Asked on Jan 12, 2012

Mold on AC vents

Cindy L
by Cindy L
What could cause the ac outlet vents in the upstairs bedrooms to get mold on them? Also, there appears to be small patches of dark dust at the corners of the vents. My house is seven years old, and this problem first started in the master bedroom but has also occurred in the other bedrooms, which are only occupied on occasion.
  10 answers
  • You have a few things going on. And like the movie the Perfect Storm these few factors is what is causing your issue. Mold needs moisture to develop. No moisture, no mold. So the first thing you need to figure out is where is the moisture coming from that would allow this mold to grow. Well in most cases the moisture develops from condensation that forms on the metal can that holds the grill into place. It also can develop on the surface of the grill if the temps of the grill are to cold and indoor humidity is high. This often happens if the air flow coming out of the grill is slow. In any case you need to figure out where the moisture is coming from. My experience tells me that the condensation is forming as a result of poor installation practices of the duct system that houses and holds the vents into place. What is happening here is that the outside surface of these cans are exposed to unconditioned space. The Attic. The attic is hot during the summer and contains lots of humidity. The cold AC that blows inside these metal boxes as its delivering the air to the room., cools the boxes down to a point that moisture collects on the surface of the metal. Much like a cold glass of ice tea that gets wet on the outside of the glass. This moisture drips down through the ceiling opening and collects on the grill causing it to provide the necessary moisture that supports the mold development. To fix this issue you need to check each grill box in the attic area and make sure not only are they sealed carefully with insulation but that a foil vapor barrier the covers the insulation is intact and not allowing moisture to penetrate through the insulation and condense. Using foil tape you can purchase at the big box stores. Tape this insulation tightly closed so no air can see the metal can that the ducts are running to. Spay foam also works well for around the bottom of the box where it meets the ceiling area. Just be careful as this stuff can become quite messy fast. Now that you hopefully stopped the moisture that is collecting on the vent box you need to lower or remove The 2nd thing that is required to support mold development. Dust. Mold in most cases will not grow on the grill surface by itself. Conditions must be real good for this to happen. And the environment that is found in most homes just does not support this. Mold needs something to eat. Like any plant it must get its nourishment from something. Clean hard finished vent grills simply do not provide this. But dust that collects on the surface of the grill face does. Remove the dust, you have removed the food source. Cleaning this is quite easy. Simply remove the two or four screws that holds the grill into place. Fill kitchen sink with any good detergent. Lysol works well. Dip the grills in and with a bottle brush clean them. Rinse and let dry. If your lazy and want to spend more time on Hometalk. Put them in the dishwasher and run through a quick clean cycle. Dry them off and put them back into place. Now you cleaned, and sealed the ducts from condensing moisture. Your not done yet. You need to replace your filter on the cooling system. The primary reason your collecting dust on the grills is that the filter system has failed to stop the dust from blowing through. Also because this has failed, the interior evaporator coil that is connected to the system also has collected a lot of dust on it. These coils much like the ice tea example cools the air down to remove the moisture in the air. This wet coil acts as a filter of sorts and collects the dust or most of the dust that blows by when the fan is running. If its also connected to the heating furnace the dust moves through the coil fairly easily and is discharged out of the grill face. In any case you should attempt to look at the coil on the side in which the air comes from to make sure that it has not plugged over with a layer of dust. This can result in poor performance of both heat and ac system and shorten its life span while doing so. Once clean and every thing is fixed be sure to replace your AC filter every month if its the cheap $2.00 ones, and at least twice a season if they are the better $10 filters.
  • Cindy L Cindy L on Jan 13, 2012
    Woodbridge, you are amazing. Thank you so much for the help. It's so good to get help from someone who really knows his business and kindly shares his expertise. We will be checking out everything you have noted. By the way, is there some other way for dust/dirt to get onto the evaporator coils than through dirty filters? We use those special filters from 3M and religiously change them every 3 months as directed.
  • Air duct leakage is the primary reason that dust gets around the filters. If the filters do not fit properly, or the door that seals off the filters is not designed to seal off that exposed edge (the one you see when you first look at the filter to change it) the dust will go around there. Air will take the path of least resistance and with the 3-M filters air wants to bypass all the time when it can.
  • Brenda D Brenda D on Jan 15, 2012
    do you have a humidfier hooked to youre furnace. if so you may need to check this and clean it. also how is youre bathroom ceiling fan vented?
  • Cindy L Cindy L on Jan 15, 2012
    No humidifier. The bathroom ceiling fan is vented through the roof. Thanks so much for responding.
  • Rwn242895 Rwn242895 on Oct 07, 2013
    How do you clean the dark mold spots that are all ready on the ceiling? Can I just spray and bleach water mixture on the ceiling?
  • You first need to determine what caused these spots and do what is necessary to prevent them from returning. Mold is supported by moisture. So what ever is causing that area to become damp or wet needs to be corrected or regardless of what you do, the mold will return. Bleach does nothing to remove mold. It is wrong to believe it does. The concentration amount of bleach it takes is not sold over the counter anyway. All bleach does is remove the color of the mold. Sure it will wash surface material away, but what is growing below the surface of the wall or ceiling is simply bleached to a lighter color making you think you were successful in removal. The only proper and correct method of mold removal on walls or ceilings is to cut the damage out and replace. Does this mean that you need to do this? Not at all. But depending upon the severity of what is present determines the method of remediation. A good cleaning with an anti-microbial disinfectant is simply the first step. Then you must using a oil based anti-microbial paint, not water based, prime the surface and allow it to dry. Then you can follow up with a fresh coat of paint. This is the simple method of cleaning up mold stains, as long as you have successfully removed the moisture source. The mold still remains within the drywall or plaster. But encapsulation by using the method described above should be enough to prevent it from returning. But I must emphasize that you have not removed the mold itself, you simply covered it up with materials that makes it more difficult for the mold to return.
    • Kelli Kelli on Jun 11, 2017


      I have been reading all your advice and have been having a horrible experience with the apartment I am in. My daughter constantly has respiratory issues and needs a nebulizer. This has only been since we have been in this apartment. In the bathroom there is a cover in the ceiling and the drip pan was not level. It has leaked multiple times since we have been here and maintenance was just clearinvite the water line until we finally realized the drip pan was not level (a year and a half later) I started noticing some mold around the back up pipe in the ceiling in the shower and decided to take my AC vents off. the picture below is what I found in all my AC vents...disgusting right? Well the complex refuses to do anything about this so I did a dust test and sent it to a lab. It came back with 3 types of mold. Cladosporium (live and dead spores) basidiospores and ulcladium. They claimed to clean the ducts but it was done in about an hour and I do not trust it. I am thinking of sending a peice of the drywall in for testing....I am really not sure what to!

  • Debra Neeley Debra Neeley on Jun 11, 2017

    Move and if you have medical bills asked to be compensated. Mold and health issues are serious.

  • Imp22013828 Imp22013828 on Jun 11, 2017

    Google. Household mold cleaners. Or ask at paint store for prep cleaner for walls. And tell them you have to wash off the mold before painting with kill.

  • Jewel Jewel on Oct 04, 2020

    You are NOT to clean that yourself. It could be highly toxic! Take pictures, send registered letter make them sign for it per your lease it states, you must send it in writing. They have 7 days to respond. You could also get it professionally tested and sue them if they don’t remedy it. And yes, move out immediately. Your daughter has mold sickness. It can then get all over your belongings as it’s in your HVAC. You must clean all ur things in concrobium and wash all laundry w EC3 laundry additive. Leather and cotton items must be checked bigtime. Pets can be greatly poisoned by mold as well. Get out!!!