0 Helpful?

Should Mastic Be Applied inside Ducts?

We hired a contractor to clean our crawl and seal our ductwork. The company used Hardcast 321 mastic. We are concerned becasue they smeared into the floor vents from above and all around the cold air return. I have attached a couple of pictures. Should mastic be applied inside the ducts like this, is it harmful/toxic?

Thank you for any info/advice.

DSC_0257.JPG671.1 KB
DSC_0252.JPG591.23 KB
Asked by K. L
Posted Oct 28, 2012 11:34 PM ET
Edited Oct 28, 2012 11:39 PM ET


2 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

The label says it's nontoxic water soluble and suitable for residential use. My company uses mastic on the exterior during new construction.

Answered by aj builder, Upstate NY Zone 6a
Posted Oct 29, 2012 7:36 AM ET


It seems to me that you are asking two questions:

1. It is OK to install mastic on the interior of a duct system?

2. Is there any reason to believe that dried mastic can release toxic chemicals into the air circulating through a duct system?

The answer to #1 is that, although most installers put the mastic on the outside of the duct, there is always a little bit of mastic that can squeeze through holes and gaps in the duct that is being repaired. Since it is routine for any holes in a duct to be repaired with mastic, many duct systems have at least some dried mastic in contact with the air stream. So that part is fairly normal.

The answer to #2, as far as I know, is that dried mastic is non-toxic. I have never heard any evidence or anecdotes of problems arising from mastic odors or mastic chemicals.

Answered by Martin Holladay, GBA Advisor
Posted Oct 29, 2012 8:14 AM ET
Edited Oct 29, 2012 8:15 AM ET.

Other Questions in General questions

Previous code for unvented cathederal ceiling insulation

In Building Code Questions | Asked by Steve Gabrielidi | Nov 25, 15

I am a paid member that cant access product guides?

In GBA Pro help | Asked by Greg Philliips | Nov 26, 15

Solar array positioning in Charleston, SC

In Green building techniques | Asked by F White | Nov 24, 15

How to separate house from garage

In General questions | Asked by Eric Chandler | Nov 26, 15

Atypical cathedral insulation

In General questions | Asked by Matt | Nov 25, 15
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!