Humidity inside your home.

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.

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.


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.

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.

Also when moisture inside your house evaporates into the air it increases the humidity indoors. If your home isn’t well ventilated, then the humidity will stay high for a long time.

 Drying clothes indoors on clothes lines or stands is a common culprit when it comes to causes of indoor humidity problems. The household HVAC system can also create humidity troubles while it artificially heats or cools the air.

 Sometimes people use humidifiers in their homes. But there are molds that only need the humidity level to be higher than 55% to start growing. So if you need to use a humidifier in your home make sure to keep the humidity below 55%.

 Not only does high humidity feed mold, but it means that puddles of water and damp materials in the home take longer to dry out. These wet surfaces can in turn create mold growth of their own.


To read more about humidity and mold click here

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