Some people believe that AC ducts are destined to become moldy. While all AC ducts will likely grow some mold and will become discolored with visible mold or visible dust to some degree, it is not normal or healthy for ducts to become excessively moldy, or to produce a strong mold odor. Mold spores from AC ducts result in health problems and even mold odors in the absence of spores have been shown to cause health problems. Keeping ducts free of excessive mold is important. The information in this blog is intended to provide readers with information on: 1) Some reasons why AC ducts become moldy. 2) The importance of proper sizing of AC units. 3) In addition it draws attention to a very common indoor air quality problem called entrainment. Entrainment is when AC units pull contaminated air from inside attics or walls or other undesirable areas. The following information is taken from a Broward County mold inspection we conducted on 11-17-07 it shows an actual example of how ducts became moldy and gives recommendations at the end.

The property inspected is a one story single family Broward county house built in 1974.ice buildup on window ac unit Client reported mold growing on AC vent registers. ac unit buzzing fan not turningThe mold started a few weeks ago. hvac blower motor rattlingThe client reported that a new larger AC unit was installed about 3 months prior to the mold starting. This inspection is for mold conditions at the AC registers only. In the above referenced area the inspector observed a new air conditioner. The serial number on the unit indicates that it is a 4 ton unit, thus is it rated to cool a 2400 sf house. This AC unit is just a little large for this house. The inspector also observed a new condenser / compressor unit. The serial number indicates that it is a 3.5 ton unit thus designed to cool a 2100 sf house.

This is also slightly large for the house. You have a newer larger AC unit. The size of this newer AC unit is a major reason for why you have humidity related cladosporium on your AC registers that were previously not moldy. The previous AC unit in use about 3 months ago was smaller than this AC system thus would have been more efficient at drying the houses air before cooling the houses air and shutting down. If an AC is to large it shuts off after the air is cooled but before the air is properly dried / dehumidified. Air that is not dehumidified and has a humidity above 60%RH to 65%RH can and often does cause humidity problems in Florida homes. The above photo is of cladosporium mold growing in your registers. Another reason why you have mold problems now is because of the two holes in your AC return. These holes in the wall studs allow the AC to pull humidity into the AC each time the AC turns on, this phenomenon is called entrainment and is a common cause of mold and humidity problems.

Yes all metal studs have these holes, but it is not typical for these open holes in the studs to be exposed and in such close proximity to an AC return. Where does the humidity in your walls come from? it comes room the attic, the attic in turn gets the humidity from outside via the soffit vents like the one in the above photo. Why did this problem not occur till recently? the older smaller AC was more efficient at drying air as stated above and thus was able to handle the humidity load created by these openings. More photos of mold on and around your AC registers. Note that the tiny black spots on and around AC registers are very typical examples of cladosporium spheasperium, or cladosporium cladosporidies mold. This mold problem is caused by elevated humidity in your ducts. This is not likely to cause a toxic reaction, but this mold may result in allergic reactions or act as an asthma trigger in sensitive persons. The real concern is if it is growing excessively deep in your ducts then you will have much more mold than you can see on the registers.

Mold growth in the ducts can and often causes health complaints. Excessive mold was not observed in your AC unit but you are recommended to have your ducts cleaned in accordance with NADCA standards or replaced.Note that the tiny black spots on and around AC registers are very typical examples of cladosporium spheasperium, or cladosporium cladosporidies mold. Based on this inspectors observations and conclusions your inspector recommends using good quality foil mastic tape or other methods to seal two openings in your AC return wall studs. Have a AC company install a humidistat and set it to around 50% to 60%RH always maintain indoor humidity between 30% and 60%RH. Most people live with a little mold in the ducts and have no problems. Mold spore levels in your air was very low. No mold odors were detected. Much of the mold appeared to be primarily in the ends of the ducts just behind the registers. Have an AC company clean the mold from the registers and from the ends of the ducts just behind the registers.

Duct removal or good professional cleaning in accordance with NADCA standards is recommended if additional hidden mold is discovered inside the ducts during cleaning. Daryl Watters is president of A Accredited Mold Inspection Service, Inc. He provides home, mold, and indoor air quality investigations in South Florida. He is also the creator of MIR forms designed to aid inspectors in the production of computer generated indoor air quality and mold inspection reports.When you say vented to the roof, do you mean into the attic UNDER the roof, or through the roof with a duct up through a roof jack into a roof hood ? Hopefully the latter, otherwise you have the likely source of the problem. You are clearly getting moisture in this area still, so more insulation will not help and may hurt - insulation does not stop moisture but does trap it, particularly if you add enough tht the freezing front moves well down into the insulation, so vapor coming up from the house freezes in the insulation (making it wet when it thaws) rather than venting into the attic and evaporating from there.

You need an expert check on the bathroom area - that the fan unit and any light fixtures are tighly sealed to the vapor barrier. Usually they are installed with about a 1/4-1/2 inch void all around for ease of installation, and not sealed at all, so moist bathroom air vents around themm into the attic. Then the fan unit needs checking for openings - many have openings in the plastic or metal case from manufacturing that are not sealed but should be. Do NOT use any type of unit that, because of big lights or heat lamp, says it has to be vented and cannot be sealed in, because moist air goes right up through it. Then the duct from the fan up through the roof needs checking for leaks (and should be insulated, at least if your attic goes below freezing ever), and should have a roof jack where it penetrates the attic - a rubber seal in a metal plate that fits tightly around the duct, so the air blown into the vent hood on the roof cannot circulate back down into the attic. Most installers just cut about a 1 foot opening in the roof (especially if they can install the duct that way without having to crawl around in itchy attic insulation), run the duct up through it into the roof hood, and walk away.

That leaves that big opening in the roof sheathing for the wet air and condensation in the hood to corculate right back down into the attic. Some installers (like my house whenn I first bought it) really take the easy route and don't even connect the duct to the hood - they just terminate it a foot or so below the sheathing so ALL the moist air goes into the attic. I would also check the kitchen and any other bathroom fans for the same leak sources or improper installation, and make sure all vent pipes are intact to above the roof, and that there are no furnace or HVAC ducts disconnected or damaged that could be adding moisture. Also look around all roof penetrations for ducts and pipes for staining on the underside of the sheathing, which would be indicative of roof hood or jack leaks that should be repaired. (Hopefully, with a new roof you would not have any). The area most affected should have the insulation moved away and checked to see if the vapor barrier has holes or tears, openings around pipes, ducts, light boxes or wiring, or was maybe totally torn out by some prior workman.

If your vapor barrier is not effective, moist household air will move into the attic almost year around, but especially in cold weather, carrying moisture into the attic, where it will condense and cause mold. Also - if you have a fireplace chase (wood boxout around metal chimney) in that area, it may connect to the house in the firebox area and be open to the attic (which is a real fire spread hazard but for some reason is not contrary to code), letting household air flow by that route. The mold should be brushed and vacuumed away, then treated - there are commercial sprays that are fungicides that commercial mold and mildew removal contractors have, a sprayed chlorine bleach and borax solution has also been shown to work but you would have to have an air supplied respirator and chemical suit to work with that, which only professional remediation contractors have. Do NOT paint the area - especially the underside of the roof sheathing and trusses. They needs to be able to breathe, not have any moisture from above locked in.

Stains in the attic (assuming this is an unoccupied area) can be bleached, and then if you want the evidence to go away and make it easier to tell if there is new staining or mold, sanded to remove the worst of them. Stains on areas visible from the outside like walls and rafters can be treated with Chlorine bleach (beware of dripping on good finishes below), painted with Kilz or similar anti-fungal primer, then painted. Stains on the underside of the sheathing visible in the soffit area can be bleached and then when dry, sanded away. Ventilation is essential, but without removing the source of most the moisture you will not win this war no matter how many times you battle it. While I would guess the fans and vents are the problem, is there anything different about the attic ventilation to this area versus the other parts of the roof - soffit covers, blocked eave openings, insulation-clogged bug screening or soffit cover openings, lack of air chutes or eave baffles, insulation pushed up against eave opening or up against roof, horizontal blocking that prevents or obstructs airflow