Q: We are trying to eliminate a mold smell that is coming through only some of the air conditioner vents in our home. Our a/c unit is under the house, in a crawl space which has some mold from moisture problems. Last night we replaced the vent pipes to my son's room, but the moldy smell remains. We also found a couple of holes in the duct work and sealed them up. Our frustration stems from not being able to figure out why the air coming through the vents in our bedroom and our closet smells clean, but in our son's room and other parts of the house the vents blow a faint mildew smell. If the mold was in the unit wouldn't the smell come out of every vent?

A: Microbial growth can occur in ductwork for a number of reasons. Based on the information you have provided and the limited areas impacted, the source of the "mold" odors may be related to condensed moisture within the air distribution ductwork or an air leak into the ductwork from the "moldy" crawl space. If the ductwork in the crawl space insulated on the interior then moisture condensing on the insulation is the likely source of the odors and it may be localized to one area of the ductwork. If condensation and resulting mold growth in the interior of the duct insulation is the mold source then replacing or cleaning the ducts is the best solution. Using closed cell neoprene insulation on the inside of the ductwork would provide insulation that is hydrophobic (water repelling). Exterior wrapped fiberglass insulation on ductwork should not impact air quality supply ductwork (it should be positively pressurized) even if it gets wet.

Investigators often see filter installation slots in air handler units that are not properly sealed and allow contaminated air from wet areas into the building air stream. Although this would generally impact the entire area being supplied by the unit, it is possible that the mixing of the air in the air handler is not perfect and one area could be impacted more that the other. The air handler should be sealed completely and no air from the crawl space should be allowed to enter the air stream.

You might also consider reducing the moisture level in the crawl space with a dehumidifier. However, the crawlspace should be sealed from the outside for this to be effective. Alternatively, you can ventilate the crawl space, placing the crawl space under negative air pressure as compared to the floors above. This should pull contaminated crawl space air outside of the building so that it could not rise into the house. I don't know if the areas of concern are directly above the crawl space or are on a second floor. This would impact the chance of moldy odors entering from around the ductwork rather than through the supply duct.
Lastly, are you sure the odor is from the ventilation system? Moisture in a wall cavity adjacent to a floor supply duct or water damaged carpet around a floor duct have been mistaken for mold sources within the supply duct. Careful inspection for alternative moisture sources is recommended.

Mark Goldman, Senior Air Quality Scientist, EFI Global Inc.

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