Help! 1/2 inch verses 5/8ths Sheetrock!

[tmix]

I have to make a quick decision.... Which is never good.
So I throw myself on the mercy of the court, ask for forgiveness and a little input.

I am just now in the framing stage of the studio, but have come across a really good deal on 1/2 inch sheetrock if I can use it.
My initial plans are building mass - air - mass 2 X 4 walls with anywhere from a 2 inch gap( on some inside walls) to 3.5 in gap (outside walls) between the walls. I have originally purchased about 40 sheets of 5/8ths rock with plans to buy more, but ran across a deal where I can get about 150 sheets of 1/2 inch for about $1 each! (no it is not stolen). My Dad is a builder and sometimes gets deals on overage that has been stored outside and might look a little discolored.
My question is how much of a hit on sound transfer will I take using only 2 layers of 1/2 inch? I could feasibly use 3 layers in some areas and just buy more 5/8ths wherever I run short. The 5/8 ths will be used where I can complete a full ceiling. The largest ceiling span I have is 14.5 feet (by 31 feet) and my ceiling joists (not installed yet) are 2 X 8 douglas fir (most spans are 10 foot and under). I could space them what ever I need to for the weight, they only support the weight of the ceiling and acoustic panels.

I could use some facts from you guys before I "good deal" myself into a corner.

Thanks.

Tom

[Aaronw]

That is a good deal considering current prices.

With 2 layers of 5/8" you have a thickness of 1 1/4". Use 3 layers of 1/2" and get 1 1/2". Or you could use a combination of both 1/2" and 5/8".

If you don't mind the extra work of a 3rd layer in some area's as you mentioned, I don't see a problem, especially from a bargain point of view.

Be weary though of drywall that might be wet or too moist. The stuff loves to grow mold. Perhaps run a few dehumidifiers to suck out any moisture.

[knightfly]

Agreed; but use 3 layers; if you're not willing to do that, be aware that you'll take about a 3 dB hit in isolation on the low end, where you can LEAST afford it.

Ditto on the moisture; drywall ain't called "wetwall", and there's a reason... Steve

[tmix]

Kind of what I thought!
I may go ahead and use the 1/2" in certain areas like the walls and ceilings of the drum booth. Possibly even the outermost inner walls .

My main concern is whether the ceilings will support the weight of 3 layers.
Most of the spans are 10 foot and under, but I do have the main room which is 14.5 feet.
I was trying to understand the span rating tables but quite honestly dont know to use them exactly. Again, I am using 2 x 8 douglas fir.

[knightfly]

2x8's at what spacing? And, is there a floor above you or are these just ceiling joists? If they're ceiling joists ONLY, if you use 16" centers instead of 24" centers you can span up to 16'6" at 10 # dead load;

Dead load is calculated INCLUDING the weight of the framing; 2x8's on 16" centers weighs about 2.1 PSF, so if your framing will support 10# dead load, the framing uses 2.1 PSF of that; which leaves 7.9 PSF. Each layer of 5/8 gypsum adds just under 2.4 PSF, so a 2x8 frame spanned for 10# dead load can just carry 3 layers of 5/8 wallboard - in your case, your longest span is 14.5 feet and #2 and better Doug Fir 2x8's on 16" centers can span up to 16'6" with 10# dead load - so you have a good margin of error, and should have no problems.

Just don't try to get by with 24" centers on those longer spans, that's only good for 13'6" at 10 # dead load. On the wider spacing, 2x8's add 1.6 PSF so a 3 layer ceiling would weigh in at 8.8 PSF, which would be MARGINAL for this application. NOt a good plan... Steve

[tmix]

Steve
Thanks!
Yes the 2 x 8s are just ceiling joists, no load on them other than the sheetrock (and light fixtures / acoustic panels.)
If I go the 3 layers I will definately go with the 16 inch spacing.

Wood is cheap.... life is not.

I appreciate the explanation of the loads, I was lost trying decide about what all factors where considered in the weight loading.


Thanks again!

Tom