People in Western countries are spending more than 90% of their time indoors. The quality of their indoor environment is, therefore, of great importance. It is affected by temperature, indoor air quality, lighting and acoustics.
Indoor air quality has improved in leaps and bounds, and today it is possible to control ventilation based on demand and in an energy-efficient way. According to Principal Scientist Pekka Tuomaala, most complaints are related to temperature.
“More than 90% of the respondents of a study by the International Facility Management Association (IFMA) reported receiving complaints about the temperature being too hot or cold. There is room for improvement in this area,” Tuomaala says.
Thermal sensation depends on many factors
Human thermal sensation is affected most by room temperature. Other factors include air velocity and relative humidity.
In addition to the physical factors related to the environment, another key factor affecting thermal sensation is the level of heat generated by the person’s body functions and metabolism. This in turn depends on the persons’ anatomy and the amount of different tissue types in their body. In addition to individual physical characteristics, the person’s activity level is also relevant – whether they are sitting, walking, exercising or sleeping. Another important contributor to human thermal comfort and sensation is clothing insulation.
Does the temperature feel comfortable?
“Construction standards refer to thermal sensation. One way to create a thermal sensation index is to conduct a survey where respondents assess their personal thermal sensation on a standard scale (where -3 = cold, -2 = cool , -1 = slightly cool, 0 = neutral, +1 slightly warm, +2 = warm and +3 = hot),” Tuomaala explains.
“Povl Ole Fanger’s PMV (Predicted Mean Vote) method has been used to calculate thermal comfort since the 1970s. However, the method has some basic restrictions. It does not make any allowance for differences between individuals, nor does it take into account the effect of the human thermoregulatory system. When assessing human thermal sensation and comfort, it is important to take into account our physiological thermal regulation mechanisms and natural ability to adapt to changing environmental conditions.”
VTT’s Human Thermal Model calculation tool
Human thermal models enable the modelling of the thermal behaviour of the human body in physical and physiological terms, as well as the body’s thermoregulatory system. Riikka Holopainen’s thesis for her degree of Doctor of Science in Technology published in 2012 presents the first application that combines human thermal modelling with a dynamic building simulation program. It is called the Human Thermal Model (HTM).
“This tool enables the use of our simulation environment to calculate the boundary conditions of the surrounding space and the thermodynamic behaviour of the human body at the same time. This approach enables us to calculate the interaction and transient heat transfer between the humans and the building structures in a more accurate and realistic way,” Tuomaala explains.
There are less than five calculation tools available globally that take into account the thermodynamic behaviour of both the space and the people in it.
Higher body fat percentage makes women feel chilly
The HTM application contains models for human anatomy and physiology. The anatomy model describes 16 parts of the body (hands, forearms, upper arms, feet, lower legs, thighs, neck, head, chest/back and pelvis) and their tissue layers (the bone, muscle, fat and skin in limbs, for example). The physiology model consists of the human thermoregulation mechanisms, such as blood circulation, sweating and shivering.
When information on the building and individual information on a person’s anatomy and physiology are entered into the application, it calculates the tissue temperatures of different parts of the body. By entering this data into a thermal sensation and calculation model developed by Zhang Hui, we can study local thermal sensations.
A person’s body structure – and the body’s ability to generate heat – is affected by gender, age, height, weight and muscularity.
“If a person has a body mass index of 25, which is the limit for being considered overweight, men’s average body fat percentage is 20 and women’s 30. Body fat is an energy reserve with a metabolic heat generation rate of 0.004 W per kilogram of fat. In comparison, muscle produces heat at a basal metabolic rate of about 1 W/kg, even when we sleep,” Tuomaala says.
“On average, men have 5 to 15 kilograms more muscle mass than women. Because muscle generates about a thousand times more heat than fat tissue, men feel comfortable at lower temperatures than women.”
Pekka Tuomaala and his colleagues used the HTM application to calculate the thermal comfort of a test group consisting of three men and three women. They selected the same activity level (office worker) for all of them, and all members of the test group wore clothing typical of office workers. Body compositions represented variants at both ends of the scale. Body mass indices varied from 20 to 30 and body fat percentages from 10 to 50%.
“When we analysed the thermal sensations of this group with our calculation tool, the results were so striking that I couldn’t believe my eyes at first. For a muscular man, the optimal temperature was 20.7 degrees, while for a non-muscular man it was 24.4 degrees. The preferred temperature of a muscular man and a non-muscular woman varied even more – as much as six degrees. Recognising this phenomenon that is caused by differences in people’s body composition represents a small revolution.”
Results of a Swiss field study that was conducted in 2006–2009 in Lausanne to study the thermal sensations and comfort of employees in an office building are similar.
“The reference temperature of 21.5 degrees that is typically used in Finland proved too cold for some employees, comfortable for most, and too hot for some. Differences between the optimal temperatures of persons varied by about five degrees. The results have been reported in an article written by David Daum and his colleagues, published in the Building and Environment magazine in 2011.”
The Tekes-funded Evicures project in the Seinäjoki Central Hospital has provided similar results on the differences in people’s thermal sensations.
Who are buildings designed for?
Considering the surprising range of variation in individual thermal sensation and comfort, Tuomaala suggests that we should seriously consider who buildings are designed for in Finland.
“There is a conflict between the currently used indoor environment quality classification and our research results. At a minimum, we should critically assess the design and dimensioning criteria,” he says.
More precise temperature control would also be more cost-efficient.
“Annual salary costs in Finland amount to 100 billion euros. According to international research scientists, the ability to adjust temperatures by +/- 3 degrees to accommodate individual preferences would save between 3 and 7% of labour costs. Indoor temperature affects productivity. According to recent reports, if the indoor conditions are good, the value and utilisation rate of a building can increase by 10%,” Tuomaala says.
“Considering the surprising range of variation in individual thermal sensation and comfort, we should seriously consider who buildings are designed for in Finland,”
Pekka Tuomaala says.