool nights. Without thermal mass, solar energy is released inside your home as soon as it is collected, which may make your home uncomfortably hot during the daytime (even in winter), but provide inadequate heating at night.
Successful passive solar design includes five elements: (1) thoughtful placement and square footage of windows (called an aperture or collector); (2) a dark masonry surface, or absorber, to absorb heat; (3) thermal mass to store heat that enters as solar energy; (4) a distribution method to transfer heat to warm rooms in your home; and (5) elements that can be used to control the amount of solar energy entering your home, such as insulating shades that you can draw closed during the winter at night to prevent warmth collected during the daytime from escaping, or overhangs or awnings that can block the high, hot summer sun and prevent overheating.
What Are the Options?
There are three general types of passive solar design. The most basic and easy to implement type is called direct gain. Direct gain passive solar design involves three steps: solar energy enters the building through south-facing apertures (such as windows); the solar energy is absorbed by masonry floors and/or walls; and the masonry thermal mass releases built-up solar energy as heat through convection and radiation to warm the room. The amount of south-facing glazing dictates exactly how much solar energy can be collected, while the amount of thermal mass inside the room dictates how much solar energy is stored and released as warmth.
There are several important design considerations to keep in mind when implementing a direct gain strategy. First, it is best if the building is oriented along an east-west axis (that is, the long face of a rectangular building should be facing south) to maximize southern exposure. If the south-facing facade is within 15 degrees of true south, the passive solar performance will not be significantly diminished. Since north-facing rooms will not receive solar energy, you should concentrate service spaces such as bathrooms and closets to the north. Living spaces such as bedrooms and living or family rooms should have south-facing apertures. However, since south-facing windows with direct sunlight can be at risk for glare, it can be useful to use white materials or paint near the windows to reduce contrast. Also, although you want to optimize apertures on the south facade, windows can also be a source of heat loss during cool nights. This problem can be mitigated by installing movable insulation that can be placed over the windows at night, or high-performance glazing to reduce nighttime heat losses.
Direct gain passive solar design can be a good option for heating during the winter, but steps must also be taken to avoid overheating in the summer. One strategy that could work on some sites is planting deciduous vegetation to the south of your home. During the summer when the trees are full, they can provide shading to keep your home cool. During the winter when the leaves have fallen, they allow enough of the sun's rays to pass through to help heat your home. You should also have sunshades installed above your windows. Correctly sized sunshades can allow the low-angle winter sun to shine directly through your windows while blocking the high-angle summer sun. In addition to fixed sunshades, it can also be useful to include options for user control, such as movable shades and operable vents in case of summer heat and conditions that are more sunny than normal. In addition, it may be necessary to install a backup mechanical system in case of extreme cold or cloudy weather. One passive solar cooling strategy that works very well with direct gain is night ventilation of thermal mass. If there are large diurnal swings in your climate (that is, it gets very cool at night compared with warm summer days), the thermal mass can absorb solar radiation during the day to prevent overheating. Then, you can open vents that bring cool air into the house at night to draw the heat out of the mass, which helps to keep your home cool.
The material that is used as thermal mass, in general, can be any kind of masonry material, such as brick, stone, or concrete. Dark-colored materials work best because they absorb more solar energy. Most thermally massive materials work effectively enough even if unpainted. Thermal mass in a direct gain scenario is most effective at 4 to 5 inches thick. If your mass is too thick, the lag to return heat to the space will be too long. If the mass is too thin, it will not be able to store enough heat, leading to large temperature swings and overheating. Also, it is critical that the thermal mass is insulated from the outside. Otherwise, it will lose heat absorbed during the day directly to the outside, rather than releasing it in your home.
Decisions that you make regarding floor coverings and windows will have an impact on how effective your direct gain passive solar design is. If you are relying on a concrete or tile floor as your thermal mass, you should minimize the use of rugs and carpets. Soft materials insulate the mass and reduce its effectiveness. Wood flooring is not ideal either, but is not as damaging as carpets and rugs. Windows work best with a direct gain strategy when they have a high solar heat gain coefficient (SHGC) and a low U-factor. A high SHGC allows more solar energy to enter your home, which is necessary for the passive solar strategy to be effective. A high U-factor, on the other hand, gives a window higher insulating properties and helps to prevent heat losses through the windows.
One drawback to consider with a direct gain strategy is that only the rooms that have direct access to southern exposure will benefit. Other rooms will either be unconditioned or will require a supplementary mechanical heating system. An added benefit of direct gain is that it also provides daylighting, which reduces the need to use electrical lights and can save energy.
You may be able to explore the potential for direct gain passive solar strategies in an existing home. If you have ample southern exposure and some form of thermal mass, all you need to do is open your blinds and shades on cold, sunny days. Even if your home has little or no thermal mass, you can still take advantage of direct gain. If your home starts to get too warm, just pull the blinds closed.
Process for Estimating the Amount of Aperture and Mass in Direct Gain Passive Solar Design
Cold climate: 0.2 to 0.4 square feet of south-facing window per square foot of floor area
Mild climate: 0.1 to 0.2 square feet of south-facing window per square foot of floor area
(For example, a 200 square foot room should have 40 to 80 square feet of window in a cold climate and 20 to 40 square feet in a mild climate.)
The rule of thumb for mass is a 3:1 ratio of 4- to 6-inch thick mass to window ratio for mass directly irradiated by solar energy, or a 6:1 ratio for mass that receives indirect radiation (for example, a 200 square foot room in a cold climate with 80 square feet of windows should have 240 square feet of mass).
The second type of passive solar design strategy is called indirect gain. Whereas in a direct gain scenario the solar collection process and living spaces occur in the same place, indirect gain separates solar collection from living spaces by a thermal mass material that distributes warmth. The thermal mass material, or thermal storage wall, consists of a thermal mass wall, an airspace of 1 inch or less, and glazing. Solar energy passes through the glass and is trapped in the airspace, which becomes very hot. Heat then passes through the thermal mass wall via conduction and is released into the space via radiation. The most common type of thermal storage wall is a trombe wall, which uses a masonry wall to absorb and distribute warmth. Another option is a water wall, which is essentially a trombe wall that uses water enclosed in containers instead of masonry.
Thermal storage walls heat up very slowly in an indirect gain scenario, so this is not a good option if you need a space to be heated very quickly (although you could consider indirect gain in conjunction with a supplementary central heating system or individual space heaters. However, this strategy is very good at moderating temperature swings and maintaining comfortable temperatures throughout the night. Indirect gain is a possible consideration as a retrofit strategy if existing conditions permit, but it is more likely to be part of new construction, with significant design considerations.
Some of the design considerations for indirect gain are similar to considerations for direct gain, described above. These include building orientation strategies, including minimizing variance from true south; shading to prevent summer overheating; and inclusion of supplementary mechanical heating, if necessary. Also critical in indirect gain passive solar design is the thickness of the thermal storage wall. If the mass is too thin, heat will pass through the wall too quickly, causing overheating during the day and decreasing the effectiveness of the strategy toward the end of the night. If the mass is too thick, the thermal lag time will be too long, which will prevent the space from being heated effectively. Another consideration is the inclusion of operable vents. During the summer, vents at the top of a thermal storage wall can be opened to pull warm air out of your home. In the winter, the vents are kept closed to prevent heat loss. Maintenance is also an important consideration with thermal storage walls. In all cases, you will need to be able to access the air space to clean the glass and remove accumulated debris. In the special case of water walls, the containers used must be able to handle excessive heat without degrading. Heat passes through a masonry wall at about a rate of 1 hour per inch, so heat absorbed in the late afternoon will still warm the inside of your house late into the night and early morning.
One key drawback of indirect gain versus direct gain is that the windows that are used as part of the thermal storage wall cannot also be used for daylighting. Daylighting is a useful strategy for reducing reliance on electrical lighting, so you will have to either determine another location for incorporating daylighting apertures into your design or include additional windows on the south-facing facade apart from the indirect gain apertures.
Process for Estimating the Amount of Aperture and Mass in Indirect Gain Passive Solar Design
The mass and the glazing should have the same area.
Trombe Wall
Cold climate: 0.4 to 1 square foot of south-facing aperture per square foot of floor area
Mild climate: 0.2 to 0.6 square feet of south-facing aperture per square foot of floor area
(For example, a 200 square foot room requires 80 to 200 square feet of trombe wall in a cold climate and 40 to 120 square feet of trombe wall for a mild climate.)
Water Wall
Cold climate: 0.3 to 0.85 square feet of water wall per square foot of floor area
Mild climate: 0.15 to 0.45 square feet of water wall per square foot of floor area
(For example, a 200 square foot room requires 60 to 170 square feet of trombe wall in a cold climate and 30 to 90 square feet of trombe wall in a mild climate.)
Thermal Mass Storage Thickness
Adobe: 8 to 12 inches
Brick: 10 to 14 inches
Concrete: 12 to 18 inches
Water: 6 inches minimum
The final type of passive solar design is isolated gain, which differs from the other types of passive solar design in that a space that is completely thermally separated from the main occupied space is used for thermal storage. The primary example of isolated gain passive solar design is a sunspace, which is similar to a greenhouse (although it must be configured for providing thermal benefits to your home, rather than for growing plants). A sunspace is a small room with floors and walls made of a thermally massive material and ample south-facing windows. It has a wall that separates the sunspace from other rooms in the house. There are two options for distributing heat absorbed within the sunspace. The first is an insulated wall between the sunspace and other rooms that can include vents to transfer heat to your home through convection. This scenario can also be implemented as a hybrid active/passive strategy with fans to aid in convection. The other option is an uninsulated mass wall between the sunspace and the other rooms, with heat transferred via conduction and radiation through the wall. This scenario can also include vents, which provide the added benefit of distributing heat with a short lag time, in addition to the longer lag time of heat radiating through the wall.
As with direct gain and indirect gain, isolated gain strategies should be designed to minimize variance from true south and should also provide shading to prevent summer overheating. Supplementary mechanical heating may be required in some climates in case of very cold or very cloudy weather, or to heat rooms that are not directly adjacent to the sunspace.
Isolated gain strategies could be a consideration in a renovation if your existing home has room on its southern exposure for an addition. Sunspaces can also be easily integrated into the design of a new home that has adequate solar resources. This strategy does have a significant impact on building form and requires space in your home to be devoted to the passive heating system. The space can be occupied; however, though sunspaces can help to moderate temperature swings and make your home more comfortable, the temperature within the sunspace itself will be widely variable.
Sunspaces are typically designed to protrude from the building on the southern face. However, they can also be wrapped by the building on three sides, providing heat in all three directions. This could be a good strategy to distribute heat to multiple parts of a house. Another, more complicated way to distribute heat throughout a building is to design a sunspace with a convective loop system, which includes a series of ducts and fans to draw air from the sunspace and circulate it through the house, eventually bringing the cooled air back into the sunspace to be reheated. (diagram on p. 264 of MEEB)
Process for Estimating the Amount of Aperture and Mass in Isolated Gain Passive Solar Design
Cold climate: 0.65 to 1.5 square feet of south-facing aperture per square foot of floor area
Mild climate: 0.3 to 0.9 square feet of south-facing aperture per square foot of floor area
What Are the Potential Benefits?
In some climates, a well-designed passive solar heating system can completely eliminate the need for mechanical heating. In most cases, some supplementary mechanical heating will be required, but can be sized much smaller and cost much less to install and operate than without passive solar heating.
How Much Does It Cost?
Direct gain passive solar heating can be integrated into early design plans in new construction in such a way that there are no added costs. If you have an existing house with south-facing exposure, plenty of windows, and some form of thermal mass, you may already have the capacity to utilize passive solar heating. All you need to do is open your curtains and let the sun in.
Indirect gain passive solar heating requires more planning and infrastructure than direct gain. It is possible to retrofit existing construction with an indirect gain system, but such a renovation requires adequate south-facing apertures and also requires sacrificing daylighting apertures for thermal performance. If you are considering an indirect gain retrofit, the potential loss of daylighting—and the resulting increased requirement in electrical lighting—must be part of your cost considerations. In new construction, the cost implications include the extra windows required as a component of the trombe wall or water wall and the cost of building a masonry wall.
Isolated gain passive solar heating requires extensive infrastructure, but can be integrated into new construction or added as a renovation if you have room on your property to expand to the south of your home.
Where Do I Start?
The first step is to consider which type of passive solar design strategy works for your home. If you have an existing home, your options may be somewhat limited. If you are building a new house, as long as you have solar resources to the south, you should be able to consider each of the three strategies. Work with your architect, contractor, or green building professional to calculate the correct size of your passive solar system.
Where Can I Get More Information?
