Acceptable material for caulking ductwork?

pjb999January 4, 2009

I want to caulk around some poor sheetmetal work where the previous owner (inexpertly) installed a humidifier, what I've seen some use is like mastic and I do have the foil tape which could be another option, but I am wondering why I couldn't use general purpose silicone - it's not flammable and the offgassing stops when it's cured. I do like to stay within code, so I'd like to know if it's ok or not.


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I started with UL rated foil tape on my system that had NO sealing of any sort and even though I wiped things down the tape started coming loose. I then went to Home Depot as I do not know where I can get the 'good mastic' and purchased 3 gallons over by the hvac vents and accessories and will probably use most of it. I am sealing my returns and everything so that takes quite a bit more. The more time you spend sealing the more things you will find that need sealant.

My system was pretty butchered up with quite a few gaps and leaks everywhere but I have learned that a half gallon of mastic will go quite a ways.

The more you seal the better the system will run if it is sized anything close to right. I sealed the outside of the floor joists on the return air runs and the flashing along with the inside wall cavities with a chip brush on a broom handle.

I would get some mastic.. a few assorted sizes of cheap chip brushes.. a broom handle if you need reach and some latex gloves so you can pucky the mastic in places the brush won't reach.

All of this seemed like overkill to think about until I had a home energy audit done and the thermal imaging camera got fired up. Small leaks here and there add up to one huge leak.

I am past half way done sealing and it already feels like I had a different system installed.

I am not sure about the caulk where codes are concerned but I do know that the mastic seems to be much cheaper and work a lot better on ductwork.

    Bookmark   January 4, 2009 at 11:41PM
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Any sealant used in HVAC ductwork must carry a label & have one of the following testing procedures;

NFPA 255
UL 723

Rating shall be 25/50

a flame spread less than 25
smoke developed less than 50

    Bookmark   January 5, 2009 at 10:49AM
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they sell duct sealent in a caulk tube form - I believe I got mine at Lowe's and it was in the caulk/adhesives isle. The stuff goes on like caulk but dries to a hard rubber like consistency. I had to use it to plug up electrical boxes that had broken plastic tabs that hold wire in place where it enters the box.

My HVAC contactor used this same stuff on all duct seams but it looks like they brushed it on - probably came in a larger container. The stuff works great and should last forever.

    Bookmark   January 5, 2009 at 1:18PM
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Thanks - yes, on some of the building sites I've worked on, they've brushed the stuff on. Not pretty, but effective I guess.

    Bookmark   January 6, 2009 at 12:02AM
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I use childers 146-cp duct sealer it comes in a 1 gallon bucket. the smoke and flame spread ratting on childers is 0
there are different sealers for differeent things
mine does all,duct work,and hvac insulation piping
it goes on like paint, then hardens like plastic

    Bookmark   January 8, 2009 at 5:26PM
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Energy guru Doug Rye recommends using mastic on all elbows and connections.

    Bookmark   January 11, 2009 at 10:47AM
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After using the mastic I wish I would have never bothered with the tape. It does a much better job I can see why the energy guru's like it. Mastic plugs leaks not just covers or slows them.

    Bookmark   January 11, 2009 at 12:14PM
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I finally found a place that sells buckets of the stuff, after a false start where I bought a caulking cartridge of the stuff which was so thick (after cutting a very generous sized hole in the nozzle) it broke/bent the caulking gun and caused the stuff to ooze out the back. I know why you hvac people use the bucket and brush method.

Seems to work well.

    Bookmark   January 13, 2009 at 12:12AM
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Circus Peanut

At -14ðF here in Maine this morning, I've decided to go this same route with the mastic for my ancient basement ductwork...

I can't find the tape anywhere - OK to use the mastic alone? Or over the silver foil tape that's already on the ducts?

How thick should the layer be, say over elbow joints? Painted on thinner like paint, or 1/8" thick or so? (I'm presuming the latter.)

    Bookmark   January 16, 2009 at 12:06PM
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I don't think there's any harm in thicker, and it's not too expensive. Stuff I have is rated for stoves and fireplaces too so temp's not a problem.

I presume the tape's a little more flexible so you may want a mixture. I don't know if painting over the tape is ideal....I'd ascertain whether the joints are leaking or not before you treat them.

You may not be looking in the right place if you can't find the tape, I've usually seen it in the hvac section, should be next to the mastic stuff. Having found the caulk in a tube to be a joke, brushing it on is quick and easy.

    Bookmark   January 16, 2009 at 12:44PM
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