Passive Building Design
Responsibility for the environment begins in the earliest stages of design and carries through the life cycle of the building. Passive design concepts can improve the standard of living within a home by providing a structure that is more energy efficient, comfortable, and healthy than a traditional building.
A passive design is one that uses non-energized design features to make the building climate responsive. It is important to note that optimum results can not usually be achieved by using just one strategy; the strategies works best when an integrated approach is used. When beginning the process of passive design, be aware of
area specific information such as climate conditions and the sun’s position (azimuth & altitude) at different times of the day and in different seasons. Other important facts include location specific information about prevailing wind direction and speed, seasonal humidity, and average combined temperature.
Importance of Vernacular Styles
Past architectural styles can give us an indication of effective passive design strategies for our area. There are several styles of vernacular architecture which are specific to particular regions in Florida. Included among the various Florida types are the Cracker style, the St. Augustine style, and the Key West Conch styles.
The classic Florida Cracker style architecture incorporates passive design strategies such as its signature dog trot, its highly reflective metal roof, and large porches that covered the front and rear of the structure.
The St. Augustine house was oriented to the south or east so that the prevailing southeast breezes present in the summer would ventilate the rooms and make the porches pleasant. Its thick tabby walls also insulated against the summer heat. In the winter, when the sun was low, light and heat flooding the loggias and porches,
with the tabby walls acting this time as an insulator against the cold.
The Key West Conch style was ideally suited to the hot, sub-tropical climate of the islands. Nails were scarce so the builders used mortise-and-tenon joints and wooden pegs. The wooden pegs, which would swell in the humid air, provided strong, flexible joints that were ideally suited to the high winds that came with Key West’s
frequent tropical storms.
The Key West homes had large windows and doors that were located to allow for maximum ventilation. Small hatches in the roof would allow hot air to escape from the house through convection. Large porches provided shade and a cool place to sit or sleep. Raised foundations protected the house from flooding and allowed increased air circulation.
As illustrated in the Florida Vernacular architecture, a home’s shape/size, interior layout, and window positioning will all affect the home’s energy use; the energy use can be reduced by 30% by just using the following suggestions.
Energy Saving Design Strategies
- Design rectangular shaped building elongated on the east-west axis Use a simple house design that will minimize the area of perimeter walls.
- Place living spaces and areas that are the most frequently occupied during daylight hours, to the south
- Place laundry areas outside of interior living space if possible
- Take advantage of natural lighting strategies
- Use the proper amount of glass and window locations
- Use properly sized overhangs (Reference: Architectural Graphic
- Plan windows for cross ventilation
Shape & Size
Generally, buildings should be elongated on the east-west axis. A simple, compact design, which minimizes the total area of perimeter walls, will use less energy because there will be less area through which heat can enter or leave a home. Note when reducing the perimeter wall area to reduce interior heat gain, it is not necessary to
take garages into consideration. Living spaces should be raised to take advantage of natural ventilation. Windows should be properly located and shaded from full sun exposure.
The interior spaces of the home should be arranged to take advantages of natural ventilation and natural lighting. A slender home with the living spaces along an axis provides increased privacy and increased levels of natural lighting and cross ventilation. Living spaces should be placed to the southeast, south, and southwest according to their sunlight requirements. Rooms should also be laid out according to the time of day they will be used. Rooms used primarily in the morning, such as breakfast areas, should be located away from the east side of the house to avoid the heat of the morning sun. Rooms used in the afternoon or evening should be located away from the west side of the house to avoid the heat of the afternoon sun. Patios, decks and porches which will be used in the afternoon or evening should be located to the east side of the house so that they will be shaded by the shadow of the house, keeping them cooler. Service spaces requiring diminished levels of light and heat, such as corridors, laundry rooms, closets, and garages can serve as a buffer oriented to the north (A Primer on Sustainable Building, 1995). Garages can also serve as buffers to shield the house from sunlight and reduce the use of windows on east or west exposures. The best location for a garage from an energy efficiency standpoint is on the west side of the house; the southwest, northwest, and east, are also efficient garage locations.
Window size, location and orientation effects solar gain, natural lighting and ventilation. In general, use one square foot of glass area for every 100 square foot of living space for adequate natural lighting. The amount of south facing glass should be equal to five percent of the floor area of the home. It is best to avoid glass the side of the house that face east and west.
Case Study Cost Impacts of Window Orientation
A study by the Jacksonville Electric Authority concluded that energy bills for an average 1,700 square foot house are about $35 per year higher when the majority of its windows face east and west, as compared to a house which had the most of it’s windows facing north or south.
Tinted glass, or solar screens can also be used to reduce heat gain. Solar screens, which fit over the windows the same as standard window screens do, have thicker filaments which help block the sun. Tinted glass, which can appear almost exactly the same as clear glass, can reduce the amount of heat entering windows by almost
A general rule of thumb for south elevations is to add the height of the window to the height of the wall area above the window and divide by four to get the best size for the overhang’s projection on the south facing windows. (Auston Sourcebook).
Shading devises are important because of the intense solar radiation. Overhangs are most effective over windows located on the west and east sides of the house. Glass areas can be shaded using porches, trees, awnings, sunscreens, shutters, and heat mirror film applied to the surface of windows. Particular attention should be given to properly sizing overhangs on the south elevation to allow shade in the summer, while still allowing winter sun exposure.
Horizontal overhangs are effective when the sun is higher in the sky. In the afternoon, when the sun is low, vertical shading devices are more effective. The sun’s position in the sky is always changing, therefore, when designing shading and shading devises the solar profile angles need to be known for the sites latitude; this information may available from local utilities and is included in most passive design books in the public library.
Air movement will evaporate perspiration, creating a cooling effect. For example, a well-shaded house with an indoor temperature of 85 degrees F and a little or no air movement will be uncomfortable. If ventilation is introduced and that 85 degree air moves across the body at 200 feet per minute, or 2.3 mph, the human body will perceive it to be 5 degrees cooler (Florida Solar Energy Center).
Design for Maximum Ventilation
- Install operable windows
- Place house slightly off the north-south axis
- Use landscaping elements to channel breezes (See CHAPTER II)
- Properly locate and size windows for cross ventilation
- Reduce internal barriers to ventilation (i.e. walls)
- Use wing walls if cross ventilation is not possible
- Use stack ventilation to create a chimney effect
The northeast part of Florida experiences from 90 to 100 days per year which are suitable for passive ventilation. Due to the high temperature and humidity levels, passive-cooling strategy poses a difficult problem for the southeast United States in general. The goal of passive cooling is to avoid overheating the structure by solar absorption. However, in order for passive cooling to be effective, consideration must be given to all climactic conditions. The most important factors to consider when planning passive cooling strategies are heat gain, humidity, and air movement (or ventilation). Ventilation is often better captured when the house slightly off the cardinal (north-south) axis by approximately 30 degrees (Florida Solar Energy Center). The introduction of landscaping elements such as hedges, fences, or decorative walls can significantly increase the ventilation into a home.
The placement and sizing of windows significantly effects the cooling effects felt from cross ventilation. It is important to keep the inlet opening as low as aesthetically possible to provide airflow at occupant levels. Ventilation is most effective when the house is fairly shallow or only one room in depth. It is also essential that windows are located on opposite walls to encourage cross ventilation.
If direct cross ventilation proves to be impossible or impractical, wing walls can be employed. Wing walls are introduced when there is a wall that is long enough
to have two windows that open to a common space. The wing walls are placed on the inside (windward) edge of the windows as shown in Figure 5. The pressure differences created by the wing walls will aPCCElerate the natural ventilation through the house as shown in the following figure.
Stack ventilation is slow moving air current of negative and positive air pressures throughout the home created by cross ventilation and the chimney effect. The chimney effect is created when an operable opening is provided at a higher elevation than the living space. Secondly, window openings need to face both the windward and leeward walls of the home; as fresh air enters the home from the windward wall a positive pressure is created. Fresh air will also enter the leeward side of the home if the negative pressure created from the chimney window is strong enough. For about two-thirds of the summer, this natural ventilation will keep the building at or below 80 degrees.
The ability to light an entire house with natural light through the day is extremely challenging, especially when the house has a complicated design. The goal of natural lighting is to provide as much indirect light (light that is reflected and not in a direct line to the sun) as possible to the interior of the building, and to prevent unwanted direct light from entering the building.
Natural Lighting Strategies:
- Light shelves
- Curved Ceiling Planes
Sidelighting from windows located in the walls of the building is the most common source of daylighting. The amount of light decreases exponentially as you move toward the interior of the room. Light colored walls and ceilings that can reflect light in a diffuse manner will aid the effectiveness of the natural light. Horizontally oriented windows high on a wall will permit the best penetration of light into a room. (Auston Sourcebook).
Skylights can be a source of excessive heat gain. Use the following guidelines, from the Auston Sourcebook, when designing skylights:
- Use a translucent glazing which reduces glare. If using clear glazing, use a ceiling diffuser at the bottom of the skylight shaft to improve light distribution.
- Use double glazing.
- Provide an exterior shading system over the skylight during the summer.
A clerestory is a windowed raised section or roof, typically consisting of operable windows, for light and ventilation. The most appropriate orientation is north or south. A southerly oriented clerestory would have an adequate roof overhang above it to prevent direct solar gain. Operable windows in the clerestory also allow heat to escape from the house during the cooling season. North facing clerestories may require a vertical wall
section on the exterior west side to prevent heat gain from the setting summer sun (Auston Sourcebook).
Sloped or Curved Ceiling Planes
Ceiling shape is the simplest mechanism for distributing light in a space. Sloping the ceiling from a high point at the window or skylight essentially has the same impact as maintaining a high ceiling throughout the space. Curving the ceiling can produce dramatic effects. The light from the window or skylight can be focused or collimated in the case of a concave surface or further diffused and spread in the case of a convex surface.
The light shelf is an extremely useful tool when used in conjunction with sidelighting strategies. This mechanism, a horizontal surface at or above eye level, serves to reflect light falling above the vision window up onto the ceiling and therefore deeper into the room. At the same time, it reduces illumination immediately adjacent to the window, where illumination levels are typically too great to work comfortably.