How to avoid flanking in between leaves?

Hopefully I didn't screw things up already- I am about 2/3 of the way through the laborious task of "cladding" the subfloor in between the joists with two layers of 5/8" wallboard. Seeing as some of these pieces were up to 96x15 1/2" I used drywall screws through the wallboard into the subfloor to help hold the wallboard as I fastened the cleats. My logic here was that the walls would be similarly constructed (drywall screws through both layers eventually into studs). I then used the cleats with caulk to seal all edges. My question is this: Should I pull the screws out now that the cleats are in place, and caulk the holes? Or should I cover the heads of the screws with joint compound/caulk? Or, the more likely choice, am I wasting time worrying about something that won't matter? Thanks for easing my paranoia... -Josh

This would kind of depend on whether you want every last dB of isolation, or to get on with it and finish - it's true that metal fasteners tend to "short out" layers of mass on a leaf, so the absolute best way from isolation standpoint is to pull the fasteners, and replace them with drywall mud (fill the holes with it) - in actual practice, though, if you have a multi-layer m-a-m barrier (wall, ceiling, etc) with complete fill of insulation, and no cracks around perimeter, etc, the only time you cause serious flanking with fasteners is if both leaves are fastened to the same studs/joists WITHOUT any resilient mounts. The minute you either use separate studs/joists for opposing leaves, you break the path and fastener penetrations become so insignificant as to not be measurable. If you were to leave the insulation out of the structure, this would change; but then, the resonances from inadequate damping of panels would make you forget about the freakin' screws... Steve

THANKS! I know you didn't wrie the laws of physics, but thats a relief! This part of the project has been (hopefully the most) difficult- particularly because my joists are sistered in spots, and are varying widths, and there are nails penetrating through the subfloor, and, and, and, etc etc... I will be decoupling the leaves so looks like I'm okay. Originally you mentioned having the insulation contact the inner leaf for damping purposes- is it okay it won't contact the wallboard between the joists? Thanks for everything Steve, Josh

Actually, it would be better if you put enough UN-FACED insulation in your cavities so that it exerts light to medium pressure on BOTH inner surfaces (meaning both leaves of your construction - gravity solves the lower contact in most cases, but the upper will improve isolation, especially footfall noise, if you can keep the insulation in contact with the panels between the joists. If you already have cross-bracing between joists instead of solid blocking, stuffing some extra insulation (not TOO tight) up over the X braces can help damp these upper surfaces; best is a complete, slightly compressed insulation fill. For ceilings, this would mean using insulation (the normal, household, fluffy fiberglass kind) that's intended for 15-20% deeper cavities than you actually HAVE - too much, and you get some loss of decoupling; too little, and it will sag over time and you'll LOSE contact with the upper leaf (your subfloor, where the extra gypsum/cleats are going) For ceiling wallboard, don't forget to leave a 1/4" gap around the perimeter for acoustic rated caulk - you don't want hard contact between ceiling and walls, even if everything is on RC or RSIC clips... Steve

best is a complete, slightly compressed insulation fill. For ceilings, this would mean using insulation (the normal, household, fluffy fiberglass kind) that's intended for 15-20% deeper cavities than you actually HAVE

So I should use the fluffy pink stuff instead of the Roxul AFB that I was planning on using between the studs and in between the joists on top of the inner leaf cieling? Or are you saying AFB with fluffy pink on top for damping purposes? Or have I been misunderstanding the insulation issue from the start and is the 2.5-3 pcf insulation for clouds/basstraps/bb absorbers only? Thanks for the clairification, Josh

No, you're right - 2.5 pcf stuff is optimum for almost ANY place in a studio where you would use insulation, either in or outside of a wall or ceiling. If that's what you have, and you can get it to "come out even", as in, complete fill with slight overfill (only enough overfill to require slight pressure against the first layer enclosing the insulation, in order for the wallboard to contact the frame) - then, you're set. If, however, another layer of the Roxul would cause moderate to heavy compression of the insulation, then you will need to get some UN-faced, normal density, house insulation to make up the difference if you want the best isolation you can get with your particular combination of joist depth, layers, etc. - If you're NOT concerned with max isolation for some reason, you can just put the batts into the joist cavities and install the ceiling layers - gravity will damp the lower panels, the extra wallboard layers between joists will lower the resonant frequency of the upper leaf, and it will cost less for insulation. The main loss of doing it the second way, will be increased footfall noise from movement above you, although it would also lessen overall TL through the structure a slight amount as well, same as in a wall... Steve Overall, the best performance will be by using as much of the 2.5 pcf stuff as you can, making up the difference with lighter stuff for panel damping - the lighter stuff should go into the cavity first, so it ends up on TOP of the heavier stuff and will therefore compress less, staying in better contact with the upper panels. From no insulation to nearly filled is the most change in performance; likely around 10 dB or so. The extra I'm referring to will not make a lot of difference in overall TL, but will lessen any ringing at resonance so the room will sound flatter in response.

Brilliant- It's all clear now (for the time being). Oh- one more quick one. Should I be insulating the cieling (subfloor) even where my inside cieling doesn't extend? In other words, should I insulate between the joists in the air space around the room? This seems like it could be pretty tricky to do... Thanks for the quick responses as usual! --Josh

Why tricky? Is there a drawing or pic I can look at? Steve

Well... I don't have a pic, but it shouldn't be any different in my situation than anybody else's. I meant tricky because I would have to insulate before framing the wall which will be adjacent to the air space. Or, I guess I could to try to insulate after I framed the wall, but that seems difficult (not undoable though...). I'll also need to figure out a way to keep the insulation against the wallboard "cladding" inbetween the joists for damping purposes (since there will be no inner ceiling below to support the insulation). String maybe? Or did I read something about impailers of some sort? That's the essence of my concern.

I would insulate those areas too - it will damp the upper subfloor a bit if nothing else. Before framing will be easier. I've not seen impaling clips used for horizontal insulation, nor for the lighter stuff; only horizontal, for 703 or rockwool batts. Lightweight lath across the joist bottoms every 2 feet usually works pretty well... Steve