Human comfort plays a major role in productivity. If people are too hot or too cold, some of the mental energy that should go to productive work gets diverted into figuring out how to cool off or warm up. The primary environmental determinants of human comfort include air temperature, radiant surface temperature, air velocity and relative humidity. Ultimately, comfort is defined by LEED as "a condition of mind experienced by building occupants expressing satisfaction with the thermal environment."24

From an engineer's perspective, the type of clothing worn also influences comfort. Wear a wool suit on a summer day, and you just won't feel as comfortable in the air-conditioned office as you would with a short-sleeved shirt. And, of course, men and women, young and old, thin and fat, all feel differently in terms of comfort. How much you move around on the job also plays a role; sitting in front of a computer all day will likely make you more sensitive to temperature swings.

Standards of comfort as specified by the American Society for Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) include consideration of both temperature and relative humidity. Most mechanical engineers learn how to design ventilation and space-conditioning systems using ASHRAE methods and guidelines. Since 2005 ASHRAE, AIA and the USGBC have teamed up to begin integrating their standards to reduce energy use and improve comfort in buildings.

The LEED rating system recognizes that human comfort plays a significant role in productivity and health in buildings, so it gives a point for maintaining established comfort standards throughout the year. In some colder northern climates, buildings need to add humidity during winter to gain comfort, since outdoor cold air tends to be very dry. In hot, humid climates such as the Southeast US, moisture needs to be taken out of the air almost year-round since high humidity even in mild weather can be uncomfortable in closed spaces. Warm summer afternoons can be particularly challenging for air conditioning if the architectural design doesn't provide external shading on south- and west-facing walls and windows. Then it's almost impossible to prevent a building from overheating in the late afternoon.

Natural ventilation systems are another approach to providing comfort and fresh air. If a building is designed to circulate air naturally from outdoors to indoors, and then to provide a "stack-effect" exiting path, where the heated air, rising naturally, exits the building near the roof, sometimes through an atrium or stairwell, it can be comfortable even if temperatures are a bit cooler or hotter than normal. Think of your own experience on a warm spring day when you can feel the outdoor air moving through the building. The sensory experience of the slowly moving air, fresh from outdoors, overwhelms any feelings of discomfort because of slightly higher or lower temperature than normal.

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