How do I dampen air ducts? And some sealant questions

Hey guys, so how do I dampen the air ducts? I will be definitely putting either standard pink insulation or that foil duct wrap stuff around my air feed and return ducts as well as dry walling around them but Im not sure if that is enough. Is that enough to stop the ducts from vibrating and resonating? Unfortunately they are coupled to the ceiling and are going to have to stay that way so I need them to not clang and make noise when someone is walking above or banging on a wall. At home depot I found a rubber product that can be bought in bulk that is intended to be used for shower liners. I was thinking of wrapping this stuff around the outside of the ducts and fastening it with some kind of adhesive.. Good idea? If so, what kind of adhesive would work best? A good choice other than expensive green glue. On that note, is there an adhesive that works as well or nearly as well as green glue when gluing two sheets of drywall together for greater mass for better isolation? Also on those notes, what is a good sealer/ caulk for just sealing up all of the gaps and joints in walk/ceiling construction for best isolation?

Hey guys, so how do I dampen the air ducts?

You put duct liner inside them.

Is that enough to stop the ducts from vibrating and resonating?

Probably not. Duct liner will help with resonance, and to a certain extent with vibration, but if vibration is the problem then you should probably treat that at the source: decouple your ducts from whatever is causing them to vibrate: If it is the air handler, then you can use canvas "boots" or "sleeves" to accomplish that: If it is the structure that they are hung from, then you can use isolation hangers to decouple them from that.

Unfortunately they are coupled to the ceiling and are going to have to stay that way so I need them to not clang and make noise when someone is walking above or banging on a wall.

Why can't you decouple them? What's wrong with using ordinary isolated hangers? In simple terms, if you cannot decouple them from the noise source, then your chances of solving the problem in other ways are somewhere between "slim" and "none". :)

At home depot I found a rubber product that can be bought in bulk that is intended to be used for shower liners. I was thinking of wrapping this stuff around the outside of the ducts and fastening it with some kind of adhesive.. Good idea?

Bad idea. If you don't know the acoustic properties of that product, and it is not specifically designed for either HVAC work or acoustic work, then it would be a bad idea. Unless, of course, you like wasting money on things that probably won't work, and also like being your very own one-man acoustic test laboratory! :)

If so, what kind of adhesive would work best? A good choice other than expensive green glue.

Well, Green Glue is not an adhesive anyway, so anything at all would work better than Green Glue if you want an adhesive!

On that note, is there an adhesive that works as well or nearly as well as green glue when gluing two sheets of drywall together for greater mass for better isolation?

Once again, Green Glue is NOT an adhesive, and it is NOT used to "glue to sheets of drywall together". It also does not increase mass, and neither does it isolate, per sé. It seems that you have misunderstood what Green Glue is, and what it does. Green Glue is a visco-elastic polymer, which is a fancy way of saying that it is gooey, rubbery, flexible, and never sets hard. It always remains rubbery and flexible, even when fully cured. It's purpose is to act as constrained layer damping in the wall, which is a fancy way of saying that it damps certain types of resonance in the wall itself. It does that partly by absorbing energy in the sheer plane, as the two sheets of drywall try to slide past each other as they vibrate. That's all that Green Glue does. And it does it very, very well indeed! But it is not an adhesive, and cannot be used to glue sheets of drywall together. Nor can it be used to glue shower rubber to HVAC ducts!

Also on those notes, what is a good sealer/ caulk for just sealing up all of the gaps and joints in walk/ceiling construction for best isolation?

The Green Glue company also makes another product, which is an acoustic sealant that is designed specifically for the purposes you mention. There are other companies that also make acoustic sealant that works the same. Your second-best option: you can also use any good quality bathroom caulk that remains flexible when it has set and does not shrink. But overall, on your main question, you really do need to decouple the ducts properly with isolation hangers, and you also need to put proper duct liner inside them. - Stuart -

Thanks Stuart. I see some duct liner products. Do you have a good link or recommendation? Any special characteristics to look out for? I saw one site with like 6 different duct liner products. On decoupling the ducts: I can. And I will do something, but I can't do those isolation hangers as it looks like they are going to drop the ceiling by a couple of inches. It's already low. So I am thinking about backing out the nails on the braces that are keeping the ductwork up and re nailing it in with some rubber inbetween. I know I would have to make sure the duct work isn't touching anywhere else either, or if it is, to slide some rubber in between those locations as well. I know that it will not work as good as being completley suspended but I believe I will get a substantial improvement. The rubber shower liner idea that I had has been done previously by my contractor who is very well respected, so I trust his opinion when he says that it is safe to do that on HVACs. So how could rubber being against the surface of the duct work do anything but help to dampen its vibration? I dont need to test it to know that..I am just wondering at this point if it would be necessary after installing duct liner on the INSIDE like you are suggesting.

I see some duct liner products. Do you have a good link or recommendation?

It depends on what you want to do, and what types of problems you have. But probably a simple 1" faced liner that is designed for acoustic properties (not thermal properties) will do. Owens Corning makes stuff like that. So do other manufacturers. The key point is that you are looking for the acoustic properties, not the thermal properties.

but I can't do those isolation hangers as it looks like they are going to drop the ceiling by a couple of inches.

How are your ducts hung now? It should not be necessary to lose much height at all.

So I am thinking about backing out the nails on the braces that are keeping the ductwork up and re nailing it in with some rubber inbetween.

Unfortunately, that wont work: the nails are still a hard connection, and therefore a flanking path. Isolation means removing ALL flanking paths, replacing all hard connections with resilient connections. Sound will simply travel along the nails, instead of through the wood. It will bypass the rubber.

I know that it will not work as good as being completley suspended but I believe I will get a substantial improvement.

Actually, if there is even one single nail still connecting, then the improvement will be minimal to zero. Sound is like water: it takes the easiest path. If there is even one single hole in a water pipe in your house, then all the rest of the pipe doesn't matter at all, since the water will come out of that hole. Same with your isolation: even one tiny little hard connection (such as a single nail) provides way more than plenty mechanical connection for all of the rest of your work to be practically useless.

The rubber shower liner idea that I had has been done previously by my contractor who is very well respected,

Great! So he has done laboratory tests, and can provide you with the data, right? And his method and materials are fully approved by your local and national building codes, ASHRAE, fire code, municipality, building inspectors, and your insurance company? Are you SURE about that? Can he show you the relevant documentation to prove that this is an accepted and proven method? I always get concerned when I hear anecdotal evidence about "wonderful" things that people have done to save money, and swear blind about. It doesn't matter how well respected your contractor is: what matters is if the method and materials are approved for use like that, and have been tested for use like that, and shown to work usefully when used like that.

so I trust his opinion when he says that it is safe to do that on HVACs.

Sprry if seem a little harsh on this point, but I would not trust his "opinion", unless he can proved the documentation to back it up. If there is nothing to go on except his own personal experience and his "reputation", then forget it. It would be a bad idea.

So how could rubber being against the surface of the duct work do anything but help to dampen its vibration?

Well, let's see now: it could add too much weight and kink the metal, or it could damage the ductwork due to some type of chemical interaction, or it could be too light to damp the vibrations, or it could have the wrong resilient characteristics, or it could do the exact opposite of what you hoped and actually AMPLIFY the vibrations, if it doesn't happen to be the right type of rubber, or it could change over time, hardening and curling away from the surface, or it could do any number of other things.

I dont need to test it to know that..

Well, sorry to disagree with that, but I would NEVER use ANY product in building construction that had not been specifically tested and approved for that application, just because I thought it might work, and thought it didn't need testing. That's kind of along the lines of "Here, hold my beer and watch this...". OK, so that's a bit of an exaggeration, but you get the idea! There are proper products that do what you want to do, and they have been tested, shown to work, and are approved. So why on earth would you use something else? It would be sad indeed if, for example, that stuff turned out to be a fire hazard when used in plenums, and your insurance company then refused to pay out when your house burned down... It's never a good idea to take shortcuts in building.

I am just wondering at this point if it would be necessary after installing duct liner on the INSIDE like you are suggesting.

They accomplish two different things. The duct liner will kill resonance inside the duct, and reduce air movement noise, but it won't do anything to prevent external impact noise from traveling through the metal and causing it to vibrate. There are several issues here. There's the noise that the duct-work could pick up from the building structure, then amplify or transfer elsewhere. There's the noise that the air handler can transfer into the duct-work. There's the noise that the fans produce: There's the noise that the air itself produces as it move through the duct. There's the noise that gets into the duct from the registers in the rooms. There's the resonance of the duct cavity itself. And probably a few more that I'm forgetting. That's why you need the multi-pronged approach, of isolating the duct from the air handler, isolating the duct from the building structure, lining the duct inside, and possibly also damping the metal itself from the outside. - Stuart -