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Sound insulation for open-joist ceiling?

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Forum topic by ADHDan posted 309 days ago 1813 views 0 times favorited 23 replies Add to Favorites Watch
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ADHDan

420 posts in 704 days


309 days ago

Topic tags/keywords: question

My shop is a 11’x17’ lower-level room in a split-entry house, directly under the kitchen. I finished the interior walls with two layers of drywall and Roxul insulation, which has provided good sound dampening in the adjacent downstairs rooms.

Now, I’d like to try to do some soundproofing to muffle the noise upstairs but I have two issues. First, there’s a HVAC vent duct running the length of my shop along the ceiling. Second, the shop ceiling is unfinished (exposed joists) and for various reasons I really don’t want to drywall it.

So, I was thinking of friction-fitting Roxul between the joists (without drywalling the ceiling) to provide at least some sound barrier, while still allowing access to all of the plumbing, HVAC, and other lines in the ceiling (and to allow me to continue using the exposed joists for hanging storage). Likewise, I was thinking of wrapping the metal HVAC duct with Roxul, roughing in some hanging supports to hold the insulation in place.

My questions are:

(1) Is there any better/more efficient way to soundproof the ceiling without drywalling everything (and for roughly the same cost as the Roxul)?

(2) Does Roxul make a batt with a surface cover that would be code compliant for an exposed-joist ceiling? I spoke with a contractor friend who said that if I don’t drywall the ceiling, I should use batts that have some sort of fire-resistant surface on one side so that they don’t immediately turn into a tinderbox in a fire situation.

(3) Is there any better way to soundproof the metal duct than covering it with Roxul?

Thanks much!

-- Dan in Minneapolis, woodworking since 11/11.


23 replies so far

View crank49's profile

crank49

3336 posts in 1567 days


#1 posted 309 days ago

I have installed a suspended ceiling in an industrial office situation then placed 6” thick mineral wool (Roxul is a brand name) batts on top of the drop in tiles. Worked quite well, but was done for looks as much as anything.
Still could get above the ceiling to service pipes, wires and such, but you would loose your “between joist” storage.
Obviously would be cheaper to tuck the mineral wool into the joist cavities. They make a rigid board version of this product. It’s used as furnace thermal insulation, but still has the damping characteristic. I don’t think there would be a fire hazard with either of the mineral wool products. Like I said, it is used in furnaces. I’m talking heat treating furnaces that operate at 2000 degrees.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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Charlie

1001 posts in 882 days


#2 posted 309 days ago

Roxul doesn’t burn. Foil and paper faced batts flame up pretty quick.

You can test this yourself. Take about a 2 foot piece of roxul and suspend it between saw horses. Put some ice cubes on the top (in the center) and get your propane torch and try to melt them from underneath the roxul.

#1, the Roxul won’t burn

#2 you’ll be there a long time trying to melt those ice cubes.

Now try this same thing with paper (or foil) faced fiberglass. Have a hose or fire extinguisher handy and don’t do it inside. :)

The fiberglass isn’t the problem. The facing is the problem.

The metal duct is going to be an issue. Nothing I know of can be wrapped tight to the duct and be effective unless you start spending a LOT of money.

View ADHDan's profile

ADHDan

420 posts in 704 days


#3 posted 309 days ago

That’s very good to know about the Roxul. Sounds like I can just sandwich it into the cavities for a cheap and easy fix. Of course, if ANYONE has ideas for insulating a HVAC duct I’m still all ears.

Thanks for the replies!

-- Dan in Minneapolis, woodworking since 11/11.

View joeyinsouthaustin's profile

joeyinsouthaustin

1205 posts in 668 days


#4 posted 309 days ago

The HVAC duct is a hard one. Air and Isolation work well. Use knock down HVAN vent to double around it for an additional 15 to 20 STC. Or line with mass loaded vinly for 35 STC Just devise something to keep those solutions isolated from each other.

-- Who is John Galt?

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ADHDan

420 posts in 704 days


#5 posted 153 days ago

Sorry to dredge this up from last year, but I’m revisiting the issue and have a few follow up questions:

How much work is involved in stalling a knock-down HVAC vent around the HVAC line running through the shop, and does anyone have a link to the vinyl recommended above (and suggestions for how to install it)?

Alternatively, could I achieve decent results by roughing in some 2×4s from the joists to create a frame that would let me friction-fit Roxul around the duct? That seems like it would be easy to do.

Thanks!

-- Dan in Minneapolis, woodworking since 11/11.

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joeyinsouthaustin

1205 posts in 668 days


#6 posted 153 days ago

If you are talking about the mass loaded vinyl, just google mass loaded vinyl. I have bought from acoustic solutions several times in the past. No recent experiences (last 5 years) The vinyl installs with any wide headed fastener with capability to hold a lot of weight, or with cleat over top with fastener through.

Depending on the opening the vent can be easy to install. It is sold in sheets with the silver foil on one side, yellow fiberglass insulation pressed into a lightweight board, about 2” thick, and usually black on the inside. HVAC pros use it to build air returns and their plenums. When I said “knock down” some places sell it in a kit version pre cut to make up square ducting or plenums. By using this you could build a second duct around the existing one. Slightly more reflective of higher frequencies than just adding insulation around the duct. IM experience mineral wool, or a cotton insulation will block more sound than fiberglass.

Here is a place to get started. I didn’t price the min wool but a 5’ by 84” roll of MLV starts around $80. You would likely only need to do the bay with the duct 3 sides. This stuff is as dense as lead and can be tricky to install overhead. Maybe pics of the area in question would help get a lead on the easiest method?

-- Who is John Galt?

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joeyinsouthaustin

1205 posts in 668 days


#7 posted 153 days ago

Sorry dan, I meant to include a link as well. Start here

-- Who is John Galt?

View Bob Current 's profile

Bob Current

300 posts in 213 days


#8 posted 153 days ago

I’m struggling with a visual of your situation and the type of insulation that is called Roxul. I am familiar with “rockwool” and standard fiberglass products. We insulate duct work 3 ways, liner that is glued and pinned to the inside of the duct work (somebody above described it. It runs from 1/2” to maybe 1” or more. Not good in a retrofit and IMO not a good choice for indoor air quality.
The other 2 are foil coated fiberglass wrap, and rigid fiberglass foil coated call duct board (much more expensive)
A good installer can make this stuff look terrific.
In what I read you seem focused on the ductwork being a major contributor to your noise transfer.
If you have return or supply openings that share your space and others that is the first place on which you should focus.
The air serves as a conduit for sound and I understand it usually travels in straight lines.
The fiberglass duct board mentioned above is no longer used in our area except for and occasional return air run.
I think someone mentioned creating a bulkhead out of removable ceiling tile and grid and those typically work well.
Maybe some manner of acoustical clouds or panels would help.
I also believe somebody mentioned air space between hard surfaces and insulation which is a valid point.
Sound has always been a source of complaints four our HVAC business so I feel your pain.
I’ll watch the replies to se how you do.
RMC

-- When you are wrong admit it, when you are right forget it.

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versa

29 posts in 1729 days


#9 posted 152 days ago

For the HVAC line, the best bet is to cut it open and replace a section with flexible hvac insulated pipe. That will break up the reverberation of the metal pipe and hinder new noise from getting into the line and absorb noise already in the line.
The second alternative is to coat the metal pipe with a mass loaded vinyl covered in aluminum foil typically used by the car audio industry called dynamat. Wrap it around the pipe and it will dampen any vibrations in the pipe.
from their website:
“Dynamat Xtreme is a very thin, light-weight, constrained-layer vibrational damper. Dynamat Xtreme has a self adhesive peel and stick backing with a high level of tack that conforms and fuses easily to sheet metal and any other hard substrate.”
There are cheaper alternatives from dynamat, just make sure it has the outer aluminum coating (it improves performance and makes it more fire retardant. even a 1 sq foot section would help, but the more of the pipe covered the better.

The third alternative, which is pretty poor, is cover it in rockwool or fiberglass. This won’t do much.

For the rest of the ceiling, rockwool 6 inches thick would be a good start, filling the entire cavity would be better. Then drywalling with risc sound isolation clips would REALLY help as well.

If you want the best I would do the HVAC splicing in ten feet of the flexible insulated pipe with 2 90 degree bends in it, use dynamat on the rest of the pipe, filling the joist cavities with rockwool, use risc clips to drywall the ceiling, and then use green glue to glue a second layer of drywall to the ceiling.

For additional references (geared toward home theater sound proofing) check the following sites:

http://www.avsforum.com/t/1509173/soundproofing-master-thread
http://www.greengluecompany.com/
http://www.acoustimac.com/soundproofing/

View ADHDan's profile

ADHDan

420 posts in 704 days


#10 posted 152 days ago

Unfortunately, any renovation of the HVAC duct itself is far beyond my time and ability at this point. I’m hoping there is some way to simply wrap/enclose it that will dampen sound. The duct line isn’t connected to the shop (there’s no vent or register); it just runs along the shop ceiling. My thought was to build a 2×4 frame around the duct that I could use to hold mineral wool (and then maybe cover with drywall).

Here are some photos of what I’m working with. The duct has actually been quite useful for the time being; I epoxy magnets on often-used tools and binder clips (for holding project plans) and use it to hold them, but obviously sound dampening is more important to me. For reference, my entire shop is only 10’ x 17’. The duct hangs below the joists running along the length of the shop for about 12’. I put a red box around it in the photos. (There is paneling on half of the shop ceiling, but I would rip it out to install the Roxul.)

-- Dan in Minneapolis, woodworking since 11/11.

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joeyinsouthaustin

1205 posts in 668 days


#11 posted 152 days ago

Boy, those pictures help… You do have a tough situation there from a sound proofing stand point. I like the advice about changing to insulated flex duct, and then boxing it out with framing, then rocking with a sound isolated s/r. The air space between framing and duct will do as much as stuffing with any insulation. Make sure the flex doesn’t touch the “soffit” That might be the best. You really do have a tuff situation there. Even GW is covering his ears. You may just want to buy bose noise cancelling ear or head phones for the folks up stairs :)

-- Who is John Galt?

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ADHDan

420 posts in 704 days


#12 posted 152 days ago

Yeesh. Unfortunately, I don’t think I’m going to be redoing the duct anytime soon; I’m handy enough but that seems like a lot to bite off considering (a) I have absolutely no HVAC experience and (b) I have a two year old kid and another on the way this summer (which limits my time and money).

Is insulated flex duct roughly the same size as what I’ve got now? If so, perhaps I could rough in a Roxul wrap and see how it works, but design it such that I could take it down and put it up relatively easily if I decide to replace the vent in the future.

On the plus side, so far I haven’t woken my daughter up even once from working after her bedtime, and as long as I’m not planing massive amounts of wood at 8 PM my wife has been pretty understanding. I think the nastiest noise-makers are the high-pitched screamers, like the planer and miter saw (the table saw and DC don’t seem to be too bad). On that note, would a Roxul wrap at least offer some dampening for those frequencies?

The walls are already well-insulated with Roxul and two sheets of drywall (one 5/8 and one 1/2), and we’ve noticed that made a difference. I’m not expecting full soundproofing, I’m just looking for a cost-effective way to dampen some of the noise bleeding through the ceiling and duct a bit better.

-- Dan in Minneapolis, woodworking since 11/11.

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joeyinsouthaustin

1205 posts in 668 days


#13 posted 152 days ago

The duct is really the problem. Hard surfaces conduct high freq noise well. Anything would help slightly. Sounds like your hands are full on this one. Duct with in a sealed box with the box no touching the duct IMO would be the fastest route to the most dampening for high freq noise.

-- Who is John Galt?

View ADHDan's profile

ADHDan

420 posts in 704 days


#14 posted 152 days ago

If I build a 2×4 frame around the duct, would I be better off covering it with drywall (or plywood) and leaving an airspace inside or covering it and putting Roxul inside (but not touching the vent)? Alternatively, if I don’t want to cover the frame (to make it easier to remove/reinstall) what about building it to hold the Roxul (again not touching the vent)? I could accept that the last option wouldn’t dampen as much as using a cover, but would it be that much worse – especially considering I’d be putting Roxul in between the joists but not drywalling the ceiling? Plus who doesn’t love the beautiful look of exposed mineral wool everywhere.

Thanks very much for your insight – I really appreciate it!

-- Dan in Minneapolis, woodworking since 11/11.

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joeyinsouthaustin

1205 posts in 668 days


#15 posted 152 days ago

2×4 frame, covered would be better than just the Roxul. Drywall better than ply, and it needs to be sealed. (taped and floated, or caulked. generally caulked better for sound. Try quietrock for a little extra umph. I have never used it, but there web site gives some ideas for quieter with out spending the money for “pure isolation”

-- Who is John Galt?

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