I recently had a new, detached workshop built - 15'x20', 2x6 @ 16" OC wall studs, 2x10 @ 16" OC ceililng joists, exterior walls are Hardiboard lap siding with 1/2" OSB sheathing. I have insulated the walls with R23 rock wool batts and the ceiling with R30 rock wool batts. My goal is to reduce, as much as is reasonable, sound transmission to the outside, particularly to my neighbors as the shop forms part of the property boundary. There are 2 small, double pane (and different thickness) windows in the building, and they do not face my neighbors.
My original plan was to use a 1/2" sound deadening fiberboard like Soundstop against the studs and then 5/8" drywall on top of that...in anticipation of this, my numerous receptacles, fixtures, subpanel box, heaters, door jambs, etc., are all installed to 1 1/8" clearance from the studs.
But I've read a lot recently that fiberboard is not really all that effective at reducing sound transmission, and that I'd be better off just using 2 layers of drywall. And the more I read, the more I see a bunch of conflicting opinions. Some say mass is the way to go, and that "mass is mass", so just add a 2nd layer of drywall. Others say you need "dissimilar materials" and/or "dead space" to absorb and reflect sound, and that fiberboard is the way to go. Or some other product sheet product, such as foam or rubber-based sheeting.
I have ruled out stuff like QuietRock and mass loaded vinyl for cost reasons. Also ruled out resilient steel channels because it looks like a PITA to install. I also do not want to move all the boxes and fixtures, so I'd like to keep the overall wall thickness at 1 1/8". And obviously, it's too late to do staggered studs, or "room within a room", etc.
So, I assume my choices are:
1) Original plan: 1/2" fiberboard + 5/8" drywall
2) 1/2" drywall + 5/8" drywall (and if this is the way to go, does it matter which layer is against the studs and which faces the shop?
3) 1/2" of one of those other rubber or foam-based sheets + 5/8" drywall
Any of these options jump out as hands down the best of the group?
I'm also looking at using an acoustical sealant such as Green Glue, but have heard that they require you remove all the mechanical fasteners once the glue dries...and no mechanical fasteners = code violation.
Thank you, and this caller will take his answer off the air.