The Attributes of Thermal Comfort
Ergonomic Criteria for the Design of "Breathable" Work Chairs
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A chair should “breathe”; its surface materials should provide comfort and allow conduction of heat and dispersion of moisture away from the surface of the sitter’s skin.
A work chair should have a neutral effect on body-surface temperatures, so that thermal comfort is not posture dependent.
What We Know
People are more comfortable when ambient temperatures are neutral, allowing the body to maintain thermal equilibrium without sweating or shivering. Increased humidity at the skin’s surface can lead to sitting discomfort.
The human body is designed to maintain thermal equilibrium with its outside environment, so that body heat produced by activity and metabolism roughly equals the amount of body heat lost to the ambient air. When this equilibrium is disturbed, the body compensates by shaking to generate more heat or sweating to transfer heat away from the body surface through evaporation.
Thermal comfort, then, is relative to body temperature. One study found that its subjects’ definitions of a comfortable temperature varied with the current temperature of their own bodies (Shitzer et al., 1978). “Ideal” ambient temperatures vary from person to person and over time as body temperature varies.
Humidity is another important aspect of thermal comfort. A seated person will usually feel uncomfortable when humidity builds up at the skin’s surface because moist skin creates increased friction coefficients (Reed et al.,1994), causing it to stick to clothing or chair upholstery and inhibiting the small movements required to shift weight off pressure points.
A comfortable chair will have a neutral thermal effect on the body by allowing the flow of air to the body or the dispersion of water vapor from it.
Design a work chair that "breathes" so that sitters experience the same thermal conditions when seated as they experience when standing.
Fabric-over-foam padding has long been the conventional approach to work chair construction. Such padding has insulating properties that prevent conduction of heat away from the body. A study of different materials used for wheelchair seat cushions found a significant rise in skin temperatures under the thighs and sitting bones of test subjects in chairs using four-inch-thick (10 cm) foam rubber pads (Fisher et al., 1978).
Foam padding can also impede water vapor transfer from the skin’s surface. One study found that, along with different foam compositions, permeability of seat cushions varied with compression (Diebschlag et al., 1988). This suggests that thermal comfort could vary for different people using the same type of work chair, depending on where and how much each person compresses the foam cushions of the seat pan and backrest.
In some climates, the cost of additional air conditioning required to counteract the insulating properties of non-breathable seating materials can be a significant expense. A report published by the Rocky Mountain Institute estimates that an office chair upholstered with a non-breathable material insulates 20 to 25 percent of the body’s surface. This can add $140 to $290US per seated worker in HVAC and utility equipment costs (Houghten et al., 1992). By helping to maintain neutral body temperatures, a chair with a breathable material requires less cooling. It may, over the life of the chair, generate enough HVAC cost savings to offset the chair’s initial cost.
Develop breathable and porous seat and backrest materials that provide comfort and allow conduction of heat and dispersion of moisture away from the surface of the skin.
The porous quality of suspension materials such as Pellicle, AireWeave, Cellular Suspension, and FLEXNET allows for unobstructed moisture dispersion and conduction of heat away from body surfaces that touch the backrest or seat pan.
A more recent development is the Pixelated Support system. It uses both a global and a local spring layer to fit the sitter’s body shape. / See Figure 1 / This contoured, layered design allows for air to flow through the layers.
Tests of subjects sitting in chairs with these breathable materials and in chairs with both traditional and highly porous foam found that lumbar and buttock skin temperatures increased most when the subjects sat in the foam-padded chairs but changed minimally in chairs with the breathable suspension materials. Test subjects’ sitting temperatures remained more constant and comparable to their standing temperatures even after 30 minutes of sitting in chairs with breathable materials. / See Figure 2 / The Pixelated Support seat, which uses a layered design, yielded net temperature increases of about +4 degrees Fahrenheit (+2.2 degrees Celsius). This increase is higher than that of Pellicle, the most porous material, yet less than half the typical temperature rise that occurred with foam seating.
In previous thermal studies of similar duration, temperature differences of up to 12 degrees Fahrenheit (6.7 degrees Celsius) have occurred as subjects sat on foam. This is a significant temperature increase to the body’s surface: A temperature difference of 10.7 to 12 degrees Fahrenheit (5.95 to 6.7 degrees Celsius) is equivalent to the temperature change between what are generally considered a hot shower and a cold shower.
A chair designed with breathable and porous seat and backrest materials allows airflow to the parts of the body in contact with the chair that is comparable to the airflow that reaches the more exposed parts of the seated body. For example, an American Society for Testing of Materials airflow test (D3574-91) of Pellicle material found no measured pressure drop from one side of the material to the other when air was directed through it at 10 cubic feet (65.6 cubic cm) per minute, signifying that the Pellicle material is virtually porous. Using even a highly porous foam in the same test restricted the airflow dramatically. / Figure 3 / Adding a completely non-porous chair shell structure would then result in total blockage.
Taking its cue from the porosity of classic wicker or rattan furniture, a chair made with breathable suspension material is virtually "transparent" with regard to airflow and heat transfer, acting as a conduit for, rather than a barrier to, the surrounding thermal environment. People who work in an office furnished with such chairs should be able to dress for the ambient conditions and not notice temperature changes when they move between standing and seated tasks.
Diebschlag et al. (1988), "Recommendation for Ergonomic and Climatic Physiological Vehicle Seat Design," Society of Automotive Engineers Technical Paper 880055.
Fisher et al. (1978), "Wheelchair Cushion Effect on Skin Temperature," Archives of Physical Medicine and Rehabilitation.
Herman Miller Research Report (2013), "Mirra 2 Thermal Study," internal report.
Herman Miller Research Report (2008), "Embody Thermal Test Benchmarking," internal report.
Houghten, et al. (1992), "The State of the Art: Space Cooling and Air Handling," COMPETITEK technical report, Rocky Mountain Institute.
Reed et al. (1994), "Survey of Auto Seat Design Recommendations for Improved Comfort," technical report, University of Michigan Transportation Research Institute.
Shitzer et al. (1978), "Human Response from Heat Stress with Relation to Comfort," Ergonomics.
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