Sustainable Building Sourcebook
Chapter: Energy
 
Passive Solar Design
 
CSI Numbers:
 
Introduction:
 

Passive solar design refers to design and material choices that use or avoid the sun's heat to promote thermal comfort and energy conservation and obviate or reduce the need for mechanical heating, cooling, and ventilation systems. It is the traditional method used throughout the world to increase comfort.

See the Earth-Sheltered Design section in the Energy chapter of this Sourcebook for a discussion of passive solar techniques that employ the earth as a major component of a building's thermal control system.

 
At-A-Glance Notes:
 
Technology:
Well-developed for both passive heating and passive cooling.
 
Suppliers:
Passive solar design is practiced to varying degrees by many designers and architects in Central Texas.
 
Cost:
The initial cost of passive design elements depends on the element. Some cost the same (e.g., light exterior walls for cooling in a hot climate) or only slightly more than conventional building costs (e.g., overhangs designed to control sun penetration in a hot climate). Some cost considerably more (e.g., interior thermal mass materials such as stone and brick). Operating costs are substantially lowered by these measures, however.
 
Public Acceptance:
There is a basic understanding and acceptance of passive heating among people who have relocated to Central Texas from colder regions. Passive cooling approaches are less well understood but are not controversial.
 
Regulatory:
Allowable glass area is contingent on window type (e.g., single-pane or insulated, frame material) and insulation levels, according to the Energy Code. It typically is not necessary to exceed the allowed amount of glass to accomplish passive solar heating in Austin. An approved analytic tool (such as that provided by the Passive Solar Industries Council) should be used to establish amount and placement of glazing, overhang size, etc. The City of Austin Energy Code does have shading requirements, which can be met in a variety of ways (properly sized overhangs, solar screens or southern low-e glass with a minimum shading coefficient of 0.50 or minimum solar heat gain coefficient of 0.445).
 
Considerations:
 

Optimum design features and material choices depend on the conditions to be mitigated in the area in which the building is located. It is critical to select elements appropriate for both the macroclimate of the area and the microclimate of the building site.

A measure that works well in one place may not work well in another. For example, sunrooms, trombe walls, and skylights are effective means of heating in cold climates, but they are likely to result in overheating in Central Texas. Central Texas has fairly consistent cooling breezes from the Gulf of Mexico, but if a building site is on the west side of a hill, it will be cut off from this breeze.

It should be easy to provide all heating needed in Central Texas with passive solar means. In fact, care must be taken to avoid overheating.

Cooling measures are much more important than heating in this region. However, passive cooling is difficult due to frequent high humidity, especially in the spring and fall and at night in summer. Because of nighttime cloud cover, heat does not dissipate well to the night sky. Evaporative cooling does not work well under these conditions. A whole-house fan may be effective at flushing out hot air in dry periods, but will simply bring in too much moisture for comfort in humid periods. Carpets, drapes, and over-stuffed furniture should be avoided as much as possible, because they absorb moisture and make occupants feel clammy.

 
Guidelines:
 
Whether for passive heating or cooling in Central Texas, the following design features are suggested. The optimum building shape is long and narrow, with the longest sides facing south and north. There is minimal glazing on east and west, because the sun strikes those surfaces at least half the day and cannot be easily controlled. A moderate amount of glazing may be placed on the north side without a major energy and comfort penalty. The most glazing should be on the south side. Overhangs on the south side can be easily sized to shade in the summer, when the sun path is high in the sky, and allow sun in the winter, when the sun path is low.

Passive Solar Heating
Two primary elements are required: south facing glass and thermal mass to absorb, store, and distribute heat.

There are three approaches to passive systems-- direct gain, indirect gain, and isolated gain. Only direct gain is applicable in Central Texas, unless the system is very well engineered.

The goal of all passive solar heating systems is to capture the sun's heat within the building and release that heat during periods when the sun is not shining. At the same time that the building is absorbing heat for later use, solar heat is available for keeping the space comfortable, but not overheated.

Direct Gain
In this system, the actual living space is a solar collector, heat absorber, and distribution system. South-facing glass admits solar energy into the house where it directly and indirectly strikes thermal mass materials, such as masonry floors and walls. The direct gain system utilizes 60 - 75 percent of the sun's energy striking the windows.

In a direct gain system, the thermal mass floors and walls are functional parts of the house. It is also possible to use water containers inside the house to store heat.

However, it is more difficult to integrate water storage containers in the design of the house.

The thermal mass tempers the intensity of the heat during the day by absorbing the heat. At night, the thermal mass radiates heat back to the living space.

Direct gain system rules of thumb for Central Texas:

Conduct a heat load analysis of the house.
Do not exceed 6 inches of thickness in thermal mass materials.
Do not cover thermal mass floors with wall-to-wall carpeting. Keep floors as bare as functionally and aesthetically possible.
Use a medium dark color for masonry floors and use light colors for low-mass walls. Thermal mass walls may be any color.
For every square foot of south glass, use 150 pounds of masonry or 4 gallons of water for thermal mass.
Fill the cavities of concrete block used for thermal storage with concrete.
It's better to have thermal mass spread throughout the living space rather than concentrated in one place.
The surface area of mass exposed to direct sunlight should be 9 times the area of the glazing/windows.

Sun tempering is the use of direct gain without added thermal mass. For most homes, multiply the house square footage by 0.08 to determine the amount of south facing glass for sun tempering.

Passive Solar Cooling
Thermal mass
To promote passive cooling in a climate that is both hot and humid, a low mass or a high mass structure may be used.

Low thermal mass - the use of materials that don"t heat well and cool quickly when the sun is not striking them. This includes such materials as a light metal or wood frame and a metal roof.

High thermal mass-the use of materials that hold heat well and change temperature slowly. This includes such materials as masonry and stone for floors, walls, and roofs.

Low thermal mass structures have typically been built in the hot, humid south, which also has high nighttime temperatures. High thermal mass structures have typically been built in the dry southwest, where the nighttime temperatures are cool, and occupants benefit from lag time between the heating and cooling of the building. However, recent studies show that high thermal mass structures provide at least a small comfort benefit in any cooling climate (a climate in which cooling in needed).

Reflectance
A large amount of the sun's heat can be reflected away from a building by the use of highly reflective exterior surfaces, especially for roofing. Reflectance depends on such elements as color and texture. The closer to white, and the smoother and shinier, the more reflective the surface and the cooler the building will be.

Shading
The most important passive cooling strategy, regardless of mass, is shading. The more of a building that can be shaded, especially the roof and windows, the better.

The most effective shading is accomplished outside the building by trees (which also cool by transpiration), or even a hill, or other buildings. One part of a building may also serve to shade another.
Further exterior shading of windows and walls can be accomplished by overhangs, arbors, and awnings.

To shade in summer (and allow sun penetration in winter) use the following formula:

kx = d . Where k = the multiplier for a given orientation at a given latitude; x = width of the overhang in feet; d = depth of shade on the wall/window

Latitude multipliers for Central Texas (30 degrees N):

           0.83 = glass facing east or west

           1.63 = glass facing southeast or southwest

2.89 = glass facing north, northwest, or northeast

           5.40 = glass facing south

Note that an overhang only two feet wide shades an entire 8" or 9" high south wall in summer.

  • Solar screens or southern-type low-E window glass (spectrally selective) further reduces heat gain. (Note that any screen, whether solar or insect, decreases the velocity of slow breezes, but screening a porch will not reduce air speeds as much as screening windows.)
  • Interior shading of windows by blinds or drapes is helpful, but not as effective as exterior shading, since heat has already penetrated the building.

Passive Solar Design, Fig. 1

Various external shading techniques

Calculating shade from overhangs

8" wall faces South

Overhang is 2" wide.

Multiplier = 5.4

2(5.4) = 10.8" of shade.

3" x 5" window at 6"-8": window is fully shaded.

8" wall faces Southeast

Overhang is 2" wide.

Multiplier = 1.63

2(1.63) = 3.26" of shade.

3" x 5" window at 6"-8": window only partially shaded.

8" wall faces Southeast

Overhang is 3" wide.

Multiplier = 1.63

2(1.63) = 4.89" of shade.

3" x 5" window at 6"-8": one-foot wider overhang gives almost full shade.


 

 

 

 

 

 

 

 

 

 

 

 

Passive Solar Design, Fig. 2
Different depth overhangs produce different amounts of shade.

Natural ventilation

In Central Texas, prevailing summer breezes come from the south and southeast off the Gulf of Mexico; this phenomenon fortunately matches well with the increased glazing on the south side needed for passive heating, making it possible to achieve both winter solar gain and good ventilation whenever needed with the following strategies:

  • Place operable windows on the south and north sides. Casement windows work best because they can be opened 100 percent and can be selected to swing right or left, whichever directs airflow best in a given space.
  • If possible, place windows on more than one side of a room. If a room can have windows on one side only, ventilation will work best if they are placed as far apart as possible.
  • For best effect, make outlet openings slightly larger than inlet openings. Place the inlets at low to medium heights to provide airflow at occupant levels in the room. Inlets close to a wall result in air "washing" along the wall.
  • Wing walls can be added next to windows on the windward side of the building to direct the breeze into the window. Wing walls create pressure differences, which accelerate natural wind speed.
  • An open interior plan (as few interior walls as possible) promotes cross ventilation across the entire building, not just within rooms.
  • A thermal chimney is a building design feature that enables convective currents to draw air out of a building. By creating a warm or hot zone with an exterior exhaust outlet, air can be drawn into, up, and out of the building, ventilating the structure. Thermal chimney effects can be integrated into the building design with open stairwells and atria.

Passive Solar Design, Fig. 3

Building orientation and window placement and sizing affect natural ventilation patterns.

Passive Solar Design, Fig. 4

Using a thermal chimney as part of a natural ventilation scheme

Materials and landscaping

Passive heating and cooling also rely on the incorporation of materials that affect heat flow. See the Energy Section of this Sourcebook for further information on landscaping, radiant barrier and ridge and soffit venting. See the Materials Chapter for more information on insulation, windows and doors, roofing, and more.

Systems and lifestyle

The ability of passive designs to provide comfort depends on internal heat and moisture loads as well as external effects from the climate and site. Appliances and electric lights (especially incandescent) add a great deal of heat and/or humidity to the living space. Cooking, bathing, and doing laundry do so as well. Judicious use and timing of systems and activities can make a big difference to occupant comfort.


 
Resources:
 
Professional Assistance:

See the Green Building Professionals Directory under "Architects." 

 
General Assistance:

American Solar Energy Society, Inc.
(303) 443-3130
www.ases.org

US Department of Energy
Energy Efficiency and Renewable Energy
www.eere.energy.gov

Florida Solar Energy Center
(407) 638-1000
www.fsec.ucf.edu

National Renewable Energy Laboratory
(303) 275-3000
www.nrel.gov

Solar Energy Industries Association
(202) 383-2600
www.seia.org

Solar Energy International
(970) 963-8855
www.solarenergy.org

Northeast Sustainable Energy Association
(413) 774-6051
www.nesae.org

Sustainable Buildings Industry Council
Washington, DC 20005
(202) 628-7400
www.sbicouncil.org

Southface Energy Institute
(404) 872-3549
www.southface.org
Technical Bulletin (PDF)

Texas Solar Energy Society
P. O. Box 1447
Austin, TX 78767-1447
(512) 326-3391
(800) 465-5049
www.txses.org