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Passive Design

BIM for Architecture, Engineering, and Construction Curriculum

Overview

A good starting point for any green building design is to look at the passive features of the design:

  • The building placement and orientation on the site
  • The massing and shape of the building 
  • The architectural features of the building, such as the placement of windows and shading features

These passive design features are inherent to the architectural design, materials choices, and configuration of the building. Decisions made when these features are designed determine the requirements for the active systems—including heating and cooling, power, and water—that will be added to the building to meet the needs of its occupants.

A guiding principle is that passive features should be designed to maximize the positive benefits of the local climate and sunlight—for example, capturing sunlight as a heating and daylighting source—and minimize the potentially negative effects, such as adding to the loads on the cooling system. 

Some of the factors that should be considered as part of the passive design include:

  • Placement of the building on the site. What parts of the site are most desirable and why? What parts of the site should not be built upon?
  • Orientation of the building relative to the sun. The location of windows relative to the sun’s path can have significant impacts on the heating and cooling loads throughout the year.
  • The location of adjacent and surrounding buildings. Will other buildings cast shadows over certain parts of the site, rendering certain areas more or less desirable? 
  • Access to buildings on the site. Consider where roads and entrances will be located and the impact on the users of the building. 
  • Terrain and topography. Is the site evenly graded, or are some areas steeper with an uneven ground surface? Are there trees, vegetation, or natural design features on the site that should be preserved?
  • Views. Are there any particular views that occupants of the building will want to enjoy?

Building Orientation


The orientation of a building and its design features relative to the sun’s path can have a significant impact on the performance of the building and the comfort of its users.

As with many design decisions, choosing the best building orientation involves tradeoffs.  If we orient a building to maximize the use of sunlight, we can improve daylighting and reduce the consumption of electricity for lighting. But the impact of increasing the solar radiation captured through the windows must also be considered. In cold weather months, this solar radiation can have positive benefits and help to heat the building. But in hot weather months, this solar radiation adds to the cooling loads and increases the power used by the cooling system.

Since the location of the sun in the sky changes throughout the year (and varies based on latitude), there is no single best answer. Analyses can be performed to determine the effects of a building’s orientation at various times through the year, and an overall optimum can be found that considers the positive and negative effects throughout the year.

Building Massing and Shape


The overall massing and shape of a building can also have significant effects on resource use. As we consider various options for enclosing the desired area or volume of built space, the relationship between the overall length, width, and height of the building determine the size and shape of the building envelope. For example:

  • Long, linear buildings provide lots of wall surfaces that can be used for windows to improve daylighting and reduce the need for artificial lighting. But these wall surfaces are also exposed to the temperature extremes of the external environment and will increase the heating and cooling loads.
  • Tall, skinny buildings also provide lots of wall surfaces relative to the area enclosed. Daylighting can be maximized, but these buildings often have the greatest heating and cooling requirements.
  • Compact buildings (with relatively similar lengths, widths, and heights) minimize the area of the building envelope relative to the volume enclosed. They tend to be very efficient from a heating and cooling perspective, but artificial lighting must be provided for interior spaces where daylighting is not sufficient.

Windows, Overhangs, and Shading


Optimizing the placement of windows is essential for good passive design. Well-placed windows provide daylighting (which helps reduce reliance of artificial lights that consume electricity) and an opportunity to capture solar radiation (which can help heat the building in cold weather months). But these benefits must be balanced with the effect of the additional cooling load created by the captured solar radiation during hot weather months.

To maximize the benefits of windows and minimize the negative effects, it is essential to design shading features—for example, roof overhangs, shutters or canopies, or brises soleils—that will admit sun light during when it is beneficial, but block sunlight when it is not needed or desired.