Drywall Joint Compound Versus Sound Proofing Sealant

Hey guys, long time lurker first time poster. Background on my project: I’m building a jam room (acoustic drums, guitar, bass vocals), in a free standing 1 car garage, which measures 12 feet wide x 19 feet long x 10 feet hight. I have two layers of 5/8” drywall with Green Glue in between them on all walls and the ceiling. The garage has a concrete slab floor. The garage doors are old school carriage style doors, made out of solid pine (barely any sound isolation here). I built a new wall a few feet behind/inside of the front garage doors, with staggered studs: 6 inch baseplate, 2x4 inch studs at 24 inches on center. My drum kit will likely be in front of this staggered stud wall, facing away from it. The guitar/bass amps will be on the opposite side of the room pointed towards the staggered stud wall. Problem: my drywall guy filled the 1/4 inch floor gap (between drywall and concrete slab) of the staggered stud wall with drywall joint compound (mud) before I had a chance to apply Green Glue sealant to that gap. He did this on both sides of the wall’s floor gap (inside facing the room, and outside facing the garage doors). It looks like he did a pretty good job of filling the gap in. There is a 2x6 inch wood baseplate behind this gap. I religiously sealed up all other gaps in the room with Green Glue sealant before he applied mud. I told him not to fill this 1 gap in with mud, but I think our language barrier got in the way of that instruction. I’ve attached a photo of this mudded gap. He is coming by tomorrow to sand the mud. Goal: I’m hoping to achieve a level of sound isolation so that our rock band can play moderately loud in the evenings without annoying the neighbors (nearest neighbor is about 30 feet away). Question: since that 1 floor gap of the staggered stud wall is filled with mud and not sound proofing sealant, am I totally screwed in terms of sound getting out? What kind of dB reduction does mud have?

Hi there "bigBurrito", and Welcome! :)

What kind of dB reduction does mud have?

The issue isn't so much how much mass the mud has, but how will that "seal" will hold up over time. Mud is similar to drywall, in terms of density, so I wouldn't worry about that. What I would worry about, is if that seal will crack. If you end up with several cracks under your wall, that can cause a surprisingly large reduction in isolation. If the cracks create an air path under wall, you can potentially lose some isolation. Basically, if air can get through, then so can sound. You do have the option of scraping out that mud, at least partially, in order to get a decent groove under the edge of the sole plate, then seal that with caulk, as per the rest of the wall. So to summarize, as long as the mud holds out, you don't have a problem. If cracks develop, then you do, potentially. - Stuart -

Wow, thanks for the lightning fast reply Soundman! I guess at this point I'll move forward as-is, and if cracks develop and sound leakage becomes an issue, I'll cut out a little groove on that bottom area and fill with sealant. By the way- any idea on the best way to cut/route a 1/4" gap in that bottom area? Seems tricky since it's at the very bottom of the wall- a saw probably won't be able to get down there.

I've never heard of mudding the bottom floor gap on drywall. But, if you put trim over that gap like most people do where drywall joins the floor and then caulk that you will have zero problems. You would probably have zero problems even if you didn't caulk the trim. It would be easer than scraping out the mud but more expensive. The trim doesn't have to be fancy You can always just use painted 1x4's like me.

Thanks macmanmatty. I agree- not sure why he put mud in the floor gap, must have been the language barrier (he knew I wanted everything sealed up tight, but he must not have understood me when I said I would be filling that gap with sealant). I'll add moulding trim like you mentioned, and I'll be sure to add sealant to the edges of the molding. I really appreciate your guys' feedback on this. My stomach dropped when I saw this issue, so near the end of my project. But looks like it will turn out ok after all. Thank you!