Million dollar question: I have studded walls (2x4, wood, 12"O.C.) and another (2x3, wood, 16" O.C.) two inches apart. My plan is to insulate the 2x4 wall with Roxul Safe n' Sound and the 2x3 with R8. I would really like to insulate both with the Roxul but it is 3" wide and my 2x3's are only 2 1/2". The question is am I better to leave the 2" gap between and insulate as the original plan, or to double the Roxul and therefore minimize my gap between the two walls? Second question is I am building a de-coupled ceiling as well. I have to span approximately 11'. I want to use 2x4's to allow another layer of Roxul between the existing joists. Do you think 2x4's are enough to support the wait of two layers of 5/8"? I will be tying them all together to try and prevent warping and they will be supported by the 2x3 wall on either side. By adding the extra layer of Roxul I will be only about 2" from my new ceiling joists. Am I better to leave the existing 5" of airspace between the old and new joists or add more 'mass' and therefore decrease the amount of airspace?

Any thoughts?

No need to double the mineral fiber. As a general rule, money on double stud walls is better spent on weight (mor drywall) than more absorption. R13 or R19 will get you the majority of the results taht insulation can give you in this type of wall, and keeping R13 (thinner insulation) but adding more weight will give a superior result than low weight and more insulation.

When utilizing a decoupled wall (and other walls, but doubly so on decoupled walls as weight both adds the benefits of weight (making the wall harder for sound to shake) and lowers the resonance point) weight should be a priority.



These rules will never steer anybody wrong when building decoupled walls

1. use as much weight on both sides of the wall as possible
2. use as deep of an air cavity as you can spare
3. use insulation, but what kind and thickness are less critical. just use at least R13 or R19 as they are cheap, and there isn't reason to believe that denser types, all-in-all, make better theater walls

Of these #1 and #3 are the most important, but spend money on weight before insulation once you have at least some insulation.

No need to double the mineral fiber. As a general rule, money on double stud walls is better spent on weight (mor drywall) than more absorption. R13 or R19 will get you the majority of the results taht insulation can give you in this type of wall, and keeping R13 (thinner insulation) but adding more weight will give a superior result than low weight and more insulation.

When utilizing a decoupled wall (and other walls, but doubly so on decoupled walls as weight both adds the benefits of weight (making the wall harder for sound to shake) and lowers the resonance point) weight should be a priority.



These rules will never steer anybody wrong when building decoupled walls

1. use as much weight on both sides of the wall as possible
2. use as deep of an air cavity as you can spare
3. use insulation, but what kind and thickness are less critical. just use at least R13 or R19 as they are cheap, and there isn't reason to believe that denser types, all-in-all, make better theater walls

Of these #1 and #3 are the most important, but spend money on weight before insulation once you have at least some insulation.

This is a long dead thread but it almost answers all my questions, so I'll not start a new one -- posting will rekindle this, I hope!

From the response above, if I reduce the air gap between my double stud walls from 2 inches to one inch, but increase the drywall in the room from one layer to two layers, have I achieved better isolation?

This is a long dead thread but it almost answers all my questions, so I'll not start a new one -- posting will rekindle this, I hope!

From the response above, if I reduce the air gap between my double stud walls from 2 inches to one inch, but increase the drywall in the room from one layer to two layers, have I achieved better isolation?

Yes!

2. use as deep of an air cavity as you can spare
Brian,

Why would depth of the air cavity matter at all, providing that there is at least no contact (e.g. at least 1/2") between the walls? Obviously 1" vs 24" would be significant, but in practicality I can't understand why 3" would be better than 1".

Sound drops off with the inverse of the square of the distance...

..so if your ear was the outside of the inner wall, the sound would drop in half (2" vs 1"). But since it's in the middle of the room, it might drop 2%..

..so if your ear was the outside of the inner wall, the sound would drop in half (2" vs 1"). But since it's in the middle of the room, it might drop 2%..Not exactly... You're trying to stop sound that's transmitted from the drywall into the air in the wall and back into the drywall from coming out the other side on the wall. By separating the drywall more you're making it much harder for the sound to travel through the air in the wall to the drywall on the other side.