Understanding Humidity

Overview

Most people recognize that water evaporates into air and when this occurs, the atmosphere becomes a mixture of gases and water vapor.

The water vapor in air is known as humidity.

Controlling humidity is not only important from the standpoint of comfort but also significant when it comes to maintaining healthy indoor environments.
 

Relative Humidity
The amount of moisture in air can be expressed a number of ways but the most common reference is relative humidity (RH). 

This value, expressed as a percentage, represents the amount of moisture in the air relative to the amount of moisture air could hold at that temperature.

In other words, air at 70 ^oF and 50% RH is holding half of what it could hold when completely saturated at that temperature.

Therefore, the relative humidity at a given temperature will range from 0% (dry air) to 100% (fully saturated air).  When you exceed 100%, moisture starts falling out of the air and it rains.
 

At a given barometric pressure, temperature is the key factor regarding how much moisture can be retained.

What is important to remember?  At the same relative humidity, warm air holds more moisture than cold air (see illustration at right).

Cooling Air
When air is cooled, relative humidity increases as shown to the right.

Cooling air that is 90^oF/20% RH causes the relative humidity to increase until it reaches 100%; the temperature known as the dew point (43.6^oF in this example).  At the dew point, moisture begins to condense out of the air and is transformed from vapor to liquid.

Most everyone has witnessed bathroom mirrors 'fogging' during a hot shower or iced-drink glasses 'sweating' on the outside.  The cool surfaces are simply condensing moisture out of humid air.

This aspect of psychrometrics is very important when it comes to analyzing indoor conditions as temperatures and humidity levels within our environments are very dynamic. 

Outdoor air creates the baseline for indoor humidity levels and we typically heat or cool this air to maintain our desired temperature range.  In the process of changing the temperature of air, we are altering the indoor relative humidity as explained above.  

Numerous activities add moisture to indoor air: occupancy (people exhale moisture and perspire), cooking, cleaning, etc.  In addition, we can intentionally increase moisture levels using humidifiers.  (Humidification is rarely required in the Dallas/Ft Worth area due to the natural ambient levels.)

Most refrigerant-type air conditioners, by design, remove moisture from the air during the cooling cycle through condensation at the evaporator coil.  This moisture is removed from the interior using a condensate drainage network.  In certain applications, dehumidifiers may be necessary if indoor relative humidity levels get too high.
 

However, from the standpoint of indoor air quality, upper ranges should be maintained below 50% (dust mite populations increase rapidly at relative humidity levels above 50% and fungal amplification occurs above 65%).  

Buy a Hygrometer and keep track of your indoor humidity levels. 
 

Target Indoor Humidity Conditions

30 - 50% RH  

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