Humidity and HVAC

High Volume Air Conditioning cools and allows heat into a space. This we all know. However, a secondary feature of HVAC is its ability to remove significant amounts of humidity from the air. Excessive humidity can change the sense of what you feel is a certain temperature. High volume air conditioning and humidity in Alabama seem to go hand in hand.

The atmosphere in your home or building can feel hotter when the humidity level is high. If humidity levels are not kept in check, the HVAC system will work overtime placing stress on mechanical parts and decreasing the life of the unit. Newer HVAC systems can even be installed with humidifiers and dehumidifiers that keep levels of moisture in line with acceptable human and manufacturing requirements.


Humidity is the amount of water vapor present in a molecule of air. The vapor is actually the gaseous stage of water and it is invisible. The reason hot air feels hotter to us is that the levels of moisture present in the air compromise the human body’s ability to sweat and then cool the epidermis with evaporation.

Humans are sensitive to humid air because the body uses evaporative cooling to regulate internal temperatures. The internal temperature or “core” temperature of the body constantly seeks and works for homeostasis. It will send water to the surface of the skin in an attempt to do away with the outside high temperature. We are more able to perceive the rate of release of heat from the body more quickly than we can perceive the temperature of the air we are breathing.

High humidity levels can cause difficulty in breathing. Typically, if a person is susceptible or has in fact been diagnosed with a breathing complication of some sort, anxiety enters the picture during the hottest and most humid days of summer in the southern regions of America.

Humidity is measured in three different domains.

Absolute humidity is defined as the mass of water vapor in a certain volume. If ideal gas behavior is assumed the absolute humidity can be calculated using: A = C · Pw/T (g/m3) , where C = Constant 2.16679 gK/J      Pw = Vapor pressure in Pa       T = Temperature in K

Relative humidity is expressed as a percentage and measures the current absolute humidity relative to the maximum highest point for that temperature. Relative humidity is the percent of saturation humidity often calculated in relation to a saturated vapor.

Specific humidity is the ratio of the mass of water vapor to the total mass of the section of air being assessed.

Humidity control is important for three different reasons.

  • Excessive humidity will cause the HVAC system to work overtime, which will create an excessive use of electrical energy from your provider.

  • Excessive moisture will cause the creation and propagation of mold, mildew, and other contaminants. This will impact the indoor air quality of your dwelling or building.

  • Humans will be more comfortable and will enjoy an enhanced state of well being if humidity levels are within appropriate boundaries.

Levels of humidity also impacts sensory perception as it relates to heat. If humidity levels are low, say below 50%, then winters will not feel as warm.

Alabama Weather

The state of Alabama is classified as a humid subtropical place under the Koppen climate classification. The state is hotter in the southern areas due to the state’s proximity to the Gulf Coast. The northern areas are close to the Appalachian Mountains resulting in cooler temperatures. There is a copious amount of precipitation with hot summers and mild winters.

In terms of relative humidity, the state of Alabama is currently experiencing a state wide average of 49%. This will rise and fall throughout the spring and summer months with a RH reading sometimes registering at 88%. This degree of moisture in the air presents challenges to the average consumer of high volume air conditioning.

Ideal Humidity Levels

What is ideal to one person, may not be ideal to another. However, when looking at appropriate levels of humidity for efficient use of energy and undue work placed on HVAC systems, there are some parameters and limits that come into play.

During the summer, research has indicated that the relative humidity level should be no more than 60%. During winter, this level falls to around 20%. It is important to note these levels are based on an indoor temperature between 68-75F for the summer and 72-79F for winter. Manufacturer’s of humidifiers have declared the most comfortable range of humidity for humans is between 40-50%. Microorganisms such as mold or mildew will be less if humidity levels are kept within this range also.

Here are some recommended humidity levels for different environmental temperatures.

Outside Temperature                                     Indoor Relative Humidity

Above 50F                                                                  <50%

Above 20F                                                                  <40%

Between 10F&20F                                                      <35%

Between 0&10F                                                          <30%

Between -10F&0                                                         <25%

Between -20&-10F                                                     <20%

-20F or below                                                             <15%

Relative Humidity and Your Dwelling

Products that homes are made of have had their value increased over the years as manufacturers of building products have sought to make them last longer and serve multiple purposes. However, even with these significant improvements, homes still have at their core mostly wood fiber products. These wood based products and others are subject to environmental conditions throughout the year.

These building products, the effectiveness of your HVAC unit, and the relative humidity of the air along with the indoor air quality rating will constantly impact how long parts of your home will last and where damage might occur in the changing seasons of the year.

Living in a humid area such as Alabama will present the homeowner certain issues and create different kinds of atmospheric conditions. These atmospheric conditions combined with less than adequate air conditioning tonnage and humidification have the potential to cost a homeowner a great deal of money in maintenance and repair.