Sheet metal trunk sitting on attic joists


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Old 03-17-16, 07:28 AM
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Sheet metal trunk sitting on attic joists

In the process of preparing for re-insulating my attic, I figured Id start sealing up and insulating the ductwork too.

We have a rectangular sheet metal trunk running in the attic with 8 flex takeoffs, one for each supply register in the second floor ceiling.

I hadnt realized it until yesterday, but the trunk is just sitting on the joists....it makes it a bit easier to seal and insulate, but I also wanted to remove some areas of plywood (which the duct is also sitting on) so I could make it easier to blow insulation into.

Is it OK to leave the trunk just sitting on the joists? Would the duct wrap insulation provide enough barrier to prevent noise and vibration from transferring through the joists and into the rooms below?

Thanks in advance.

-Chris
 
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Old 03-17-16, 07:49 AM
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Would the duct wrap insulation provide enough barrier to prevent noise and vibration from transferring through the joists and into the rooms below?
Isn't that what you have now ? Removing the flooring from under the duct isn't going to change the noise or vibration any.
 
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Old 03-17-16, 09:08 AM
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I would hang the trunk line from the rafters above using metal strapping. This would separate/isolate the duct from touching the ceiling joists and you could raise it up allowing more room for insulation underneath.
 
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Old 03-17-16, 10:26 AM
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Thanks...I thought about trying to suspend it but the way they built the trunk, there is a seam inside the duct chase (aka finished walls in our bedroom) that I would need to disconnect so I could add a straight section to allow for the trunk to be lifted higher.

There is a bit of room to raise it now (maybe an inch or so)...I suppose I could hang it that high but it seems senseless to do all that work and only gain an inch.

I was wondering (hoping) if there would be something that could be put underneath the duct as an isolator, if noise and vibration turn out to be a potential problem.
 
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Old 03-17-16, 10:34 AM
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I don't read anywhere that you're having a noise or vibration problem so I don't see one coming just by removing some plywood.
 
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Old 03-17-16, 11:09 AM
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They make an inexpensive fiber type board called Homasote. That would work well under the duct. It's available at just about any home improvement center by the sheet.

440 SoundBarrierŽ is a special-density, structural board made from 100 percent environmental HomasoteŽ cellulose fiber, a homogeneous composition manufactured with uniformly distributed protection against termites, rot and fungi and resistance to moisture. Homasote is ideal for sound proofing.
 
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Old 03-17-16, 01:10 PM
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Thanks for the Homasote recommendation. Seems that I can get it locally too!
 
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Old 03-25-16, 06:26 AM
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Just wanted to update everyone - thanks for the advice. After installing most of the duct wrap, its clear we shouldnt be having any vibration/noise issues with the ductwork. The insulation itself helps pad everything, and where I removed the plywood that was holding the trunk up (parallel to the joists), I added some metal strapping to prevent the duct from sagging.

All is well so far!
 
 

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