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Stylized Text: Humidity.

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- rain water 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 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.

Picture of thermometer.
Photo by Chuck Withers Florida Solar Energy Center

How do you know if your humidity is too high?

If amphibians are comfortable for extended periods of time, then that should be a clue. If mold appears on wall surfaces such as in the upper right corner of this photo, then that is a clue that has come too late. The best way to know is to spend $20- $30 at a local electronics store and buy a temperature and humidity sensor. There is really no need to spend more than this for indoor air monitoring.

The photo was taken in a new unoccupied building during late fall in Florida. The mold at the bottom of gypsum walls was due to a roof leak that ran down into an exterior wall. Air conditioning equipment was not operable at this time.

Control Moisture and Improve Air Quality Control.

The best target to maintain indoor relative humidity is between 45%- 55%. 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 or heated home indicates that the air quality control could 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.