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Bookshelf - Thermal Comfort
  • Design with Climate Bioclimatic Approach to Architectural Regionalism (Design with Climate Bioclimatic Approach to Architectural Regionalism)
    Design with Climate Bioclimatic Approach to Architectural Regionalism (Design with Climate Bioclimatic Approach to Architectural Regionalism)
    by Victor Olgyay
  • Manual of Tropical Housing and Building Design
    Manual of Tropical Housing and Building Design
    by O. H. Koenigsberger
  • Lessons from Traditional Architecture: Achieving Climatic Buildings by Studying the Past
    Lessons from Traditional Architecture: Achieving Climatic Buildings by Studying the Past
    Earthscan Publications Ltd.
  • Tropical Architecture: Critical Regionalism in the Age of Globalization
    Tropical Architecture: Critical Regionalism in the Age of Globalization
    Academy Press
  • Taking Shape: A New Contract Between Architecture and Nature
    Taking Shape: A New Contract Between Architecture and Nature
    by Susannah Hagan
  • Hot Homes: How to Live with Heat
    Hot Homes: How to Live with Heat
    by Suzanne Trocme
  • Cool Homes in Hot Places
    Cool Homes in Hot Places
    by Suzanne Trocme

 

Elements of Intelligent Design - Thermal Comfort

 

Setting The Scene...

What is it about tropical resorts that make them so utterly enjoyable? Can you not close your eyes and easily picture your favorite sunny, beachy, vacation spot - do you not feel yourself awash in a balmy breeze, the coolness of the shade, the light filtering through louvers? Now, take a mental step back and think of really how hot and humid it is...does it seem like it? If you were to close windows and doors, is it likely to be as pleasant? To get further into this scenario, take the exercise a step further and imagine your house in this place - would you have the same sensory experience? Would you be as comfortable? Probably not, unless you have a home that has been designed for a similar site, by someone who took you and your innate physiological and psychological comfort to heart.

Climate is one of the most important and base-level ways that people experience 'place', and appropriate design responses to regional and local climatic conditions are what create distinctive building practices and the resultant establishment of 'place'. The critical factor in play is the level of thermal comfort that is achieved by designing according to location. The corollary to that is the inability of a building that is not contextually designed to satisfy comfort requirements of its inhabitants without total and excessive reliance on mechanical cooling.

Exclusive dependence on mechanical systems for creating universal, standards-based 'comfort' levels is the undoing of the human connection between built environment and place. Sealing one's body in a building, inside against outside, eliminates the experiences that place-based design inherently creates and, ultimately, decreases the level of satisfaction that inhabitants derive from being in the building. When a preference for individually adjustable building environments is demonstrated (as shown in research by Gail Brager, Professor of Architecture, UC Berkeley, and Professor of Environmental and Life Sciences, Macquarie University, Sydney), it would behoove architects to incorporate, if not emphasize, climate-based design solutions.

 

Keeping Your Cool...

Thermal comfort is maintained when the body is in thermal equilibrium with its surroundings. Body HeatThere are four mechanisms involved in the transfer of heat (gains and losses) in buildings and bodies that play a role in establishing thermal comfort. These need to be addressed in design, both in terms of how to avoid creating undesirable conditions and how to use them, passively, in a beneficial way. They are:

  • Conduction - The process of heat transfer through materials by direct molecular interaction. It is primarily dependent on air and surface temperatures. Heat transfer by conduction can be altered by materials selection (conductance) and insulation of surfaces exposed to solar energy. Example: Through contact of two items, electric stove top element heats pot.
  • Convection - The process of heat transfer by means of flowing and mixing motions in fluids (liquids and gasses). It occurs when human skin is in contact with air or water. It is primarily dependent on air/water temperature and air/water motion. Convection conduits (windows, doors, movable walls, landscape/hardscape) are used to control the amount of air that reaches and enters a building. Example: Warm air rises, mixes with other air in the space, and circulates.
  • Radiation - The process of heat transfer through the air by electromagnetic waves from warmer objects to cooler ones. It is primarily dependent on surface temperature of materials. Example: Solar radiation from the sun heats the earth without making contact.
  • Evaporation - The process of heat transfer by physical transformation of material. It occurs only in one direction - where the body loses heat to the environment. It is primarily dependent on air and surface temperatures and air moisture content.

 

Thermal comfort is, then, dependent on the balance between heat produced from the Building Heatbody; heat and radiation received by the body; and heat lost from the body through radiation, convection, evaporation, and, occasionally, conduction. People's perception of thermal comfort varies according to their own adaptability to different conditions and their activities, age, clothing, and culture. The temperature of the ground or of a building's walls and other surfaces can differ considerably from that of the surrounding atmosphere, as surface temperatures are greatly affected by color (reflectivity), heat capacity, and conductivity. Parameters that affect thermal comfort are thus separated into two main categories of influence - Environmental and Physiological - as follows:

 

Environmental Influences

  • Dry Bulb Temperature (DBT) - DBT is a prominent environmental factor as it is the controls heat dissipation by convection as the result of the relative difference between skin and air in contact with it.
  • Relative Humidity (RH) - RH affects the rate of evaporation. Excessively low or high RH creates discomfort.
  • Mean Radiant Temperature (MRT) - MRT affects the heat lost or gained through radiation and/or conduction.
  • Air Speed (Vel) - Increases in Vel accelerates heat loss from skin to air through convection and promotes the rate of evaporative cooling.

Physiological Influences

  • Metabolic Rate (Met) - Met is the rate at which the body converts food into energy, or heat, which varies by individual, and is a function of activity level. Values for Met range from 9.5 when running in a sports activity to 0.7 when sleeping (see ASHRAE 55, Appendix A).
  • Clothing Insulation (Clo) - Clothing thermally insulates the body, reduces radiant and convective losses/gains, and, thus, allows adjustment for warmer or cooler temperatures. Values for Clo range from between 3 and 4 for polar weather suits to 0.01 for hose (see ASHRAE 55, Appendix B).

 

Putting It Together...

The integration of environmental and physiological influences can be quantified in a chart format to create what is known as the 'Comfort Zone' (CZ), which is defined as conditions which 80% of a building's occupants will find thermally acceptable in still air and shade. The most recognized formats that have been used to plot local climate data which then is used to define the CZ are (1) the 'Bioclimatic Chart' (BC), developed by Victor Olgyay in 1963 and is applied in cases of passive cooling methods (i.e., without mechanical assistance); (2) the 'Building Bio-Climatic Chart' (BBCC), or Psychrometric Chart (PC), developed by Barruch Givoni in 1976, modified by Givoni and Murray Milne in 1979, and again modified and enhanced by Watson and Labs in 1983; and, finally, (3) the American Society of Heating, Refrigerating, and Air-Conditioning Engineers' (ASHRAE) 'Comfort Chart'. The latter two systems take mechanical methods into account and are now typically used in the design of HVAC systems.

 

Click on images below to view examples of Comfort Zone Diagrams

Olgyay's Comfort Zone Givoni-Milne Bioclimatic Chart Psychrometric Chart: Comfort Zone Pschrometric Chart: Interpretation Psychromatic Chart: Climate Classifications

 

To utilize and interpret the BBCC, if a plotted point falls within the CZ, conditions are comfortable in the shade and in still air. If the point falls outside of the CZ, corrective measures are necessary to bring conditions into the CZ. If the point is to the left of the CZ, additional solar or surface radiation is needed. If the point is to the right of the CZ, additional air movement is needed. If the point is below the CZ, additional moisture is needed, and if above, dehumidifications is needed.

These graphical presentation methods have been used with some success as general indicators of acceptable, comfortable conditions, but lack the precision required in all locations as a result of localized situations, cultural differences, etc. To better evaluate site- and building-specific cases, computer-based analytical and simulation programs have been developed. Some of these will be explored and used during this and subsequent phases of this project, and will be added as discussion items as they are encountered.

...more to come...