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Thread: Mold/Mildew in Cold Air Return

  1. #1

    Default Mold/Mildew in Cold Air Return

    After encountering a pungent, musty oder upon entering one of our upstairs bedrooms for the second time in the past couple of weeks, I decided to find the source of the odor. I previously suspected it was the humidifier's stagnat water, but that proved wrong.

    I finally discovered that the cold air return, located near the ceiling on the 2nd level (basic colonial layout), was the source of the odor. I checked the bedroom next to this odorous one and found that its cold air register also had a musty odr, but not as strong.

    I went into the attic to check the roof and attic space near these registers and noticed a dark mold or mildew on the inside surface of the exterior wall sheathing, extending about 6" up from the top plate of the wall below, and perhaps 30" wide or so. The same was true a few feet over, where the other bedrooms cold air return is. I noticed no leaks in the roof, and was prepared to take some siding off to inspect for leaks there.

    Then I realized what was really happening. The cold air returns for these rooms, which are just hollow spaces between studs and drywall, are located up against an uninsulated exterior wall! Seems like this would be a no-no from a common sense standpoint as the warm interior air will condense on the cold exterior wall surface in the wintertime. This past winter in MIchigan has been long and cold, meaning I had a longer period of exposure to the moisture than I've had in the past few years.

    There is one more cold air return on the first floor below one of these two known "smelly" ones. I haven't inspected that one yet as there is a very heavy china cabinet blocking me from getting close enough to it to see if it smells.

    Is it legal to place a cold air return on an exterior wall? Shouldn't my local inspector have caught this?

    I plan on spraying the mold/mildew with a 50-50 bleach-water mixture and letting it dry out for a few days. Then what should I do to correct the situation? I'm inclined to stuff a batt or R-13 or R-11 in the wall cavity if I can, and suffer the consequences of worse air circulation in these rooms will hopefully preventing this from happening again in the future.

    Any other suggestions?
    Scott Sabin

  2. #2


    I suspect you are right about the source of the moiture that is feeding the mold. This is a health hazard and should be addressed.

    Cold air ducts if on the outside wall requires insulation installed between a cold air duct and the outside elements of cold to prevent the condensation that you are experiencing. A lot depends on the age of your home and skill of the inspector and heating installer during original installation.

    Best way I know of to cure your problem is the knowledge that mold will dry out and disappear quickly if moisture if not present. Mold is a growing vegetation and must have water.

    I would find an inside wall in the same room that you bad cold air duct is located. Then cut out the top plate over a stud space and nail a covering over the ceiling joist making a duct of the ceiling joist running to the main cold air duct rerouting the cold air duct as discribed. Then block of the bottom of the old cold air duct to stop warm air from entering that duct with the mold. The mold will then dry out and die. Once it dries out and dies you need to then wash that area down best you can with your bleach while wearing a mask to avoid sickness due to this mold.

    Good Luck


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