So recently this summer, we noticed some mold(?) growing around one of my new air conditioning registers. It looks like when I had my ceilings redone when I moved in the beginning of 2014, this register wasn't bolted down as tight as it should have been.

What should I do to clean it? I bought some mold remover spray, took off the register and sprayed it and sprayed the effected area on the drywall. Is this enough or does this look bad enough to have professionals come do it?

Had the same thing and replaced the boxes in the ceiling with insulated ones. Worked perfectly.

Is this in a bathroom?

I've had good luck with the kitchen / bath spray cleaners that contain bleach. Spray a little on a cloth and lightly dab and lightly rub. If you get aggressive dabbing and rubbing you'll take the paint off. Also, you might have to go at it a couple times, like once today, then once tomorrow or something like that. If you get the paint / sheetrock too wet the paint will come off. Be gentle and you'll be fine. Once complete look at your register and see if the "fixed" louvers (not the moveable ones that open / close the register) are directing the air too close to the ceiling. The "fixed" louvers can be bent down to direct the air more down and less across at the ceiling. All of my registers that developed this type of mold / mildew just needed to be adjusted down and none of this mold / mildew ever reappeared after the adjustment. Adjustment (assuming your register is sheetmetal and not plastic) can be done with some pliers or your fingers. If you use your fingers be careful of sharp edges, if you use pliers use a rag between the pliers and the fins so you don't scratch up the paint.

No, it is living room.

Thanks for all the suggestions guys. I got a mold remover spray from Home Depot (it said drywall was under the applications). Sprayed it really well and wiped it down. Unfortunately it took off some of the paint and texture, but I just repainted it after it was dry and installed the register. Looks good so far, I'll keep an eye on it and update this thread if it comes back.

You need to insulate around the box where it meets the drywall. Hot air leaking around the box meeting the cool air from the duct is causing the mildew.

Do they make like a gasket to put around the air vent?

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Do they make like a gasket to put around the air vent?

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I've sealed all of mine with a thick bead of silicon caulk. Works like a champ. Just work it in between the box and the sheetrock.

If the condensation you've had is severe enough you might need to replace or patch some sheetrock if it's showing signs of water damage such as swelling.

You can use a thin strip of self-adhesive gasket to make your own to put around the inside of the register. 1/4 inch or so. I wonder if it's also being caused by some air leaking from where the duct meets the register box and filtering down beside it. That mildew seems excessive.

To add to what I posted above...

When you've got a leak one of two things will happen. It looks like in your case you've got a leak between the register box and the sheetrock. Exactly how the air flows around that leak is a function of some high end physics that's above my head, so for detailed analysis consult your local rocket scientist. However it sums up that you can either be blowing dehumidified cold air into your attic, or you can be creating a lower pressure zone (created by the high velocity air flowing across the opening) and drawing in HUMID air from the attic.

In your case you're drawing in warm & very humid air from the attic. It's the venturi effect. That humidity load is far greater than the cooler air can hold. The moment that warm wet air is in the vent it is flowing across cooler surfaces, your grille and the sheetrock, and begins to lose its moisture to condensation. Makes sense too... This air is entering the edge of the flow on the edge of the duct/vent because it's entering from the edge.