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What is a suitable humidity level to prevent mold?

According to the EPA.gov,

“Sometimes, humidity or dampness (water vapor) in the air can supply enough moisture for mold growth. Indoor relative humidity (RH) should be kept below 60 percent — ideally between 30 percent and 50 percent, if possible. Low humidity may also discourage pests (such as cockroaches) and dust mites.

Humidity levels can rise in a building as a result of the use of:

  • Humidifiers
  • Steam radiators
  • Moisture-generating appliances such as dryers
  • Combustion appliances such as stoves

Cooking and showering also can add to indoor humidity.

HVAC System

One function of the building heating, ventilation and air conditioning (HVAC) system is to remove moisture from the air before the air is distributed throughout the building. If the HVAC system is turned off during or shortly after major cleaning efforts that involve a lot of water, such as mopping and carpet shampooing or cleaning, the humidity may rise greatly, and moisture or mold problems may develop.

Condensation

Condensation can be a sign of high humidity. When warm, humid air contacts a cold surface, condensation may form. (To see this, remove a cold bottle of water from a refrigerator and take it outside on a hot day. Typically, condensation will form on the outside of the bottle.) Humidity can be measured with a humidity gauge or meter; models that can monitor both temperature and humidity are generally available for less than $50 at hardware stores or on the Internet.”

Read more:

https://www.epa.gov/mold/mold-course-chapter-2