WALL QUESTION - new room over garage

[egcc]

I'm building an addition, a new room over my garage. It's intended use will be recording space, but also a home theater room when the house is sold sometime in the future. I want to contain sound leakage. I'm trying to decide the most effective way to build the walls. My thoughs are 2x4 staggered studs on a common 2x12 base, each side 4 inches of insulation, with a 4 inch air gap in the middle. The interior walls would be flex channel with layered drywall, the exterior will be one layer of sheeting and vinyl siding.

Questions - would building two completely separate walls, not sharing a common base, make a significant difference ? Also, rockwool or mineral wool instead of fiberglass insulation ?

By the way, the floor is 12 inch joists, will have 4 inch insulation, drywall on bottom (garage ceiling) and 5/8 sheeting on top (new floor). The roof is custom built with 2x8 rafters, 4 or 6 inches of insulation, 5/8 sheeting and shingles on top, flex channel with layered drywall for the new ceiling.

I intend to build gobos and maybe some semi-potrtable frames for large hangers if necessary. Not sure exactly what room treatment will be necessary yet. The room dimensions with 1 foot thick walls are about 18' wide, 22' long and the ceiling height would average 9' or so (8' by walls, follows rafter contour up then flat section across middle of room about 10').

Any comments or suggestions welcome.

[knightfly]

I'm working on some general drawings of what goes where for best isolation, should be ready in a couple of days - I'll post them here when finished.

Your basic plan is a good one, and yes separate stud frames will improve on Transmission Loss, as long as EVERYTHING else is done right. Generally, you want (ideally) two completely independent high-mass envelopes around the total area, separated by as much air space as possible, with as much mass in each as possible, the air space having several inches of 3 lb/cu ft Rockwool or mineral wool in it.

That being said, it's almost impossible to achieve ALL that, but the closer you get the better the result.

More when I finish the drawings... Steve

[egcc]

Well, the plan is now basically "room within a room" - except for the floor and ceiling...once again overall goal being sound containment...

Walls - two separate walls around the entire room. The exterior is 2x6 framing, with 6" insulation. There will be a small airgap between the exterior and interior walls. The interior wall will be 2x4 framing, 3" rockwool insulation, and resilient channel with 3 layers (drywall/soundboard/drywall).

The roof is 2x8 rafters, similar to cathedral ceiling shaped inside.

Question - The interior sidewall plate at the top would be mitered to fit the slope of the rafters. Should the interior sidewalls be permanently fixed to the rafters or not? How should they be attached, how do you anchor the interior sidewalls at the top to keep the wall vertical? While we're at it, how would they be anchored at the bottom on the floor as well?

[knightfly]

Here is one type of sway brace sold by kineticsnoise.com - check out their site by deleting the part of the link after .com -

http://www.kineticsnoise.com/kwsb.html

These can be used for upper wall support of inner walls - if your local code doesn't mandate it, I would try to NOT connect the frame between the ceiling and the inner wall plate, nor would I go to the effort of mitering the top plate. If you have compound miters at the top of the wall due to non-parallel walls, you can temporarily mount a sheet of drywall high enough up on the frame so that one end of it touches the ceiling, then measure the distance to the other end of the sheet from the ceiling, cut a diagonal line on the new sheet so that the higher end of the new sheet has the same gap as the other end, then use a rasp to fine tune the edge for better fit. Caulk like crazy. If that explanation sucked as bad as I think it did, let me know and I'll try again with a drawing.

If the commercial sway brackets are too expensive (not sure what they cost) you could make something similar of you're handy with tools. The main thing you want is no HARD connection from inner to outer wall.

If you bed the sole plates of the inner walls with 1/8" or 1/4" neoprene and use acoustic sealant on both sides of the neoprene, the weight of the wall should keep anything from shifting. The fewer fasteners you put thru the plate, the less sound can get through. One possible way to handle this would be to use the sway brackets top and bottom - they could be installed by putting the sheet rock on AFTER the wall frames are tipped up. You would need diagonal braces temporarily nailed to each wall frame until the first sheet of wall board is fastened, in order to keep them square. Then, once you have some wallboard on the studs the braces could be removed. Having sway braces top and bottom on each side of the inner room would keep anything from shifting, even if gravity weren't enough.

One way to build your own sway bracing would be to build a pocket of some sort on the outer wall frame, large enough for a 2x4 to slip in with 1/2" gap all around - then put 1/2" neoprene in the BOTTOM of the pocket, cut a second piece of 1/2" neoprene that will surround the 2x4, push the 2x4 firmly into the rubber-surrounded pocket, and nail the other end to the inner wall frame. If you did this on all sides, the frame could go nowhere but still would be isolated.

It sounds like your floor may end up being the weakest link - if the sheet rock isn't already on the underside of jousts, putting Resilient Channel on the joists first would help quite a bit. Hopefully, the lower air cavity (called a garage) would help dissipate what leaks thru that way.

I wasn't really clear as to what your total wall materials will be - I'm hoping those outer walls have no inner paneling, so you can keep to the 2-leaf wall ideal.

Hope that answered a few things for you... Steve

[sharward]

The link above doesn't work anymore, but this one does.