Duct mastic vs silicone

**jhcrowman** · 03-10-2005, 11:53 AM

I'm getting ready to assemble some ductwork. Advice from an HVAC relative is to use silicone caulk to seal takeoff collars, duct joints, etc. Everything I read here references mastic/mesh and/or UL181 tape.
What would best practice be? If mastic, is the stuff available at home centers good, or do I need to go to an hvac supply house?
thanks!

**amickracing** · 03-10-2005, 12:28 PM

Well, let me be the 1st to say "you should have called a pro". Be prepared to hear that about a dozen more times.

Ok, enough sarcasm... I'd go with mastic (aka pooky). Silicone is somewhat messy (at least when I put it on), some of it can really stink, and it's somewhat time consuming to put on.

Mastic gets smeared on with a cheap paint brush, doesn't really smell and cleans up with water.

About the only time I use caulking to seal duct is if it's going to be exposed in some fancy building (mostly commercial). If it's in a residential setup and exposed, I still pooky.

**ct2** · 03-10-2005, 08:05 PM

What kind of Duct is your company installing on this job?

The company I work for uses about 4 wraps of packageing tape
on flex

**41gasman** · 03-10-2005, 09:41 PM

CT2.Your kiding right.Just yanking are pee pee maybe.Just joshing around with us RIGHT.....Packageing tape? Are you talking the clear stuff or the brown???

jhcrowman. In regards to sealer applications.I use silicone on the furnace or air handler sets.For one and only reason.Aesthetics.Yes, Silicone can be messy.It will get every where if your not very carefull.But a clear silicone looks SOooo much nicer.alot more professional than a bunch of grey or white pasty looking stuff smeared all over.I did one time have(get this guys)a inspecter all most tag me for using silicone on the sheet metal.Why you say? Well in the galvinized sheet metals manufacturing companys specification sheet he had at the office.It said silicone will etch the galvinizing!!!!!. WELL we would'nt want that now.Well as it turned out he was a really nice guy.He was going to LET me slide just this one time.Anyway silicone on the furnace and air handlers and any exposed take offs.Mastic on all the ducting seams.Also the mastic you can buy at the big boxs of this world works just fine.
41GASMAN

**framehvac** · 03-10-2005, 09:46 PM

You should have called a pro right

I like silicone,mastic is very messy.

**duct dr** · 03-10-2005, 09:51 PM

On metal duct, silicone. On duct board, mastic.

**square2round** · 03-10-2005, 09:55 PM

The only time I've used silicone is to seal the gaps where copper goes into the A/H. I've never used it to seal duct...but that's just my experience.

[Edited by square2round on 03-10-2005 at 09:58 PM]

**framehvac** · 03-10-2005, 09:58 PM

Originally posted by square2round
I've never used silicone for sealing. That's just my experience.

So what do you use silicone,mastic or tape?

**oloenneker** · 03-10-2005, 09:58 PM

Originally posted by 41gasman
CT2.Your kiding right.Just yanking are pee pee maybe.Just joshing around with us RIGHT.....Packageing tape? Are you talking the clear stuff or the brown???

I think that he is talking about that UL 181 listed Mylar tape that you use for the flex connections. It looks and feels just like packing tape, but it is black and has the UL181 listing on it.

As far as Silicone is concerned, I HATE it when I have to changeout a furnace that has been siliconed....Bloody murder trying to scrape all that stuff out to get the unit free from the can....Arrgh..
I actually use foam weatherstripping for the furnace connections, and for my sheetmetal joints, they are S and Drive connections with a layer of foil backed mastic tape on the seams, including my round WA pipe stuff... That mastic tape is the best, no mess no fuss (except for that blue liner tape on the mastic you gotta pick after you make the joint...
They sell it at the box stores as well... Kinda expensive but well worth it.

[Edited by oloenneker on 03-10-2005 at 10:03 PM]

**mo-flo** · 03-10-2005, 09:59 PM

have used aluminum type stuff before made by alcoa,...comes in a tube and when it sets up..........good-luck trying to get it apart.Blends in nice with the metal too.As far as preference,.....i don't really have none one way or the other.Both work just fine......

**square2round** · 03-10-2005, 09:59 PM

Originally posted by framehvac

Originally posted by square2round
I've never used silicone for sealing. That's just my experience.

So what do you use silicone,mastic or tape?

I use tape and mastic. It may be overkill, but I feel better.

**framehvac** · 03-10-2005, 10:03 PM

Originally posted by square2round

Originally posted by framehvac

Originally posted by square2round
I've never used silicone for sealing. That's just my experience.

So what do you use silicone,mastic or tape?

I use tape and mastic. It may be overkill, but I feel better.

I use to do that to but then I tried silicone,I love it.

**jdmcjd** · 03-10-2005, 10:06 PM

Has anyone read Bryant's white paper on evaporator coil
failures ? one of the main culprits is acetic acid.
Go to the site and read up
thanx

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