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Mold Growth in Ducting

Mold growth in ducting is a controversial topic.  If you believe what duct cleaning companies tell you, mold is growing in everyone’s ducting.  If you believe what the EPA tells you, mold growth in ducts is a rare phenomena.  What is the truth?  Unsurprisingly, the duct cleaning contractors are a bit overzealous in their claims.

Mold growth in HVAC ducting is quite rare.  There are several reasons for this.

1.)  Metal is not a good food source.  Most ducts are constructed of either metal or plastic.  Neither of these materials are particularly efficient sources of food for mold.  Thus, for mold growth to occur, a layer of dust must be present on the metal or plastic ducting. Granted, many older homes have a substantial layer of dust within the ducting, which brings us to our next requirement, moisture.

2.)  Ducts are typically dry.  Mold growth requires elevated levels of moisture to grow.  Because ducts are used to transport air throughout the home, they are typically the driest areas within a house.  This is due to both the elevated temperatures (in cool climates) and increased air speed.  Unless the home suffers from extremely high humidity, the ducts are unlikely to have conditions to mold growth.  And if a home does have sufficient humidity to cause mold growth in the ducts, it is assuredly occurring throughout the rest of the home as well.

Exceptions.  When does mold growth occur in ducting?

Mold in Ducting

Though uncommon, mold growth does occasionally occur in HVAC ducting.  Recently we inspected a home with extremely high levels of humidity.  This cause systemic mold growth throughout the entire home, including the duct work.  In this particular case, the mold growth was significant enough to require a complete replacement of the ducting rather than simple cleaning.

Crawl space flooding can also lead to extensive mold growth problems in ducting.  As you can imagine, submerging duct work in flood water will almost assuredly lead to mold growth.  In these cases, cleaning is typically not a viable option and full replacement is warranted.

Water Noted Coming Through HVAC Ducting

Water Damaged HVAC Ducts & Vapor Barrier

Air conditioning.  In hot/humid climates, such as the south east, condensation can occur on the outside of the ducting.  This develops when the warm, humid air interacts with the cold surface of the ducting, leading to condensation on the outside surface.  If dust or debris is present, mold growth can occur.  Though this will not directly effect the air quality, as the mold growth is outside the ducts, it can indirectly cause elevated levels of mold growth in the air.



Reducing moisture in the ducting is the most critical step in preventing mold problems.  Many companies will try to sell you on preventative coatings or pre-treatments.  However, within days these coatings will become covered with a layer of settled dust, rendering them useless.  Don’t spend a penny on cleaning up duct mold problems until you have a clear idea of the cause of the moisture problem.  Moisture reduction typically comes down to two areas.

Project Report > Inspection for HVAC Ducting

Work #: 104027

General Information

  • The client is concerned about odors emanating from the HVAC ducting in the motor home’s bedroom.

Furnace / HVAC

  • Very strong odors noted when rear furnace was activated.
  • There is some small evidence of water droplets inside of the furnace enclosure.
  • All visible portions of HVAC ducting in question appeared to be well connected and undamaged.
  • Small amount of construction debris and one dead insect noted in HVAC ducting.


  • Assess furnace for possible contamination from waste water or other decaying organic materials.

Master Bedroom:

  • Small gap around HVAC boot and subfloor, which may allow odors from the undercarriage or road to enter the bedroom.
  • RH: 49.3%     Temp: 65.7F     CO2 = 497ppm     CO = 0ppm


  • Seal gap in subfloor with appropriate materials.

Project Report > Inspection for Improper HVAC Ducting

Work #: 102110

General Information

Poor Ducting

  • The home owner is concerned about odors that are most severe when HVAC system is running.

Furnace / HVAC

  • The furnace is located the crawlspace below the Family Room.
  • The furnace has recently had a large pleated filter (poorly fitted, filter box not well sealed) and UV catalytic filter added.  These units are known to produce ozone which may be a respiratory irritant.  No specific ozone testing was done during the inspection to determine the levels of ozone in the home.
  • The furnace ducting does not appear to be adequately insulated or sealed.
  • The pleated filter is very poorly fitted.
  • The supply duct for the Family Room, is a 6” round metal duct that is directly taken from the HVAC supply plenum, creating higher pressure flow to this area.
  • The furnace has reportedly been recently cleaned and serviced.
  • The addition of a fresh-air intake will greatly improve the ventilation in the home.

Improper Ducting


  • The accessible HVAC ducting should be sealed and appropriately insulated.
  • The furnace filter should be replaced with one that fits correctly, or the filter box modified professionally to ensure it is tight-fitting and leak proof.
  • Investigate the amount of ozone being generated by the UV filter and if it may be contributing to the irritation to the home owner.

Family Room:

  • Door to garage does not have self-closing hinges.
  • RH: 33.3%     Temp: 69.5F     CO2 = 554ppm     CO = 0ppm


  • Install appropriate self-closing hinges on the door to the garage to prevent the accidental influx of carbon monoxide from vehicle engines.


  • Kitchen range exhaust hood does not duct to exterior.
  • Scented candle burning on counter.
  • RH: 33.9%     Temp: 68.1F     CO2 = 465ppm     CO = 0ppm


  • Removal of all scented candles, oil diffusers, or other fragrance emitting products may decrease irritation to sensitive individuals.
  • Installing the appropriate ducted range exhaust hood will help to exhaust humidity and odors generated by cooking.

Dining Room:

  • RH: 34.6%     Temp: 67.8F     CO2 = 489ppm     CO = 0ppm 

Hall Bathroom:

  • No bathroom exhaust fan currently installed.
  • One scented oil diffuser noted.
  • RH: 38.4%     Temp: 68.3F     CO2 = 515ppm     CO = 0ppm


  • RH: 34.2%     Temp: 39.5F     CO2 = 515ppm     CO = 0ppm

Guest Bedroom:

  • Large section of missing insulation noted in ceiling cavity near hall bathroom.
  • RH: 34.0%     Temp: 68.9F     CO2 = 487ppm     CO = 0ppm


  • Inspect attic and install insulation correctly, as needed.

Master Bedroom:

  • Missing insulation noted in ceiling along exterior wall, batts have likely been pulled from eaves to prevent soffit blockage.
  • RH: 35.8%     Temp: 39.2F     CO2 = 545ppm     CO = 0ppm

Master Bathroom:

  • No door separating from master bedroom.
  • No exhaust fan currently installed.
  • Minor straining noted on vinyl flooring near toilet supply line from previous leak.

Basement Main room:

  • RH: 35.0%     Temp: 66.7F     CO2 = 452ppm     CO = 0ppm


  • RH: 41.4%     Temp: 66.5F     CO2 = 421ppm     CO = 0ppm


  • No ventilation currently installed.
  • History of previous water intrusion noted along foundation wall, owner’s report that this has been resolved.
  • Vapor barrier installed along soil, but appears to be only 4mil thickness.
  • The area is very dusty.
  • No sealing or insulation noted along common wall to the conditioned basement space.


  • This area is currently not functioning correctly with respect to the building envelope.  The area should either be:
  • Ventilated to code standards of 1 square foot net free area per 300 square feet of foot print, the floor joists and common wall air sealed and insulated, and ducts insulated OR:
  • The foundation wall insulated with 2-part spray foam to at least 24” below grade, and the vapor barrier replaced with a sealed 10mil product securely affixed to the foundation.

Basement Bathroom:

  • Exhaust fan ducting via kinked flex and into dryer duct.
  • RH: 39.0     Temp: 65.0     CO2 = 444     CO = 0


  • The exhaust ducting for the bathroom exhaust fan must be re-routed via rigid metal ducting to a dedicated exhaust point, and no longer shared with the dryer exhaust.

Laundry Room

  • Dryer duct mixed with bathroom exhaust ducting.
  • RH: 36.4%     Temp: 65.2F     CO2 = 457ppm     CO = 0ppm


  • Dryer ducting can no longer be shared with bathroom exhaust ducting.

Crawlspace with furnace

  • This area has recently had passive ventilation added.
  • No air-sealing or insulation was noted to common wall to basement, or under floor cavity.
  • This area is not a conditioned space, but the building envelope has been maintained as if it were a conditioned space.


  • All areas of under floor cavity and common wall to basement must be air-sealed and insulated.

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I have 1 inch insulation on a basement duct. When I examined the ducting I found some black substance on the yellow insulation. Is this a problem?

You’re very likely seeing dust particulates rather than mold growth.   When air flows through the ducts, some of it escapes through the cracks in the joints.   When this air hits the insulation, the dust particles are filtered out and deposited on the insulation.  The dust itself is harmless.  However, the leaky ducts cause higher energy bills and therefore should be sealed.  You can see a photo of the phenomenon on attic insulation here (scroll down to “False Alarms”)