The HTM model reveals huge differences in people’s thermal sensations

The HTM model reveals huge differences in people’s thermal sensations

The Human Thermal Model (HTM) developed by the VTT Indoor Environment research program provides unprecedented accuracy in simulating the interaction between the human body and its environment. Identification of the thermal sensations typical of different user groups enables designing buildings that are both energy efficient and best suited for their users.

The research has focused on the effects of a person’s age, sex, body fat content, and physical fitness. The ambient and surface temperatures of modeled spaces were determined. People were brought into the spaces and their individual anatomies were visualized. During their interaction with the space, the model enabled calculating thermal sensations by body parts.

In the HTM model the human body is divided into 16 sections: head, neck, upper and lower body, upper and forearms, palms, thighs, lower legs, and feet. These are further divided into tissue layers: bone, muscle, fat and skin. Heat transfers between different body parts are modeled by visualizing the blood circulation between them.

– In engineering jargon humans could be referred to as continuously self-regulating systems. Should the temperature of the internal organs rise for some reason, the body opens valves to increase surface circulation, which in turn increases skin temperature and makes the body release more heat. If that is not enough, the body resorts to sweating. This way an individual transfers heat from their body to the environment. Correspondingly, when internal organs cool down, circulation in skin tissues is reduced, notes Pekka Tuomaala, Senior Research Scientist at VTT.

Fat does not keep you warm – on the contrary

According to Tuomaala, studies show that many of our characteristics determine our individual thermal experience. Metabolism, the internal heat production of the body, depends on the composition of a body, particularly the amount of muscle tissue. Therefore, differences between individuals may be considerable: a resident of an elderly home may wear a cardigan while a more muscular young nurse wears a short-sleeved nurse’s top. The difference between individual ideal temperatures at the same level of activeness and thermal insulation of clothing is several degrees.

– A muscular person copes much better with cold than a chubbier colleague: the difference between the temperature found agreeable can be up to six degrees. Thermal comfort is also largely related to age and sex: men and young people have relatively more muscle tissue whose heat production can be up to 1000-fold compared to fat, Tuomaala continues.

Design and dimensioning must correspond to the end users

Simulations run with the model have already yielded a lot of new and important information on factors affecting thermal sensation. It allows qualitative analysis of both environmental (temperature, relative humidity, air flow) and personal effects (clothing, metabolism). The model makes it possible to estimate the optimal temperatures for different user groups of a building.

– It seems evident that the design and dimensioning criteria used in selecting temperatures must be reassessed according to occupancy. The end users of schools, day care centers, elderly homes and public premises need different temperature levels, Tuomaala summarizes. In his view that also calls for changes in indoor air quality classifications and recommendations.

While indoor air quality is subject to ever greater and increasingly individual requirements, ever tighter limits are set for the energy efficiency of buildings. Matching these goals is not easy since conventional design and dimensioning principles do not always result in the best conditions from the end users’ point of view.

According to Tuomaala, HTM makes it easier, for instance, to choose between different renovation alternatives because it allows comparing the impacts of alterations like adding extra envelope insulation and replacement of windows or the heating system. Besides, HTM helps identify situations where buildings are either overheated or cooled in relation to the actual needs of users. Thus, the method helps avoid unnecessary heating and cooling.

More information:

Pekka Tuomaala, Tel. 040 720 1724, pekka.tuomaala@vtt.fi

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