I have insulation in a decoupled room (see pic 1)
My question is I want to put another layer of insulation (roxul acoustical fire batt 3"). The problem I have is when I tap the joist after the new layer is in (see Pic 2), I can feel the vibration on the original joist. When I take the insulation out I do not feel the vibartions. Will this just kill the whole "decouplization" of the room?

I am not sure if more is good here; any input would be greatly appreciated.

Thanks

Tom

Also I want the extra insulation but not if the sounds will vibrate from the new joist to the old joists.

I have insulation in a decoupled room (see pic 1)

I am not trying to upset anyone here, but I don't see how the wall is decoupled. I see it attached straight to the joists, hangers or nailed, it is still attached. If your just concerned with the vibrations I would turn a radio up pretty loud and see if you can hear it elsewhere in the home. Scott

After looking again i think i see.......the hangers are joined to the rafters of the theater itself, and the others are the floor joists. I would stick with the radio test.

Actually I tried something that I think answered my question. I attached a Bass Shaker to the new stud (the one that IS attached to the home Theater room) and did not feel any vibrations between the joists. Then I snugged some Roxul insulation between them and BAM it was vibarting at almost the same rate as the one attached to the Bass shaker. So unless anyone has any ideas i am going to leave well enough alone and pray for the best.

Thanks
Tom

I am guessing that the roxul insulation is fairly rigid? Do you have some fiberglass, unfaced, that you could test to see if you have the same results?

The fire batts are actually pretty rigid. I remember some time ago I tried an experiment. I had some scraps of the 4 inch thick fire batts and I wondered if they would fit in a 2x4 wall (3x1/2 inches) So I took a couple of 2x4s and layed on two sides of a 2x2ft scrap of the fire batt on the floor. I placed a piece of plywood on the fire batt and then stood on the plywood. OK, so my 210 lbs could not get the plywood to bottom on the 2x4s. I was really surprised that it wouldn't compress. Your experiment with the bass shaker doesn't surprise me.

I'd take out the fire batts and go with the standard fluffy pink fiberglass.

Edit: I was testing the Safe batts not the AFB batts

The fire batts are actually pretty rigid. I remember some time ago I tried an experiment. I had some scraps of the 4 inch thick fire batts and I wondered if they would fit in a 2x4 wall (3x1/2 inches) So I took a couple of 2x4s and layed on two sides of a 2x2ft scrap of the fire batt on the floor. I placed a piece of plywood on the fire batt and then stood on the plywood. OK, so my 210 lbs could not get the plywood to bottom on the 2x4s. I was really surprised that it wouldn't compress. Your experiment with the bass shaker doesn't surprise me.

I'd take out the fire batts and go with the standard fluffy pink fiberglass.

This is completely opposite my experience after the room has been finished.

My decoupled ceiling w/ Roxul in it significantly dampens sound transmission.

Steve:

A couple of thoughts.

1) In Tom's situation he has a double ceiling joist decoupled construction. The mechanical vibration appears to transferred from one joist to the other by the Roxul. Maybe he is compressing it between the joists. (I looked again at your pics and i didn't see how you layed it up in the ceiling)

2) There are a number of Roxul products.

The safe and sound batts have a density of 2.5 lbs per sqft
The AFB batts that Tom (and you) used has 2.8
What I was using and what I tested was the Safe batts which have a density of 4.5 lbs per sqft.

I just looked at the tech spec for the Safe batts and they show a compression rating of 10% at 144 lbs per sqtf.

So If I was standing on a 4 sq ft of 4 inch safe it would have taken more than 576 lbs to get 4 inches into a 3 1/2 inch space.

I am guessing that the roxul insulation is fairly rigid? Do you have some fiberglass, unfaced, that you could test to see if you have the same results?

in addition to generally better low-freq performance, boring old low-density fiberglass has the advantage of being floppy enough to allow contact without coupling, while (as a couple of folks mentioned above) more rigid insulations can couple.

i'd recommend boring old fluffy insulation.

Steve:

A couple of thoughts.

1) In Tom's situation he has a double ceiling joist decoupled construction. The mechanical vibration appears to transferred from one joist to the other by the Roxul. Maybe he is compressing it between the joists. (I looked again at your pics and i didn't see how you layed it up in the ceiling)

2) There are a number of Roxul products.

The safe and sound batts have a density of 2.5 lbs per sqft
The AFB batts that Tom (and you) used has 2.8
What I was using and what I tested was the Safe batts which have a density of 4.5 lbs per sqft.

I just looked at the tech spec for the Safe batts and they show a compression rating of 10% at 144 lbs per sqtf.

So If I was standing on a 4 sq ft of 4 inch safe it would have taken more than 576 lbs to get 4 inches into a 3 1/2 inch space.

Mayhaps. All I can say is that the stuff I have has contact with both my floor joists and sperate celiing joists in the same cavity, and the blanket assertion that it will transmit sound energy hasn't held true. Perhaps your points above help explain it.

Here is what I hope to be a better picture. The current Roxul insulation does not touch the new Joists anywhere. My problem is that I want to add a second layer of Roxul (This set was free) between the new and old joists. That is when I feel vibration. I am going to try and staple/nail/glue...some roxul to just the old joist so there is at least something up there.

I guess my question is do I need the extra Layer? I have about 6 inches of "air space" above where the Drywall will be and the insulation that is currently there. Is this "good" airspace or bad? Will the sound leak out the sides and down the wall (the theater walls are 1.5 inches off of the Brick Masonry basement walls. Will this matter. I should mention it is not that I am anal but I do live in a Townhouse with a neighbor on each side!

Thanks

another Idea I have is to take the extra insulation and attach it to the ceiling covered with fabric (like a big room treatment. Is this a better idea? No Vibration!