Doubling up subfloor w/greenglue and OSB...and drywall?

Ok, I'm trying to improve an attic room for mixing in. It's original 1" T&G subfloor, I'm going to put another 'subfloor' over top of it with greenglue to help transmission to the second floor(I don't have access to the 2nd floor ceiling). Soooo...just wondering - I can get excellent pricing on T&G OSB and greenglue, which would be better, one layer of 5/8" T&G OSB with greenglue, or 2 layers of 3/8" T&G OSB with greenglue between each layer? What's the best way of butting the OSB up against the kneewall? Small gap and acoustic adhesive or caulk? The kneewalls will eventually be double drywall with greenglue, all edges caulked etc...

...or would there be an appreciable difference with one layer of 3/8" OSB vs. one layer of 5/8" using greenglue?

I would put down a layer of 1 or 2" 705 - and then 2 layers of plywood over that with green glue - being carefull that the screws for the 2nd layer of decking did not penetrate the 705......... That's a floating floor that will not act as a drum head - and the 705 gives good isolation. Rod

Hey Rod; thanks for the reply - I should've specified I don't have the headroom to really 'float' the floor, I would've loved to do it properly as you suggested but I'm already at 7' 4" ceilings right now...maybe carpet underlay, OSB? Or carpet underlay, drywall, greenglue, OSB? I figure I can lose ~1-2" of floor including whatever carpet/laminate goes on the top...and I'm not thinking I'm going to get 100% isolation, I'm just looking for the maximum reduction within the working parameters I've got...

ok now that i've spent some more time up there it seems apparent that I'll be doing the floor last, after the ceiling/cathedral/kneewalls in that order...so I've got some more time to think about it. I sealed the subfloor inside the kneewalls with GG/drywall/GG/drywall. Each layer mudded and caulked around the edges, and I'm using up a bunch of old pink insulation around the very outside edges for thermal insulation, and laying an initial layer of roxul on top of the rest of the space in there...still might plan out some storage in the kneewall areas if I can seal it all off right. Now, my thinking for the rest of the floor is carpet underpad/5/8" OSB T&G/GG/Drywall/GG/3/8" OSB T&G ...that would make a solid top layer that if properly sized to the room, wouldn't have to be drilled into the existing subfloor. I would be putting carpet or maybe laminate over top. Questions: 1. Is the solid rubbery carpet underpad any better than the crazy-coloured foam one? 2. Would usingl strips the height of the new floor around the edges help to isolate the new floor from the walls? 3. If drywall is sandwiched this way(with Green glue on both sides) and the OSB T&G is properly assembled, is it safe to use as a layer that would be walked on? 4. How would I calculate how much this would weigh? I'm thinking of using 1/2" for the drywall layer just so it doesn't tip the scales...the entire second floor is sistered 2x6's that were purposely reinforced, and a load bearing wall runs down the middle of the house front to back, so i'm not worried, but I know once a couple racks of equipment and some folks get up there it can get heavy..

Question #5 - Will this be too heavy? By my rough math - drywall @70lbs for a 4x8 sheet, 5/8" OSB@60lbs per sheet, 3/8" OSB@40lbs per sheet, maybe 30lbs for a 4x8 area of the rest(carpet underlay, carpet or laminate) will all equal approx. 6 1/4 lbs/sq. ft... a pretty heavy non-live load without a lick of furniture or gear in there... if drywall did go in the flooring 'sandwich' I figure it has to be between 2 other hard materials(not carpet underlay or fiberglass) if it's going to stay in one piece...so it's hard to come up with a subfloor that can utilize it and still balances performance with weight with shortest height increase possible...rubbery underpad and one layer of OSB might be as good as it gets, and that's approaching 3lbs sq ft... The existing subfloor is pretty flat, what about GG/1/2" drywall/GG/ 5/8" OSB as a compromise subfloor? carpet underlay and carpet on top for footfall reduction. Just musing out loud really. Any thoughts?

tungstengruvsten wrote:

Just musing out loud really. Any thoughts?

Maybe no drywall layer? Rubbery carpet underlay or Owens Corning Acoustic Mat, then 2 layers of OSB with Green glue in between? That would keep me under 2" of used up headroom once final flooring and everything is in...

First of all - the green glue is great for sound - but if have direct connections through all of this to structure - your weak link is going to be flanking - not passage through the surface - Forget carpet pad - it's useless for what you're doing - and it creates a perfect home for dust mites -, etc. SO if OC Acoustic Mat - Add up what you want to do (per OC installation instructions) OC QuietZone acoustic floor mat 3/8" 2 layers 3/8" exterior grade plywood - 3/4" (you can't use OSB because it will never hold together with screws that short - they cannot penetrate the floor pad. That's 1 1/8" - floating - but the 2 sheets of 3/8" plywood don't have a whole lot of mass - so they aren't going to do a whole lot to help you with airborne sounds. Adding green glue will help quite a bit - but not a much as more mass would (along with it). Also, the acoustic mat is pretty much a simple decoupling mechanism - it will stop foot fall noise, etc. - which is exactly what it says in it's literature:

"Noise Control Floor Construction Installed beneath a double layer of exterior-grade plywood or 1 1/2" gypsum concrete and finished flooring, Quiet Zone acoustic floor mat isolates and minimizes structure-borne noise from transferring to adjacent areas or rooms"

Do what you want - just make sure you understand exactly what you're getting in return. Sincerely, Rod

I came here from your stall mat thread. Yes you are creating a triple leaf. With no access to the ceiling below, you are stuck between a rock and a hard place. Rod's recommendations are 705 or OC acoustic mat. Stall mats are nothing like those products. Here is a thread specifically about Acoustik Mats. Note how the thread evolves to triple leaf effect. Check the design load for the ceiling for weight capacity. Ceilings are otten designed for no transient loads. Good luck with your studio! Andre 080710 edit: corrected html and english errors.

Thanks guys for the suggestions and explanations - between 3 or 4 sites and 'experts' advice in various threads, there is a lot of conflicting information - after doing extensive digging it only becomes apparent all the reasoning in certain practices, and also the discrediting of other practices/material that seem perfectly logical. soooo...after finally finding 705 for a good price(favour from a friend in construction), I'd like to learn more about Rod's first suggestion to float the floor. I would like to use 1" 705, the room would take about 320sq. ft of it...on top of that, I can afford another inch or two maximum...my thinking is: 5/8" T&G OSB on top of 705, with green glue and 3/8" OSB on top of this. Finish with carpet or laminate. Proper gaps around the edges and caulking etc of course... The 5/8" OSB should allow screws to hold the two layers together...is the green glue in between worth it? Rod states that this wouldn't act like a drum head - do I still need a neoprene layer or will the 705 decouple and still be strong enough to support the floor and gear on top(not a ton, no big consoles etc...) Thanks again for the help-

the compressive strength of 705 is 200 psf (pounds per square foot) you'll be fine no matter what you put on it.. Personally I don't like the holding power of OSB - it isn't designed with that in mind - I would rather see you use 5/8" CDX as your base - and then 1/2"MDF (Medium Density Fiberboard) over that. The green glue is well worth the investment. Rod

by 'holding power' do you mean the way the wood grips a screw? I see that as a worthy upgrade...I was only after OSB due to it's price and waterproof characteristics....well, mostly price. If I remember right, CDX is the rougher/cheaper plywood, not sure if it's at H. Depot or Rona around here but I'll have to go out looking... Normal 5/8" plywood is $30/sheet compared to $9 for the 5/8" OSB...hopefully CDX is somewhere in the middle!!