Principles of Bioclimatic Design
Jan 11, 2023
Bioclimatic design may be a new term and concept that is unfamiliar to the general public. The core idea behind bioclimatic design is exactly what the term describes – a combination of human biology and climate. Human physiology functions are partially determined by our body temperature and its abilities for thermoregulation. If our core body temperature rises even 2-degrees above its regulatory base of 98.6oF we will experience fever and then hyperthermia at 104oF, which is life-threatening. In the other direction, if our core body temperature drops even 2-degrees below its base we will experience cold shivers and then hypothermia at the dangerously low temperature of 95oF. The hypothalamus, which sits just above the brainstem at the lower part of the brain, is the mechanism that controls and regulates body temperature just like a thermostat.
The climates of different places in the world have unique characteristics that our bodies may be comfortable or uncomfortable with depending on temperatures, radiation, airflow, and humidity. When we consider the climate variables of a place and the natural thermal comfort levels of the human body, we make decisions on how to best design habitats and shelter to accommodate thermal differences. From an architectural design perspective, we identify these as bioclimatic strategies and categorize these strategies within passive design. Passive design implies that the static elements of our buildings are able to deal with the climatic variability to maintain human thermal comfort. Active design strategies introduce mechanical heating and cooling systems into the building to achieve human thermal comfort. The more we can maintain human thermal comfort through passive design, the less energy we will use and the lower the home utility bills will be.
Each climate type has different characteristics that constitute different approaches to the design of buildings. When considering the range of climate variables, the resulting building designs may have different expressions of form and geometry as well as different material systems. Human thermal comfort is achieved by different design strategies based on a theoretical model of a comfort zone. Human thermal comfort is identified by a combination of six variables: 1) temperature (T), 2) relative humidity (RH), 3) airflow (m/s), 4) mean radiant temperature (MRT), 5) metabolic rate (MET), and 6) clothing factor (CLO). Each of these variables has a range of conditions that determine a quantifiable calculation for human thermal comfort.
Temperatures between 68oF and 78oF are within the human comfort zone, while temperatures below 68oF are too cold and temperatures above 78oF are considered too warm. Relative humidity can range from 20% to 80% and accommodate human comfort, though a narrow range of 40% RH is most ideal for respiratory health. Breezes of 1-2 meters per second (m/s) are within human comfort range while anything faster or stronger becomes distracting and then uncomfortable. In humid conditions, airflow helps to remove moisture from the surface of the body assisting in the heat transfer balance for thermoregulation.
Mean radiant temperature (MRT) is a slightly more complex variable because it requires a detailed calculation dependent on the geometry, materials, and radiant sources in a given space surrounding the human body. To begin with, radiant sources can be anything that is providing additional radiant energy to a space or removing radiant energy from a space. For example, the sun shining through a window directly into a room is a radiant heat source, while a cold window with a winter scene of snow and ice outdoors is a radiant attractor that more rapidly removes heat energy from a space. Radiant sources and attractors tend to shift the actual temperature that our body experiences, so going by the ambient air temperature alone is not a sole indicator for comfort. All materials in a space are also types of radiant sources, and the configuration of different materials provide a distribution of radiant conditions based on a view-factor from the standpoint of the human body. Material view-factors are typically determined by analysis of a fisheye-lens image (simulated or real photograph) of both looking up and looking down to constitute a 360-degree composition of the radiant conditions of a space. All in all, MRT can be incorporated as a variable into designs and renovations of spaces and inform material selections for specific human thermal comfort goals.
The final two variables in the human thermal comfort model can be modulated based on human activity and clothing. Different activities will displace our metabolic rate, which results in either raising or lowering our overall body temperature. When we are sitting and at rest, the metabolic rate is at its typical or normal level. While if we are exercising and moving our body quickly for a period of time, the metabolic rate will increase and so will our body temperature. The surrounding thermal conditions of a space can help accommodate different metabolic rates by design. Gymnasiums and workout rooms should be designed for maintaining cooler temperatures when metabolic rates from human activities increase. Libraries and sitting rooms should be designed for maintaining average temperatures to accommodate human comfort at the normal metabolic rate.
Clothing can also be adjusted to shift the human comfort levels and is identified in the comfort calculations as the clo-factor. One clo-factor is a 3-piece business suit! Human comfort science was developed in the 1950’s-60’s when indoor air-conditioning was proliferating for office buildings, so the measure in most of the early models was a man sitting at a desk while wearing a business suit. Clothing can certainly be modified to adjust our comfort as we experience this daily whenever we go outside. In Japan in the early 2000’s there was an energy crisis that instigated a shift in business office culture in Tokyo and set a new requirement for office employees to dress casually in short-sleeve cotton shirts rather than 3-piece suits. The intent of this cultural mandate was to reduce the intensity of air-conditioning required to cool office buildings and resulted in an extremely large and rapid energy use reduction. Never underestimate the power of cultural shifts in comparison to technological advances!
The translation between human comfort and passive design strategies originated with the psychrometric chart. Psychrometric charts are used to determine the air temperature and humidity mixtures in mechanical heating and cooling processes. Baruch Givoni and Murray Milne, two architectural designers and academics from the 60’s and 70’s, developed a version of the psychrometric chart known as the bioclimatic chart. This chart overlays the human thermal comfort zone onto the psychrometric graph and identifies additional zones of architectural design strategies that can serve to bring a given environment into the comfort range. It is relatively complex to explain in words, but the graphs display quite clearly different temperature and humidity zones relating to different climate types (i.e., hot-humid, hot-arid, cold, etc.) and how to address those climate conditions through passive design.
Some examples of passive design strategies that can be implemented for human thermal comfort include the following: shading and roof overhangs, natural ventilation (cross- and stack-ventilation), evaporative cooling with outdoor ponds, thermal mass walls, sun-spaces, double-roof systems, rain-screen or shade-screen wall systems, and others. Passive design strategies help to serve human thermal comfort in particular climates seasonally and diurnally. Before passive design strategies with materials and systems are implemented, good bioclimatic design always begins with proper siting, orientation, massing, and spatial layouts of a house or building. Bioclimatic design principles correlate with solar orientation and radiant conditions of the climate, which means that the constant movement and changes in the path of the sun need to be accounted for in the design. This is accomplished first and foremost through building orientation, massing, and geometry.
If you are interested in discovering the optimal bioclimatic design strategies for your home, reach out to AIDA, LLC today for a consultation. You can always find more information and healthy home resources at Aletheia Ida Design and Architecture, LLC (AIDA, LLC) at www.aletheiaida.com.