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Discussion Starter · #1 ·

I live in Texas, and my A/C units are in an unconditioned attic. There are two units. One has five 8" ducts and the other has three 10" ducts and one 12" duct. Almost all of the ducts are metal ducts with a few short sections of flex duct. There are no return air ducts, air goes directly from the filter to the return air plenum to the unit.

When I was finishing up my radiant barrier project, I noticed that I felt some leaks coming out of some of the ducts, near the plenum. There was mastic glommed all over the ducts so I stripped it off. The ducts are attached to the plenum with sheet metal screws (good), but there are some gaps and the gaps were sealed with duct tape. (very bad). The duct tape degraded over time and air ended up leaking.

For these leaks I strip off the mastic and duct tape, put down fresh foil tape. Then I have been using "duct sealant", which is grey stuff the consistency of paste. Should I be using mastic over the metal tape?

Also, there are some gaps between the plenum and duct and also between the plenum and the A/C unit. Some of the gaps can be a quarter inch or so. How should this kind of gap be sealed up? Are foil tape and sealant sufficient? Should I use some kind of caulk? Or perhaps a strip of aluminum to bridge the gaps?

A lot of the seams in the ducts are sealed with duct tape. Many of these seams look perfectly fine. Should I strip off the duct tape on the seams that look okay and replace with foil tape and sealant?

Which seams, if any, is it absolutely necessary to use mastic on?

TL;DR What's best practice for sealing ducts?


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2 Posts
my ac was poorly installed when it was replaced after being hit by lightning.
the flexible interface had a 1" for 8 years. it probably wasted over 8000$ worth of energy.
finally i decided to try to seal it..
first i tried silicone.. that tore out and flaked off...
it was hard to remove. globbed off...
bad idea.
then i got brave and decided to try to develop something with urethane foam in a can.
i sealed the lower return duct with it. its a horrible mess. it gets in y our hair and on your skin and just stays there for weeks. it will peal the skin off.
you should not attempt this unless you are extremal experienced.. and do not value your skin much.
i noticed it ruined my cloths the first time . so i used disposable socks and shoes and rags as cloths the next time... i think i wore them to a party and told the story.. i may have been proud.. my bad.
anyway despite the colossal mess.
the first time i used "urethane foam in a can" i was blessed with immediate success. the "urethane foam in a can" globed and fell off and expanded .
it becomes non workable in a few seconds..
but i was still able to make it stay in the gaps and cracks.
i drew 3 or 4 beads and then kind of tried to hold it up in the vertical cracks as it expanded to the point where it is so light that it stops falling off in globs.
but now. I've tried in 3 times and
i seem to have developed a procedure that works great and can even work in places that you cant reach!!!
i tried making a troff of aluminum duct tape just below the gap
this gap ran along the back of the unit and the upstairs return duct was less than 2 inches from the unit and it was almost impossible to work it..
eventually i used a 22" section of "refrigerator ice maker hookup line" which is 1/4" plastic tubing.
i used it as an extension on the canned foam spout tube. i heated the ice maker plastic tube till it became almost transparent then forced it on the end of the canned foam tube .. i should have taped it in place but it stayed on miraculously.. after i was done. i tried to clear the nozzle to save what was left of the foam and i could not get the plastic extension tube to stay on..
oh yea. i had to straiten the plastic extension tube by holding it over the kitchen burner.. again. just plane dangerous without experience.. but i have been doing stuff like that for afew decades and i don't cars if i get a few 3rd degree burns. but those suckers make you wish you hadn't tried it... i didn't get burned this time. but you need experience or you will learn how hot that gets.

i ended up grabbing the cardboard tube off the garage floor in desperation and shoving it into the tight space just below the gap in the duct work.. then i fed the extension to t he far end and just filled it up with a nice squiggly bead of foam..
all that with the ac set to 100 degrees so it wouldn't go on..
and it worked perfectly..
i will never use acrylic caulk or silicone again if i think theres a chance urethane foam will work.
p.s. i used the cheaper non expanding foam.. which seems to expand about 27 times its initial volume.. maybe less that sounds crazy.. thats what i calculated.. its about 6 times the hight of when it comes out of the tube.. maybe i should say its 3'ish cause it expands so fast at first then real slow.

i would like the mention the name of the installers because they never admitted their incompetence. they left the gas line so it would have burned the place down, they broke the watter heater and didn't say anything. they even somehow clogged the condensation line and the gap in the duct work was insane.. plus they flooded the house with toxic gas t hat made my eyes burn for days. they were trying to use fluorocarbons to blow hard silver solder out of the my unsuccessful repair of the existing system. i could have fixed it but they came along and someone authorized them to "fix it".. they later needlessly removed the entire system unable to repair the leak and replace the compressor. why? do you think maybe they were hopping to steel the copper? i cant say because i kept the old machine. the old machine was old but it had afew parameters that i feel made it better.
but to summarize :
i used a cardboard cylinder. like the cardboard tube from a 2 or 3 foot long spindle of Christmas wrapping pappier.
i think the car board tube i used had some kind of plastic film on it at one time.

i think next time i will try cutting a 3 foot cardboard tube in half lengthwise then cup it over the crack or gap, then tape it in place firmly not sparing any tape..
then just fill it with a liberal amount of foam

i hope i don't cast aspersions on the good guys cause they made it work for over 35 years before that.
but the creeps wanted 7000 for their job.

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I am having some of same issues as Cakemeister. I bought 1st home last year and trying to learn to be a DIY'er. Last night my sons and I were installing a radiant barrier in attic and I noticed cold air coming out around the plenum. I traced down the duct pipes and more cold air leaking at some connections. The duct pipes are all covered with insulation wrap. The plenum has foil tape at the edges but it has come loose. Should I stick some duct seal in there and cover with foil tape or what? Also, I dug the loose fill insulation out around one of the air ducts where it goes thru the ceiling and the insulation and ceiling was cold and some air leaking there. Should the duct pipe be sealed to ceiling sheetrock from the attic side or just loose fill insulation around it? Should the plenum have insulation around it?

Thanks for your help.

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14 Posts
My old system which ran the whole house is like this. I am currently installing a 2 ton unit for my first floor and blocking things off to my second floor unit.

I too felt cold air coming from mine while in the attic installing attic foil. I also found out that the large square sheet rock space running down the center of my house had all of the ducts uninsulated. This wouldnt be much of a problem other than the space was open on the bottom into the crawl space. If the ducts werent there you could access my 2nd froor, crawl, and attic by climbing through the space without cutting or breaking anything.

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when i have a place where tape adhesive has failed and its just flopping loose not sticking or sealing the gap, ,,,

(i first try to determine that it hasn't been stretched and simply been pulled apart by receding foundation from the unit sitting on the floor and the duct suspended from the ceiling rafters. )

if it wasn't pulled apart or it it might be not then i remove any part of the failed tape that i can

if some old tape is permanently stuck , i just work with it in place

before i apply new tape, i try to prepare the surface for the adhesive by tarring or cutting a fresh piece of the tape from the roll i intend to use.

then i wad it up into a ball with the adhesive side out so its nice and sticky

then meticulously dob the tape-ball around the place i intend to stick the tape to.

if it seems like its sticking pretty good then i know i have a pretty good chance of getting it to last a little while

i have to be careful because if its a plastic membrane, it can adhere permanently and make a new problem(tare holes in something or bend sheet-metal etc..)

i may never use tape again if the urethane foam in a can hold up for and long period of time like a year.. i expect it to last 25 minimum

i didn't mention the tactics i used with the tape.. i assumed that everyone knows. but there you go.

the first time i tried tape on a duct plenum, i cleaned the surface with a suitable cleaning solvent for the conditions,(then rinse it and dry it)

after cleaning, i just stick the tape on there and assumed it was designed to adhere to most hvac surfaces.

but now, i always use what i call "the sticky tape ball" system..

also. if your foundation is receding, it can pull tape off of junctions where the unit in sitting on the floor and the plenum is suspended from the cealing... so you have to put as much excess as you can somehow.. then if it does pull off.. you might want to try keeping that tape on and try to scrunch it up into an accordion type expansion flap...
but metal tape can fatigue from vibrations so i wouldn't put much faith in that..
. but then who knows why it really failed.. it can't hurt t o do that if it seals the leaks.. i would just keep monitoring it..

i would try to develop a system to use foam forms with round ducts,, but this house is so old that i just have custom sheet metal duct work.

maybe someone will be inspired and try to develop some system's to make foam work with round ducts. 2 half circles taped together.. or a cardboard of plastic troff coil..

we'll see..
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