Relative Humidity
The amount of moisture in the air measured as a percentage of the air’s capacity to hold moisture the relative humidity.
The key concept here is that warm air can hold more moisture than cold air. So, 50% relative humidity means that the air is 50% saturated, but this is all that is known unless you know the temperature of the air.
Condensation occurs when the relative humidity reaches 100%.
There are many factors involved in indoor comfort (such as age, sex, attire, and activity), but the primary factors of concern are temperature (dry bulb temperature) and humidity (specific humidity). Much research has been conducted on the parameters of comfort for typical people, most notably by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (www.ashrae.org).
Most people are comfortable indoors if the space temperature is near 75°F and the relative humidity is near 50%, depending on the season.
During the cooling condition, summer with the air conditioning on, an increase in specific humidity results in a need to lower the space temperature to stay comfortable. Envision sweat drying off of your skin--when the surrounding air is humid, you don’t dry as fast. As a liquid dries, it cools the surrounding air; when your skin doesn’t dry as fast, the surrounding air is, in effect, warmer.
The converse is also true in a cooling condition; the thermostat can be raised (space temperature increased) and occupants are still comfortable with low humidity. This concept can be useful in lowering energy usage of air conditioning equipment, especially in residential applications.
In the heating season, the specific humidity of outdoor air is much lower, resulting in a potential for uncomfortably low indoor humidity levels. Heating systems don’t add humidity to the air, in effect lowering the relative humidity indoors if these systems provide ventilation.
Psychrometrics Page 3 discusses the HVAC systems and how psychrometrics is critical to understanding their performance.
| Psychrometrics Page 1 - Introduction Page 2 - Indoor Comfort Page 3 - HVAC Systems Page 4 - Building Enclosures Page 5 - Definition of Terms |