How to avoid flanking in between leaves?

[Billyboy]

Just thought of an other question-

Where the stringers cross the joists- I should just cut the wallboard to fit snugly, and caulk the corners? Or, should I attempt to leave an 1/8" for a bead of caulk? Or am I splitting hairs here?

I have some nails protruding through the subfloor by less than 1/8". Should I trim or impale the wallboard? They won't even penetrate halfway if I impale...

Thanks for your time-

Josh

[Sen]

Sen, for absolute MAX isolation the entire room should be floated - floated floor, inner walls resting on floated floor, inner ceiling resting ONLY on floated inner walls - this can get pretty grim in the calculation department though - best isolation is when no one wall, floor, or ceiling has any hard contact with any other.[/quote]
Yep, that's what sounds logical...it's like "If one wall "gets" why should the rest of them "get" it but.....

it's safer to NOT float walls on top of a floated floor, but just fasten firmly; for one thing, the extra weight of the walls and ceiling will lower the resonance of the entire structure and so improve isolation, mainly through the floor, because of the improved m-a-m values.

EXACTLY my thoguhts!!Thank you Steve.I always thought of the first part (in bold), but the further comments are great too.
Thank you sir
cheers

[z60611]

Sen

it's safer to NOT float walls on top of a floated floor, but just fasten firmly; for one thing, the extra weight of the walls and ceiling will lower the resonance of the entire structure and so improve isolation, mainly through the floor, because of the improved m-a-m values.

I'm not sure about the second part of that. Sound TL is improved by forcing it all to go through mass. Just because you have a lot of weight around the edges of the floor doesn't mean that the middle 95% has mass. Imagine a drum -- lots of stiffness/weight around the edges, but the middle still bounces quite a bit.

[knightfly]

Bob, I shoulda clarified what I meant by "safer" - I'm by no means a vibration engineer (don't even PLAY one on TV :=) so am not qualified to tell anyone how to float a wall without causing more problems than they solve - there are shear factors when resting a wall on resilient mounts, just as there are for a floor - the difference is, the wall is being bombarded with sound waves which are (kinda) perpendicular to its surface, but the resilient MOUNTS would see more of a SIDEWAYS deflection (attempting to SHEAR the mounts, instead of compressing their thickness)

I've not had nearly enough study time on this subject to keep from screwing up someone's build for them, so I won't recommend they float a wall til I know a lot more. I also wouldn't recommend they "experiment" with this, unless they have more money/time than sense (or "cents" :=) - one possible outcome is that you make your wall resonant at yet another frequency different from the normal m-a-m ones, and so weaken it possibly to the point of having to tear it down and start over.

I can only speak for myself, but my list of things I want to do over and over includes beer, pizza, sex, music, but NOT building my studio... Steve

[Billyboy]

Steve-

I forgot to ask you about the wall detail you posted. Was the marked up drawing taking into account that I will not be floating the floor anymore? In other words, since the wall is now resting on the concrete floor (you were correct, PT is in order), should I go back to using RC, or do I not need to because of the sway brackets?


Much appreciated,

Josh

[knightfly]

Josh, with or without a floated floor if you're using separate frames you wouldn't see much difference in performance between RC and no RC - If anything, you'd probably see a bit worse bass isolation and a bit better mid-high isolation. Generally this is NOT what you want to do for music isolation.

I personally would bed your inner wall plate in either sill seal or a double layer of 30# roofing felt, lag it to the floor, space/sway bracket it to the ceilng joists, and mount your wallboard using 1/4" spacers around the perimeter of the wall, do each layer, pull the spacers and caulk; use half the screws on all but the final layer of wallboard - be sure to find a way to keep the insulation batts in snug contact with the first layer of wallboard in the inner frame, to help damp these layers so ringing isn't a problem.

Your ceilings should be each separate from the other room's wallboard layers, and dropped inside the walls (1/4" spacers, then pull the spacers after fastening and caulk at each layer) - insulation fill should be snug at most; gravity will keep the insulation against the wallboard on the ceiling, which is good.

For best isolation, insulation in walls should be at least 2.5 PCF, up to about 4 PCF.

Hope this helps... Steve

[Billyboy]

Steve-

Thanks! That's exactly the info I was looking for. Good info on the lag bolting- I hadn't considered means of attatching the lower part of the walls.

I truly appreciate all of the help you, John and the rest of this forum has to offer. The fact that you guys spend time out of your busy days to share your hard-earned knowledge is a testament to humanity.

Cheers,

Josh

[knightfly]

Josh, it's always nice to be appreciated - thanks for your kind words.

I'm not clear on your question earlier -

"Where the stringers cross the joists- I should just cut the wallboard to fit snugly, and caulk the corners? Or, should I attempt to leave an 1/8" for a bead of caulk? Or am I splitting hairs here?

I have some nails protruding through the subfloor by less than 1/8". Should I trim or impale the wallboard? They won't even penetrate halfway if I impale..."

If you're talking about the extra layers of wallboard up against the subfloor between joists, these should have ZERO air gap between subfloor and each layer - so unless you can use some sort of hydraulic jack to impale them FIRMLY against the subfloor, I'd find a different way. Maybe clip each nail and grind it flush with a small hand grinder, or grind a sharper point on each nail so it "impales" easier? Air gaps here are NOT your friend -

Fastening these inserts - if the subfloor is separate planks and has any irregularity (like grooves) I would first smooth this using drywall joint compound - once the compound is in place, you can put up the first layer using the compound as glue - then do a stripe along each joist (but NOT in the rest of the area) of joint compound, and immediately press the second sheet firmly upward and fasten using a cleat attached to the vertical side of each joist - this will keep the pieces of drywall from sagging or falling down later.

Once this is all in place, you can caulk along the edges of the cleats and where cleat meets drywall insert for a hermetic seal.

After all that, did I misunderstand your question? Steve

[Billyboy]

After all that, did I misunderstand your question?

No- I think you covered it. I was specifically trying to get an answer on how to interface the wallboard and joists/stringers. I.E. should they butt up against the joists/stringers, or should there be a gap a la wall construction for caulk to fit inbetween the wallboard and joists?

It will be quite a bit of work, but worth it I'm sure. The subfloor is planking, and there appears to be lots of nails protruding, again only 1/16" to 1/8". I'll just have to perform a test to see if I can get things flush via impaling...

immediately press the second sheet firmly upward and fasten using a cleat attached to the vertical side of each joist - this will keep the pieces of drywall from sagging or falling down later

Can you describe this "cleat" to me? Do you mean a small strip of wood which I'd tack to the joist (parallel to it) to hold the edge of the wallboard? Or am I misunderstanding.

Don't mention the kind words- you're more than deserving.

Thanks yet again,

Josh

[knightfly]

You're correct in what you think a cleat is - just a long strip of wood, typically 3/4 x 1 - can be ripped from 2x stock, set rip to 3/4" and you'll end up with 3/4 x 1-1/2 stock.

A gap is fine, hard to get perfect; then caulk or mud before next layer; main thing is keeping that area from becoming a void, so mud or caulk but NOT air.

Fastening full length cleats while putting up the last layer will keep air gaps from developing between layers, and also keep the layers from falling down should any adhesive fail. Cleats should be left in place even after glue/caulk/mud dries, so that any later failure of adhesives won't cause the inserts to fall down and create a triple leaf system.

If you decide to screw the cleats in place instead of nailing them, you'd need to countersink and drill for each screw - not really necessary, I'd just lay a bead of caulk before nailing the cleat, then push hard and nail. A small finish nailer works well for this if you have compressed air available - other wise you may need a helper to hold the cleats FIRMLY in place while nailing them... Steve

[Billyboy]

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

[knightfly]

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

[Billyboy]

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

[knightfly]

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

[Billyboy]

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