Internal insulation provides better acoustical (noise) control. Flexible duct is very low cost. These products are engineered specifically for use in ducts or as ducts themselves, and are tested in accordance with standards established by Underwriters Laboratories (UL), the American Society for Testing and Materials (ASTM), and the National Fire Protection Association (NFPA). Many insulated duct systems have operated for years without supporting significant mold growth. Keeping them reasonably clean and dry is generally adequate. However, there is substantial debate about whether porous insulation materials (e.g., fiber glass) are more prone to microbial contamination than bare sheet metal ducts. If enough dirt and moisture are permitted to enter the duct system, there may be no significant difference in the rate or extent of microbial growth in internally lined or bare sheet metal ducts. However, treatment of mold contamination on bare sheet metal is much easier. Cleaning and treatment with an EPA-registered biocide are possible. Once fiberglass duct liner is contaminated with mold, cleaning is not sufficient to prevent re-growth and there are no EPA-registered biocides for the treatment of porous duct materials. EPA, NADCA and NAIMA all recommend the replacement of wet or moldy fiber glass duct material.
2. If air quality is so important and duct cleaniliness is so uncertain, why not adopt doubling filtering; that is, in addition to the existing main furnace filter, add a filter at each air outlet. I understand replacing so many filters would be a burden, but air outlets can be designed for easy filter replacement. The builders and home design engineers should bear responsibility to provide good and low maintenance homes, rather than prioritizing on fancy or costly featuers for homes.
Air duct cleaning service providers may tell you that they need to apply chemical biocide to the inside of your ducts as a means to kill bacteria (germs) and fungi (mold) and prevent future biological growth. They may also propose the application of a "sealant" to prevent dust and dirt particles from being released into the air or to seal air leaks. You should fully understand the pros and cons of permitting application of chemical biocides or sealants. While the targeted use of chemical biocides and sealants may be appropriate under specific circumstances, research has not demonstrated their effectiveness in duct cleaning or their potential adverse health effects. No chemical biocides are currently registered by EPA for use in internally-insulated air duct systems (see Should chemical biocides be applied to the inside of air ducts?).

I had someone out to clean my ducts last weekend. He took a blue bag up in my attic. He came back down in just a few minutes and said I had mold and he even showed me a picture of something saying it was mold but I couldn’t really tell. I refused his service to take care of the mold. He went back up in the attic for a little while and then came down. I never heard any sounds that he was actually up there doing anything. How do I know if he actually cleaned the air ducts?

Metal is way better than flex duct or ductboard. It gets better airflow, holds up to critters and duct cleaners. I know of no hvac company here that cleans ductwork. (there probably is one somewhere) The mfg's of duct cleaning equipment keep trying to sell us hvac people on their benefits and profit potential but most all hvac people know that unless you have metal ductwork you are NOT doing the customer any favors.
Did you know that cleaning your ducts and vents can help airflow and increase energy efficiency? According to the Environmental Protection Agency (EPA), keeping your air ducts and vents clear can increase energy efficiency and indoor air quality. While the cost to clean ducts and vents might sound high, the benefits to homeowners–especially those sensitive to allergens–is worth considering.

Chemical biocides are regulated by EPA under Federal pesticide law. A product must be registered by EPA for a specific use before it can be legally used for that purpose. The specific use(s) must appear on the pesticide (e.g., biocide) label, along with other important information. It is a violation of federal law to use a pesticide product in any manner inconsistent with the label directions.


You may be familiar with air ducts that are constructed of sheet metal. However, many modern residential air duct systems are constructed of fiber glass duct board or sheet metal ducts that are lined on the inside with fiber glass duct liner. Since the early 1970's, a significant increase in the use of flexible duct, which generally is internally lined with plastic or some other type of material, has occurred.
Professional cleaning involves getting the dust and debris outside, so they use outdoor ventilation equipment. It is so important that homeowners make sure the professional contractor is using good equipment then, as they could otherwise spread the spores into the home and further agitate sensitive family members. In the case of mold, a cleaner will only be able to tell you it is there, followed by needing an additional mold professional to come out and test or remove the mold. Air duct cleaning industries do not require state licensing, so make sure to check for a company with references.

There are many interesting comments from several informed individuals. The question that has not been answered is how do duct systems get dusty and dirty? As a RESNET Home Energy Rater, I perform pressure tests on homes and duct systems, and the one common thread is . . . most air delivery sytems have large amounts of air leakage. The solution is to seal the duct system, and pay particular attention to the return duct system and ductwork near the air handler (the 'blower'). As others have said, a captive (sealed) duct system with proper filter maintenance should be more then adequate for most occupants. For those who have allergy, asthma, or other breathing problems, a filter with a higher MERV rating (number should be listed on the package) or a HEPA filter are good choices. These filters will capture smaller particulates and should improve the indoor air quality. Again, make certain that the entire duct system is sealed, preferably with mastic, second choice UL-181 foil tape - never use cloth duct (also known as duck) tape.
Having evaporator and condenser coils cleaned could cost between $100 and $400. If your coils can be accessed in-place, you'll be looking at a lower service cost. If your technician needs to remove the them first, the cost should be around $400. Having the evaporator and condenser coils cleaned can have a significant impact on the efficiency of your system. If your coils are excessively dirty, they won't be able to do their job and your system will have to work harder. Keeping up with the hygiene of these components can save you money in both the short and long-term. For example, replacing evaporator coils can cost anywhere from $650 to $1200.
On the other hand, if family members are experiencing unusual or unexplained symptoms or illnesses that you think might be related to your home environment, you should discuss the situation with your doctor. EPA has published the following publications for guidance on identifying possible indoor air quality problems and ways to prevent or fix them.
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