While designing buildings, one of the factors to be considered is how to make occupants feel comfortable in enclosed spaces. Thermal conditions majorly impact this comfort level and have wide-ranging repercussions on how buildings are experienced and used on a daily basis. Read on to learn more about the thermal comfort meaning, why it’s important, factors affecting thermal comfort, how building thermal comfort is achieved and what the comfortable temperature for human beings is.
What is Thermal Comfort?
Thermal comfort is a subjective component of building design that denotes an occupant’s contentment with the conditions of an indoor environment. A person is said to be thermally comfortable in a space when he/she can function in an optimal manner under the given thermal conditions. With the time spent indoors rising rapidly in the modern era, it has become increasingly important to ensure thermal comfort in buildings, compelling architects and occupants to analyse ways of optimising it.
Why is Thermal Comfort in Buildings Important?
- Energy Efficiency: Buildings that are naturally comfortable thermally have a lower dependence on mechanical ventilation systems like Air Conditioners and heaters. Apart from reducing electricity and utility bills, this also enforces sustainable building practices.
- Enhanced Physical Health: Good indoor air quality and thermal comfort have noticeable impacts on the physical health of occupants. These impacts include regulated biological cycles, blood pressures and heart rates, reduced breathing problems and headaches and better digestion.
- Improved Mental Health: When a person is thermally comfortable, he/she can focus better on other activities. Increased productivity, better attention spans, a general upliftment of spirits and lower mental and physical fatigue are some of the significant benefits.
- Building Wellbeing: Thermally comfortable buildings typically maintain good indoor air quality and sufficient natural ventilation. These help prevent issues like build-up of moisture and stale air, which can further avoid problems like mould development, water seepage and structural failure.
Factors Affecting Thermal Comfort
Environmental Factors
- Temperature: Wet-bulb temperature is the lowest temperature that air may achieve through evaporative cooling, while dry-bulb temperature is the standard air temperature as determined by a dry thermometer. Both of these values (denoted in degree celsius: ℃) affect the amount of body heat exchanged with the atmosphere. Temperatures that are higher or lower than optimal values can make spaces uncomfortable by disrupting metabolic rates.
- Relative Humidity (RH): It is the ratio of the actual vapour present in the air to the amount of vapour content that the air can potentially hold. It is denoted as a percentage that affects building thermal comfort. High RH values can increase sweating and make the air feel sticky, while very low RH values can make spaces feel dry and warmer than they actually are.
- Winds: The direction, speed, duration and amount of wind all have an impact on thermal comfort. While the lack of winds or presence of minimal winds can make rooms feel stuffy, high speed winds (especially dusty winds) can also make spaces uncomfortable. Wind speeds are typically denoted in km/h (kilometers per hour) or m/s (metres per second).
- Thermal Radiation: This includes direct and indirect rays from the sun, as well as artificial light reflected by internal objects. Even if all the other factors are optimal, direct glare in the eyes caused by radiation can lead to thermal discomfort.
Personal Factors
- Clothing: Heavy and dark fabrics absorb more heat and demand lower indoor temperatures and vice-versa. For instance, a person wearing a black business suit will require a temperature about 9°C lower than a naked person.
- Metabolic Rate: Every person has a different metabolic rate. Older people have slower metabolisms and prefer higher temperatures. Gender, body shape and physical health also have an impact on this factor. Women have been noted to prefer a 1°C higher temperature, while thinner people also opt for higher temperatures as they can dissipate more heat. During an illness, metabolic rates may be lower and such people have narrower comfortable temperature values.
- Acclimatisation: People are physically adjusted to the climatic conditions of the region in which they reside. For example, a person from Bangalore might find Mumbai too hot, but a local Mumbai resident may not feel the same. The human body needs around 30 days to get adjusted to any new climatic zone.
What is the Human Comfort Temperature?
The comfortable temperature for human beings can vary widely based on objective environmental factors or subjective personal factors as explained above. According to ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) standards, the human comfort thermal values can be found when all of the below factors are fulfilled:
- Temperature: 20°C – 25°C
- Relative Humidity: 25% – 75%
- Wind speed: 0.2m/s – 0.8m/s
However, these values are interdependent on each other and can also vary substantially depending on the location, season and personal factors.
Thermal Comfort Zone as per ASHRAE
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Thermal Comfort Indices
Thermal comfort can be analysed, calculated and/or depicted by thermal comfort indices. A good thermal comfort index takes into account all of the environmental factors, including temperature, relative humidity and air speeds. Some popular indices used today are:
- Bioclimatic Chart: Developed by Victor and Aladar Olgyay in the 1950s, this index finds the optimal comfort zone by plotting a graph with values of relative humidity (x-axis) against dry-bulb temperature (y-axis) and other lines for indicating radiation and wind speed.
- Psychometric Chart: Various values like dry-bulb and wet-bulb temperatures, absolute and relative humidity and air speeds are plotted on a graph to find the thermal comfort zone in this chart.
Depiction of a Psychometric Chart
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- Corrected Effective Temperature (CET): A globe thermometer is used to measure air temperature, which is compared to values plotted on a graph against radiant heat, air speed and wet-bulb temperatures to find the comfort zone.
- Universal Thermal Climate Index (UTCI): This index takes into account all the four environmental factors to analyse the amount of heat exchanged between the body and the environment and find the thermal comfort zone.
How can Thermal Comfort be Achieved in Buildings?
There are several ways in which thermal comfort can be ensured in buildings, the most important ones being:
Passive Design Features
These refer to features that do not rely on any mechanical or electrical equipment for heating and cooling spaces. Passive house design instead incorporates architectural strategies to optimise natural light, ventilation and humidity. A few important factors considered here are:
- Building form and orientation, which affects the amount of air and sunlight entering a space.
- Size, number and location of doors, windows and other openings that allow cross ventilation.
- Shading devices such as overhangs, louvres and window sashes used to restrict the entry of excess heat, humidity and/or dusty winds.
- Materials and techniques utilised for building insulation in cold climates or cooling in hot climates.
Active Design Features
Active design features refer to the inclusion of devices that rely on electrical or mechanical energy to operate, particularly HVAC (Heating, Ventilation and Air Conditioning) systems. ACs, fans, heaters, coolers and dehumidifiers are a part of these systems and can be used along with passive design features to provide a wider range of thermal comfort in buildings.
Personal Changes
Building occupants can make many changes in their personal lifestyles to be thermally comfortable. A few crucial ones are:
- Clothing: Wearing light fabrics in summer increases natural air ventilation and breathability and vice-versa.
- Food Habits: Eating food with lower calories in warm environments can help occupants feel more comfortable as it can reduce sweating.
Ensuring Thermal Comfort in Your Building
Thermal comfort is a significant part of building design and has several big impacts on mental and physical health, as well as the building’s wellbeing. Brick & Bolt is a tech-enabled construction company in India with over 7000 completed homes. The company’s team of qualified architects customises floor plans and elevations for each project to ensure optimal thermal conditions and customer satisfaction. Get in touch with Brick & Bolt today to enjoy the benefits of thermal comfort in your dream home or commercial building!