Project 1-susp black growth HVAC

Suspicious black growth is appearing on my HVAC vent registers!

Mold Solutions GroupMold Solutions Group
We see this all the time. Clients will send us pictures wondering just what the heck to do about it. The cause and solution is usually very simple. When you see this happening, there is most likely is some sort of cross talk between two opposing environments. Sometimes there may be an insulation deficiency in the attic above, around the register/duct in the ceiling. This will also result in elevated temperature contrasts and create an environment for mold growth. Check your attic and repair the insulation if necessary. To put it simply, your air ducts are probably leaking. The air inside an air duct (in the summer) is cool. The air outside of the air duct is warm. When the two temperatures collide, you get condensation! The registers typically have some dust/debris on them in some fashion (This is by no means a slight upon your housekeeping!). This dust/debris coupled with moisture from condensation is enough of an environment for a small fungal problem which results in a blackish growth. Sometimes the growth will appear around the vent register on the ceiling drywall. This may be either a clogged return filter making your system function inefficiently and/or a leak where the duct line connects to the register boot.


  • First of all, safety! See above for proper PPE requirements. Always read the label of products before use.
  • Remove the register. The register can be then be taken outside to be cleaned.
  • Now, determine the cause. Generally there is a leak of some sort where the duct line attaches to the register boot. Make sure that this area is properly sealed by using either a duct mastic (a paste that is applied around the connection that seals) or a metalized HVAC tape. Never use plain old duct tape! Contrary to it’s name, duct tape is the worst as it has a tendency to breakdown and fail over time.
  • Check your return filter. If it is dirty replace it. Sometimes the problem is a result of your HVAC system struggling for air. When this happens, the system will draw air from places it typically would not. Many HVAC specialists recommend using the cheap filters for the returns. The reasoning behind this is that the more expensive filters, even though they filter out more particulate, actually make your system work harder to move air. You want to keep that air moving!
  • If the staining extends to the ceiling, look at the edges of the drywall that are exposed once you have taken out the register. If they are clean, chances are the growth is mainly topical (just on the surface of the drywall). This area can be cleaned in the following manner:
  • Purchase from your local hardware store 2 grout sponges. Grout sponges can be found in the tile section. We use these because they are larger and last longer.
  • Purchase from Lowe’s or Home Depot a bottle of “Mold Control” (usually found in the paint section but we have also found it in the roofing section. Why we don’t know). This product comes ready to use and can be acquired in spray bottles or gallon jugs. For your purposes, the spray bottle will be sufficient.
  • Spray the cleaning product onto the surface you are treating.
  • Now, take one of the sponges and clean the area. Once cleaned, set that sponge aside.
  • Re-apply the cleaning product to the area of concern and take the second sponge and gently wipe the excess off.
  • Let dry and you’re done with the treatment.
  • If staining is left behind, don’t worry, sometimes this happens. You will want to apply a couple of coats of a quality primer and then re-paint your ceiling. When repainting, please keep in mind that a ceiling white is one of the hardest colors to match for some reason (We know, sounds silly right?). Make sure you feather the edges of the area you are painting into the existing to minimize color differences as best possible.

Sometimes this phenomena will occur around other penetrations in your ceiling such as light fixtures or bathroom exhaust fans. This may be an indication clogged filters at the HVAC return (change them) or something may be wrong with your attic and/or house ventilation. In these instances, you may want to have us check it out. A larger issue may be present.

To use one of our own houses as an example: A new roof was put on. The roofers, sadly, were quite the incompetent lot and messed up the intended ventilation in a vaulted ceiling. Now, black stains occur around the light fixtures as the cavity between the drywall ceiling and the roof deck struggles to move air. Typically, this is not a weekend warrior repair.