The louvered metal grille that directs conditioned air from a duct into the room is called a register, and mold around an air conditioning register can have several causes:

1) If the cold air coming out of the register is too cold compared to the room air, condensate will form on the surface of the register and migrate over to the surrounding drywall. Wet drywall will begin to grow mold within a few days. The register in a bathroom is more likely to have mold growth because the high humidity in the room provides more moisture for condensation.

    The difference between the room air and cold air coming out of the register is called the “temperature differential” or “split,” and more than about a 22º F split can cause condensate to form. 

2) Duct air leakage above the register or at the connection of the duct to the register can also cause condensate to form, even if the temperature split is not excessive, because the air in the unconditioned attic or wall cavity may be significantly warmer. If squirrels or rodents get into the attic, they may  shred openings in ducts for material to make their nests. Workmen in the attic also occasionally cause duct damage that leads to leakage.

    Home inspectors like us that have an infrared camera are able to detect duct connection leakage before it causes ceiling problems. The cold areas show up as blue in the bottom photo below of a leaking register connection to duct.

3) Closing the louvers of the register completely—to shut off air conditioning to a room—can also lead to mold growth because it increases pressure behind the register that can cause leakage at the connection.

   Mold around an air conditioning register is a good reason to call a licensed air conditioning contractor for evaluation and repair, before the problem causes damage that requires more expensive mitigation. What appears to be mold at first glance, however, might be dust adhering around the duct leakage point behind the register.

    Also, see our blog posts What is wrong with an air conditioner when the air flow out of the vents is low? and Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting.

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

    Visit our HEATING AND AIR CONDITIONING and MOLD, LEAD AND OTHER CONTAMINANTS pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.