This article is, in part, a reprint from the Florida Solar Energy Center

Advanced Home Watch we have been experiencing high humidity readings while visiting our Home Watch Clients in spite of the fact that the room temperature is low.  For example, we were at a customer’s home and the thermostat was set to 76 degrees and the room temperature was 72 degrees, yet the relative humidity in the room was 60%.   How can that be?  Our customer uses a humidity alerting device and receives text and email notices when the humidity exceeds a set threshold of 55%.  She is a seasonal homeowner and did not understand how the humidity could be so high when its 72 degrees in the home.  First, one needs to understand that cooler air does not mean lower humidity in a tropical climate such as ours in Southwest Florida.

Humidity is a term that everyone uses, but technically speaking, not always accurately. We sometimes assume air is at high humidity when we are uncomfortably warm. Sometimes we are not aware of high humidity when the air is cool around 70°F.

Personal comfort varies depending on one’s health, clothing, activity level as well as the air conditions and velocity around a person.

Relative humidity is expressed as a measure of the amount of water vapor the air contains as a percentage of the total amount it could contain at a specific temperature. If you think of air as a paper towel for a moment, it can only hold a specific amount of water at a specific temperature. However, unlike a paper towel, the total amount of moisture the air can hold changes as the temperature changes. As the air becomes warmer, it can hold a greater mass of moisture.

Consider this example of how the relative humidity changes as temperature changes with indoor air at 75°F and 60% RH. If this air cools down (not from an air conditioner) to 70°F, the air can no longer hold as much moisture, however, the actual amount in the air has not changed, so the relative humidity increases to about 70%.

Don’t Go Down with the Ship.

To better understand relative humidity control, it helps to consider moisture transfer in buildings. Closed buildings that are air-conditioned are like a large boat with a leak. Small leaks are acceptable as long as the bilge pump can pump water out faster than it comes in. Moisture can move into buildings in 4 primary ways.

Bulk- rainwater or plumbing leakage

Capillary- liquid water absorbed by building materials

Diffusion- water vapor molecules passing through pores in building material

Air transport- water vapor in the air moved through pathways in materials due to pressure differences.

Interior generated moisture from people, animals, cooking and bathing should also be considered as a way moisture comes into the indoor air.

Moisture continuously moves across the building materials. It naturally moves from a location of higher vapor pressure to one of lower vapor pressure. In hot and humid climates, moist outdoor air diffuses through porous materials or pathways into the conditioned air. It can be a tortuous path for some building assemblies, but moisture can and does move its way inward. In cold weather climates, the indoor air typically has more moisture at a higher temperature and the direction of travel is opposite that of humid climates.

Let’s go back to the boat analogy for a second. The boat is our building, and the leaks represent any of the four ways moisture gets into our building. As more moisture moves into our building, it becomes more important that we can remove it. The air conditioner is the only “bilge pump” we may have in the building. Air conditioners cool the air, but they also remove moisture. When the rate of transfer into the building or materials exceeds the rate that the air conditioner can remove moisture, moisture increases, temperature likely stays about the same and humidity rises.

Of course, the best remedy is to control the ways that moisture can get into the building. Just like the boat, we would fix the leaks before buying a bigger bilge pump. In buildings a bigger air conditioner would actually make the situation worse since the air temperature would be satisfied more quickly, the unit would run less resulting in less moisture removed. A dehumidifier is a simple way to remove moisture in a building already built. However, the long-term cost of operation and maintenance should be compared to the one-time cost of controlling moisture entry at the source of the problem. One should also be aware of how much moisture is actually entering the building per day since a small 40 pint / day unit may not be enough to handle many situations.

How do you know if your humidity is too high?

The best way to know if your humidity is too high is to spend $15- $30 at a local electronics store and buy a temperature and humidity (hygrometer) sensor. There is really no need to spend more than this for indoor air monitoring. At Advanced Home Watch we recommend purchasing two or more to place throughout your home to get a balance of your overall temperature and humidity.  

Control Moisture and Improve Air Quality Control

A target to maintain indoor relative humidity is between 45%- 55%. The International Home Watch Alliance suggests that indoor humidity should be maintained at 50% or lower and this is the standard that Advanced Home Watch suggests as well. Bacteria, viruses, fungi, and some pests such as dust mites begin to thrive in humidity outside this range. Indoor relative humidity above 60% for several hours in an air-conditioned indicates that the air quality control should be improved.

Monitoring the indoor humidity is not a guarantee that mold will not develop. Indoor air at 75F and 60% RH that comes in contact with an exterior wall with an interior surface temperature at 63F will have a surface RH of 90%. Exterior walls that are not insulated well during long cold periods can have interior surfaces low enough to maintain surface humidity above 90% long enough for mold and mildew to develop. A period of just a few days can be enough for mold to begin to grow under the right conditions.

Ways to help mitigate humidity issues.

One way to control the humidity in the home is by using the Air Conditioning system to remove humidity.  The temperature threshold will need to be monitored to ensure that the AC is operating below the temperature of the room.  For those of you with Wi-Fi and internet-connected thermostat control systems, you can easily adjust your settings remotely and monitor the humidity remotely.  Absent a Wi-Fi internet control system, a simple way to help control humidity is with the addition of a dehumidifier.  While dehumidifiers will help it likely won’t be enough to dehumidify your whole home.  You will need to place them strategically throughout the home and they will need an outlet to drain into to, such as, the kitchen sink, the tub, the shower, bath sink or routed to the outside with a hose.  Select dehumidifiers on the basis of the square footage that you are trying to dehumidify.  Pick one or more that’s appropriate for the space.  Set the dehumidifier to a 45-50% humidity level and maintain an overall reading of 50% or less.  Another simple way to help manage high humidity is to use those vent fans and exhaust fans in your home located in the kitchen or your bath areas.  Always keep your AC fan setting to AUTO, never, place it in the ON position.  Use those ceiling fans as they are designed to help air movement.  If you have them turn them on.

Advanced Home Watch is a professional Home Watch service serving Venice, and South Sarasota FL area.  We perform a humidity check when we conduct a Home Watch observation, and we make suggestions to our seasonal homeowners on how to deal with high humidity issues.  Call us today for a free home evaluation at (941) 218-0008 or visit us at for more information.  We provide an essential service that performs visual observations of your home or property, looking for obvious issues that can be the difference between damage and disaster. 

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