# Help with a 2nd Floor Home Theater



## daustin (Oct 11, 2010)

Hello all, 

I am planning to build a home theater on the second floor of my home, the only available empty room which happens to be above the master bedroom. Space is an issue here as the room is contained within an A frame roof peaking at 12 feet above the finished floor. 

The room measures 22'6" by 15'8". There is only one communicating wall the other walls are exterior or knee walls to the roof slope. I was planning on using the clips on the walls and ceilings with 2layers of wallboard on the theater room side. The current rafters will have to be extended 3-4" to accommodate the required insulation for my area which will make the room a bit smaller. In theory the finished room will be 22'3" by 12'10"(making the knee walls 55" high instead of 38" as they are).


With this room being above the master bedroom sound proofing the floor as much as possible is a must, currently there are 2x10 joist and 3/4" subflooring. If at all possible I would rather not remove the subflooring if possible, I was thinking of using the floor joist isolators by cutting the existing subfloor around the joist and placing heavy carpet padding in the 'void's' between isolators then using two perpendicular layers of 3/4" subfloor glued together resting on the isolators. I also want to build the new knee walls on the floating floor instead of the current subfloor to help reduce noise. 

Any thought’s or suggestions are greatly appreciated.


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## Ted White (May 4, 2009)

Perhaps laminate massive ply to the floor with damping compound.


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## bpape (Sep 14, 2006)

Unfortunately, you'll want to pull the subfloor for a few different reasons.

First, you'll definitely need to insulate the cavities between the new room and the bedroom below.

Second, if you build another floor above the existing subfloor, you're going to be creating a triple leaf situation which can actually make sound transmission worse (the leaves would be the ceiling of the BR, the existing subfloor, and the new subfloor.)

You can use isolators if you want but I'd do it do lay another layer of joists perpendicular to the existing ones and do your new double subfloor on top of that. This will decouple the structure some as well as making the single air cavity deeper to lower the resonant frequency of the cavity.

Bryan


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## Ted White (May 4, 2009)

I agree you don't want the triple leaf. http://www.soundproofingcompany.com/library/articles/triple_leaf_effect/ Here's an article that explains why.

So unless you are willing to remove the existing subfloor, really the only option is to make the floor much more massive and damped. 

The floor would _certainly _be better if decoupled, however.


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## daustin (Oct 11, 2010)

I would love to lay another set of joist, however being 6'5" even using 2x4's as a joist with a double layer floor (approximately 7" of new flooring) the ceiling would be at 7'4" for only 4' wide then drop at 0.85" per 1" until the knee wall. Ceiling height is also a problem too; the room is being built in a triangle that is 12' high 23' wide. 

I can remove the floor, however being that this originally was a storage area the floor should  already have insulation in it; I will check that later today. 

If I do remove the flooring would the joist isolator's work with the double layer floor or would I still need another perpendicular set of joist?


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## Ted White (May 4, 2009)

If you remove the subfloor, you can use rubber isolators and install a double ply floor (damped) on top of the isolators


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## bpape (Sep 14, 2006)

You'll just need a LOT of them if there's no full length joist to tie the subfloor to. In fact, you'll not be able to fasten the subfloor to anything. If you screw down through the isolator to hold it down, you're just reconnecting everything structurally.

Bryan


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## Ted White (May 4, 2009)

bpape said:


> In fact, you'll not be able to fasten the subfloor to anything. If you screw down through the isolator to hold it down, you're just reconnecting everything structurally.
> 
> Bryan


Exactly. This is why I suggested double ply (damped) to float on the isolators. Screwing the two sheets together as shown here:


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## Ted White (May 4, 2009)

Depending on the load, you may move the rubber isolators to 16" or even 12" spacing. You would plan on double 3/4" subflooring. Overlap (stagger) the seams.


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## bpape (Sep 14, 2006)

Yup. That was my point exactly. You'll need to get the supporting points closer together or the floor will be very live and potentially even sag in places if you don't. 

Bryan


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## Ted White (May 4, 2009)

I am all too familiar with sags where I don't want them... oh wait...


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## daustin (Oct 11, 2010)

Wow you guys are fast, I really appreciate all the input. 

I think that I will remove the existing subfloor and use the joist isolators with a double subfloor. Next question would it be better to build the knee walls on the new floor or the existing floor and cut out the subflooring for just the area of the room, leaving the current floor for the spaces behind the knee walls.


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## Ted White (May 4, 2009)

I'd place the new walls on the new floor. This is less of an issue when the walls are thoroughly damped, but you may as well


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## bpape (Sep 14, 2006)

Given the situation, I'd likely do the double subfloor on the whole thing and build the wall on top of it. I would recommend having a small gap between them (not much, maybe 1/8") between the subfloor where the wall sits on it and the area behind the walls that's not in the room. That will at least help a little with preventing that section from vibrating in sympathy with the floor that's in the room. 

Bryan


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## Ted White (May 4, 2009)

bpape said:


> Given the situation, I'd likely do the double subfloor on the whole thing and build the wall on top of it. I would recommend having a small gap between them (not much, maybe 1/8") between the subfloor where the wall sits on it and the area behind the walls that's not in the room. That will at least help a little with preventing that section from vibrating in sympathy with the floor that's in the room.
> 
> Bryan


Personally I'm not a big fan of the gaps. In theory it has merit, but from a practical standpoint it's tough to implement. Given that all surfaces are massive and damped and all subjected to the exact same energy, I don't see the value in engineering in the gaps. I'd rather not reply on a sealant to close that gap, I'd rather have the mass of the drywall in contact with the subfloor.

Personally, that is.


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## bpape (Sep 14, 2006)

On the front (room side), yes. The gap I was referring to was just something like a blade kerf on the back side of the wall, outside the room. That area of floor is not subject to the same airborne pressure waves as the inside. 

It's not a lot but it is certainly easy enough to do.

Bryan


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## daustin (Oct 11, 2010)

Are there anythought's on bracing the joist, since removing the subfloor will allow the joist to deflect since the floor will be floating at this point.


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## Ted White (May 4, 2009)

You can and should brace away...


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## Ted White (May 4, 2009)

bpape said:


> The gap I was referring to was just something like a blade kerf on the back side of the wall, outside the room. That area of floor is not subject to the same airborne pressure waves as the inside.
> 
> Bryan


I see what you're saying. Good idea


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## daustin (Oct 11, 2010)

Update: The boss, my wife, has shot down the removal of the subflooring in the room. So are there anyother ideas with out the floor joist isolator's? I was thinking possibly 2 layers of 2 or 4 mm rubber underlayment glued to eachother and the new subfloor with green glue. It is still the plan to build the new knee walls ontop of the new floor.


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