People "feel comfortable" over a wide range of temperatures and conditions... depending upon age, weight, sex, and level of physical activity. A sedentary person could feel "cold" at 74°F if the humidity is low, while a factory worker could feel warm at 65°F if they are performing heavy manual labor. The HVAC system designer usually has to make a compromise, and must design to accepted criteria for human comfort. During the summer, interior office space is usually designed to hold a maximum temperature of 75°F and a relative humidity of 50%. But, some areas of the country have very low humidity, even during these summer months. This means the space temperature could be set at 78-80°F and provide the same level of comfort.
The comfort levels are also dependent upon air movement. Someone sitting in a space with inadequate air movement is likely to feel "closed in." A certain amount of fresh air must be introduced. This is defined by ASHRAE standards, which provide CFM/person and CFM/sq.ft. guidelines for the building type and intended use.
There is no one temperature and humidity condition at which everyone is comfortable. People are comfortable at a range of temperatures and humidities. Research conducted over many years on large numbers of people by the American Society of Heating, Refrigeration, and Air Conditioning Engineers concluded there is a range of combined temperatures and humidities that provides comfort to most people. This chart shows indoor air temperature on the vertical axis, relative humidity on the horizontal axis, and a shaded area known as the "comfort zone." Notice that most people are comfortable at higher temperatures if there is a lower humidity. As the temperature drops, higher humidity levels are still within the comfort zone.
For many years it was general practice to design for 72-75°F and 35-40% relative humidity in winter, and 75-78°F and 50-55% relative humidity in summer. In 1974, for energy conservation reasons, the federal government recommended lowering the winter temperature to 68°F and raising the summer temperature to 80°F for government buildings and encouraged private industry and homeowners to adopt similar practices.