Best Way to Seal Ducts?

[cakins]

I just had a new HVAC system put in a 1956 house that I'm totally redoing and upgrading. The old ducts are 75% in a basement and about 25% in a crawlspace. They are sheet metal except for a few feet of the new runs in the crawlspace which were put in with 6" flex, duct-taped and banded to the boots and takeoffs.

I asked the contractor about sealing everything tight to which he replied that it's actually good to let things leak a little in the crawlspace for heat in the winter so pipes don't freeze. I know that's not right, and I know the efficiency of the whole system depends on it being sealed, so I thought I'd give the mastic approach a try.

Is that the best method to seal sheet metal ducts? What about where the flex meets the boots? The sheet metal is uninsulated in a basement which I will probably start heating and cooling also by cutting in a register.

I went to an HVAC supply store, asked for mastic, and literally got this response: "What's mastic?" After I explained their business to them, :-), I found some on the shelf. It's directions spoke of using a fiberglass mesh tape to do it right. Of course they didn't carry that.

Is there anything else I can use for the fiberglass tape? Do I really need it?

Any direction and help would be greatly appreciated.

[mattison]

Duct sealer is what you want not mastic. You can use this on the sheet metal and where the flex meets the boot.

[cakins]

Please explain - for those of us not in the industry :-)

Duct sealer is ... what? Can I get it at my home improvement warehouse? Or is it a specialty item? Is it tape, goop, etc?

[Ed Imeduc]

Try this www.
http://www.mcgillairseal.com/textDocs/products/uniMastic.htm

and here
http://www.hardcast.com/reference/texts/SECTION%2015820_2003.doc

There are so many Id say Idont know where to start.
Hardcast has 3 to start with for ducts then there is Rectorseal duct sealant.

ED :D

[cakins]

Yes - Rectorseal is what I saw at the HVAC supply - but that IS mastic, right? Or a type of, at least.

So, to seal where the flex meets the boot would I still use the tape and band first, then just spread the duct seal on top of that?

[Ed Imeduc]

Code here on flex pipe pull the inside out and over the take off band it there. Then pull the insulation part down and over it all Band it then use mastic to seal the V/B cover and insulation to the duct.

ED ;)

[mattison]

Sorry about my vagueness. I don't like plain mastic since it's so thick but if you just go to a hvac store and ask for duct sealer you'll get a thinner type and it spreads like peanut butter. I'm not sure if the box stores sell it or not.

If you choose to use tape don't use the old grey duct tape. Use aluminum tape. It will outlast the old grey tape by years.

[hvac01453]

this term is correct. It is usually a water base white colored paste, and is applied with a chip brush. If you have holes or cracks so large as to need self sticking fiberglass tape, you can get that at Home Depot in the sheet rock section. The mastic works great an not messy. Duct sealant I hear lots of guys use laugh when they hear someones using it because it gets all over everthing and a mess to clean off. I've heard some call it buffalo snot. Ive also seen it in chaulking gun tubes at the supply house. The bands are fine, you shouldn't need to mastic them as well. Its usually where sheetmetal leaks on joints to help maximise the static pressure.

[cakins]

Thanks for the input everyone - it's helpful so far.

One question - What is a chip brush?

[scottg]

your HVAC suppliers should also have a mastic tape sold in 2" and 3" sizes. I use the 3" it is a gum back tape that has a plastic backing that you peel off and then stick to the duct you can also use it for your flex duct connections.

Proper way for flex connections is to pull back outer sleeve and fiberglass. attach vinyl flex to pipe connector then wrap with a U.L. tape (U.L.181-BFX) then attach band clamp over tape joint then slide insulation back up tight to 90 degree metal joint then slide outer sleeve up and secure with a duct tape do not put clamp band on outer section this crushes insulation and causes duct to sweat during cooling mode.