December, 2019
Inspection News and Views from the American Society of Home Inspectors

Duck Duct Problems


Jeff May writes a quarterly newsletter that puts a spotlight on indoor air quality (IAQ) issues. This month, ASHI shares Jeff’s Winter 2018-2019 newsletter topic: Duck Duct Problems.

A duct system can cause plenty of indoor air quality (IAQ) problems if the system is improperly installed and/or it has been poorly maintained. When ducts contain biodegradable dust and mold growth, odors, mold spores and surrogate allergens may be carried on air flows into habitable areas.

Hidden treasures?

I’ve found Cheerios, dog food, toy soldiers and even a used condom in ducts.

One owner who had just renovated her home hired me because whenever her heat was running, she detected an unpleasant odor coming from the duct in her home office. She met me at the door with a long list of questions. The contractor who had worked on the house for her must have had some problems getting along with her because when I removed the floor register from her office supply and looked into the flexible duct with a mirror and flashlight, I found the yellow-stained remains of a “pool” of urine in the duct bottom. Wonder how that got there?

In my own home, when our two children were little, my wife took a milk carton out of the refrigerator to give them some at mealtime. She tripped on a toy on her way to the table and fell to her knees in front of a supply duct. The milk carton tipped as she fell; before she could right the carton, a few cups of milk poured into the duct. That was a cleaning job and a half, but I was luckier than the fellow who had trouble sleeping in his bedroom due to an odor coming from his bedroom supply. Inside a basement duct elbow for the supply to his bedroom was the foul-smelling residue of a cup of coffee with cream that had gone rancid.

Panned bays

These are usually full of biodegradable dust, some of which in older homes may be ancient. In addition, panned bays can be open at joints and sheet-metal gaps to a musty space.

Such “ducts” are difficult, if not impossible, to clean (especially if they contain wires and cross bracing). I generally recommend that the sheet metal from such “ducts” be loosened or removed and cleaned, and that the framing be cleaned and paint sealed (or preferably lined with aluminum flashing at the joists and foil-laminated sheet foam at the subflooring (Photo 1).

Duct board ducts

Duct board ducts just can’t be cleaned and when biodegradable dust collects in such ducts due to inadequate filtration, mold growth ensues. For families with allergies, such ducts should probably all be replaced.

Flexible ducts

Flexible ducts are another type of duct that is difficult to clean. And the “shelf life” of this kind of duct can be short if the duct is installed in a hot attic. (The vinyl wrap is about 30% liquid plasticizer, which vaporizes out of the plastic, leaving a very thin, brittle plastic film that shreds and allows the insulation to fall.) For families with allergies, soiled flexible ducts may have to be replaced (Photo 2).

Basement supplies

I generally do not like to see hot-air supplies in basements. For one thing, if the basement is moldy, the supplies may pressurize the basement and force basement air up into the habitable rooms. In addition, in the winter after the blower shuts down, warm air can flow passively through a supply into the duct and out of the supplies in habitable rooms.  If the basement is moldy, byproducts of mold growth as well as mustly odors may be circulated within the system. 

Leaky ducts

In basements, crawl spaces and attics that contain mold growth, leaky return ducts can entrain mold spores and musty odors, and spread them into the system and into habitable spaces.

More problems with ducts in crawl spaces

Crawl spaces are not conditioned, so in cold weather, ducts lacking exterior insulation can be cold. If the temperature of the duct is below the dew point of house air, moisture may condense and if dust is present, mold growth will ensue. A similar situation may occur in the summer when the duct surface temperature is below the dew point of house air during humid weather (Photo 3).

Ducts insulated at the interior

Fibrous lining material captures biodegradable dust: landscape for mold growth.

Ducts in a separate air-conditioning system

In a home with hot water or steam heat and a ducted air-conditioning system in the attic, ceiling supplies and returns should be closed during the heating system. Otherwise, moist air will flow into the ducts and supply the moisture for mold growth. This is a particular problem with a return in a hallway ceiling outside a bathroom and/or if humidifiers are used.

Keep ducts cleaned

Occupants may keep surfaces in a building spotlessly clean, but they don’t think about ducts; and yet, the air they breathe passes over duct surfaces. If such surfaces contain biodegradable dust and/or mold growth (even in new construction), occupants will be exposed to these substances (Photo 4). As home inspectors, therefore, it’s worth speaking to your clients about duct maintenance, as well as the pros and cons of the types of ducts you see in your inspections.

If any of your clients have allergies or asthma, it would be worth their while to have the ducts professionally cleaned before they occupy their new home. Tell them to spend money on getting their entire HVAC system cleaned, including the blower cabinet, blower and cooling coils, rather than depend on the cheap, quickie job from a truck-mounted vacuum used to clean only the ducts.

Jeff May (May Indoor Air Investigations LLC) is a retired member of ASHI and has given many presentations at ASHI meetings. Jeff has been investigating moisture and indoor air quality (IAQ) problems for almost 30 years, and he has analyzed by microscopy over 40,000 air and dust samples. He has trained other home inspectors to do the same. Jeff served as an Adjunct Professor in the Department of Work Environment at, University of Massachusetts—Lowell and he is author or coauthor with his wife, Connie, of four books on IAQ published by The Johns Hopkins University Press availible on He and Connie are working on a second edition of My House is Killing Me!, which is scheduled to be published in 2020.