What can I do to stop offgassing of chemicals used in mastic sealant in my condo's AC duct system?


What can I do to stop offgassing of chemicals used in mastic sealant in my condo's AC duct system?

Asked by Leslie Cedar

Since the mastic sealant was used in the AC vents and on part of the fiberglass duct, I am smelling a chemical odor from the AC. Methanol, ethylene glycol, and ammonia were listed on the MSDS, and I have been having some physical reactions as well. I was also told that formaldehyde may be offgassing. The product is "ccwi 181" by Hardcast Carlisle. I ventilate as much as possible, but with the Florida heat, an AC system is necessary.

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Mary Cordaro's picture

Usually, for the average person, duct mastics do not outgas for longer than a week or two, depending on the product.

However, before I discuss the specific mastic and what to do about the odors, it's important to first rule out other sources of chemical or other odors.For example:

  • Were additional changes made to your AC (which I'll now refer to as HVAC) system, other than the newly applied duct mastic?
  • Did anything else change in the house, such as new interior or building materials or finishes?

Depending on your answers to those questions, there are a number of possibilities.

Air flow changes in your home

When building pressures coupled with HVAC carry pollutants from one area to another, pollutant pathways are created, resulting in chemical smells or reactions that you may not have previously experienced.

  • One typical pollutant commonly distributed by HVAC systems is mold.
  • That's why the building scientist / environmental inspector on our team calls some HVAC systems in humid environments "mold distribution systems."

Depending on many factors, such as closing doors to rooms with no returns while running your HVAC, changes to your ducting, or existing/remaining leaks in your ducting, negative pressure can result.

If your home becomes negatively pressurized when you turn on your HVAC, then your house "sucks." This means that all the air inside one or more rooms literally sucks in, pulling in pollutants and odors from damp, dirty and smelly places such as wall cavities, basements and attics. The contaminants are sucked into the interior through penetrations like outlets and lighting fixtures, holes for electrical and plumbing, and penetrations under baseboards.

Formaldehyde in new materials

Formaldehyde may be present in certain materials in your HVAC system, and/or in fiberglass insulation in walls, attic and crawl spaces.

  • For example, if you have any metal ducting, it may be lined with "duct liner," which is simply exposed fiberglass insulation that contains formaldehyde.Duct liner is also a source of fiberglass particulates that are classified as carcinogenic by the EPA.
  • Some HVAC systems may also include duct board. Duct board is similar to particleboard, is used to construct ducts, and contains formaldehyde.
  • You might be reacting to duct board or duct liner when the HVAC's air stream blows over those materials.

Or, if the HVAC company added new fiberglass insulation to soffits, the attic or crawl space, or did not completely air-seal new plastic "flex" ducting (which also contains fiberglass insulation between the layers of plastic), you may be experiencing reactions, particularly if the fiberglass is not formaldehyde-free.

Our team has found that in some conditions, formaldehyde can take years to drop down to safer levels; however, it never completely dissipates enough from duct board. High humidity would increases levels of formaldehyde from sources.

Negative pressure can also move chemicals from one area of the house to another, such as from new interior materials and finishes.

Finding pollutant pathways

If there have been any changes to your HVAC system besides just the newly applied duct mastic, then I suggest you call an environmental inspector with extensive building science training and experience, or a BPI-trained consultant with several years of experience to conduct a check for pollutant pathways caused by duct leakage and pressures with both pressure-testing equipment and equipment called a "duct blaster."

The combination of these two testing procedures will uncover the pollutant pathways so they can be eliminated with the appropriate solutions, determined by the testing professional.

If you've ruled out other sources

If you have ruled out other possible sources of odors or pollutant pathways,
then unfortunately, there is no easy solution for the dust mastic outgassing problem.

This is because:

  1. Return and supply air flows over the mastic. Duct mastic is designed to seal up holes so there is less cooling and heating loss. So it's impossible to prevent the return and supply air from flowing over the mastic when you turn on your HVAC.
  2. Mastic may be sandwiched between layers of duct material. Therefore, it will take much longer to completely outgas, because it is shielded from air that causes it to dry and cure quickly. This also makes it impossible to get to all of it, without taking apart the ducting or other materials.
  3. Barriers don't stick readily to duct mastic. Otherwise, you could cover over the mastic with less-toxic foil tape. But you'd still not be able to get to it all, even if there were a practical way to seal over it.
  4. Outgassing takes time, and longer in humid environments.The duct mastic in your ducting and behind the vents is low-VOC, and the manufacturer says it takes about a week to outgas. Given #2 above, it can take much longer for the VOCs, including methanol, to completely dissipate. If the mastic was applied too thick, this also increases outgassing time. I don't believe this product contains formaldehyde.
  5. Some chemically sensitive people cannot tolerate latex and biocides: The duct mastic contains latex co-polymers, which some chemically sensitive people can't tolerate, even after outgassing. It also contains biocides, which prevent the product from getting moldy. However, some chemically sensitive people are extremely sensitive to biocides. Biocides last for the life of the product and do not have an odor.

Speeding up the mastic's curing process

When the outdoor heat and humidity are tolerable, try running the HVAC on fan mode only with the windows open. Simultaneously run the kitchen and bathroom exhaust fans, which will exhaust odors and pull more fresh air from windows (as long as they fully exhaust only to the outdoors).

With fresh air coming in from the outdoors, you may be able to tolerate the mastic, and running the HVAC on fan mode will speed up the curing of the mastic.

Or, install a window air-conditioning unit

If you continue to react to the mastic after one or two months, or if you cannot bring in fresh air from windows or exhaust fans because it's too hot and humid outdoors, I suggest that you install a window air-conditioning unit, and when you're home, keep your HVAC turned off.

Then, when you're away from home, run your HVAC on fan mode and keep your kitchen and bathroom fans running. (If it's not a security problem, keep windows open too.)

The more you use the HVAC and dilute with fresh air, the faster the chemicals will dissipate. When you return home, immediately turn off the HVAC, open up windows and turn on exhaust fans in bathrooms and kitchen to eliminate chemical odors as quickly as possible. (If you are chemically sensitive, put on a respirator specifically for chemicals before you enter the house to air it out.)

Fresh air dilution if the best solution for the long term

Ongoing fresh air dilution, whether it's hot and humid or not, may be the best solution for the long term, too.

Given your hot/humid region, you might consider adding fresh air dilution mechanically. If you have your house pressure-tested and duct-blasted, ask about installing an ERV (energy recovery ventilator), an energy-efficient device that attaches to your HVAC and brings in fresh air while exhausting stale air.

You may need to dehumidify as well. The more fresh air you add, the more oxygen and healthier indoor air quality (without sacrificing comfort or using a lot of energy). Dilution lowers the levels of toxic chemicals from all sources, including from the duct mastic. Be sure that your entire HVAC system, including all the return and supply ducting, is airtight as well.

A duct sealant we've had good success with

For future HVAC air sealing, DP 1010 is a product that does not contain latex, biocides, or long-lasting VOCs, but it does contain ethylene glycol.

The HVAC specialist on our team has had great success using DP 1010, including for some chemically sensitive people who are not severely sensitive.

Caveat about all duct mastics and chemical sensitivity

You may be sensitized to duct mastic chemicals, given your recent exposure. If this is the case, make sure you take extra precautions if you introduce new duct mastic into your HVAC system/ducting.

No matter how nontoxic a product may be, it will always outgas until all the chemicals completely dissipate, and all duct mastics contain chemicals.

It may be best to test yourself on a fully cured and outgassed sample of the product first. Some chemically sensitive people cannot tolerate any duct mastic or plastic "flex" ducting, and must instead use nontoxic versions of foil duct tape and metal ducting. If you do tolerate a specific duct mastic, and particularly if you are chemically sensitive, it might be best if all future work on your HVAC takes place when you can be away from the home for at least a few days, and the weather permits you to open windows for several weeks.

Important note about air sealing and foam sealants/insulation

Never use foam sealants or foam insulation to seal up holes or insulate. Sealants and insulation manufactured with any amount of polyurethane foam contain toxic organophosphate flame-retardants that pollute the interior environment for the life of the product, and they are odorless.

Our team finds it important to specify HVAC materials that are completely flame-retardant-free, and free of many other chemicals commonly found in HVAC materials. We often find that even highly energy-efficient systems are not built with all nontoxic materials.