Expanding foam or some better idea

Hello all I have hit a snag in my design for sound proofing my garage. On the inside is me and on one side is my mother-inlaws bedroom. I will be doing a room in a room so I will be stripping the drywall from the inside of the garage before building a second wall on the inside. Her bedroom on the side closest to me has a built in wardrobe with shelves so I cannot add extra sheets of drywall to her side of the wall. Q: How do I make the leaf that is created by the layer that is her bedroom wall better at blocking sound from my side? A1: Do I add expanding foam to the back of her wall from my side? A2: Do I cut drywall into small rectangles and glue them to the back of her drywall from my side in between the wooden frame? A3: Do I ask nicely for better ideas? Thanks in advance for any help/ideas you can give.

I am not an expert but I do read many of the posts on this forum and I would say option 2 for a start but I am sure one of the mods will post for you to be 100%

A1: Do I add expanding foam to the back of her wall from my side?

The common type of polyurethane "foam in a can" is not much use acoustically, since it is closed cell (there are some types of polyurethane that are open). Good for sealing out drafts and thermal insulation, not very useful at all for sound: it has no real mass, and extreme gas flow resistance.

A2: Do I cut drywall into small rectangles and glue them to the back of her drywall from my side in between the wooden frame?

Roguejackal is right! This is your option But "Yes and no". Yes, cut strips of drywall to fit in between the studs from the back, but no do NOT glue them! And not "small rectangles" either, but large strips that cover the entire surface. Cut each piece to be just a fraction smaller than the space between the studs (a couple of mm gap on each side), hold them in place with cleats, and caulk around the edges for an air-tight seal. Depending on how much isolation you need (based on your numbers that you measured when you did the initial calculations) you might need to add two layers of 5/8" (16 mm) drywall like this. And of course, you do need to seal the rest of that wall, all around, absolutely air-tight. Even a tiny crack will cost you isolation.

A3: Do I ask nicely for better ideas?

That's what this place is all about! :) - Stuart -

Fizbain wrote:

Hello all A2: Do I cut drywall into small rectangles and glue them to the back of her drywall from my side in between the wooden frame?

I wouldn't make an issue if they were glued in place. The testing that was done on glued/laminated panels was a comparison of a full panel, not cut up little pieces. And one thing that can be proven is that nothing stays the same, every time a dimension changes, the ability to interpret it in the new context changes. The smaller pieces are rigid, little room to flex like a full panel might do, so the gluing might actually make a better piece of mass versus little pieces that can vibrate/rattle.