what is the STC of ultralight drywall? - AVS Forum
Dedicated Theater Design & Construction > what is the STC of ultralight drywall?
cgott42's Avatar cgott42 10:22 PM 09-14-2013
How does it compare to standard drywall?
I goofed and bought the ultralight instead of the regular, wondering how much difference it is and worth loading,unloading,reloading,andreunloading

BIGmouthinDC's Avatar BIGmouthinDC 06:05 AM 09-15-2013
you should be using two layers of 5/8 drywall. take back the 1/2 inch light weight.
cgott42's Avatar cgott42 06:27 AM 09-15-2013
Fwiw it was 5/8 lightweight, still necessary to bring back?
Or together with the green glue and 2nd layer is it reasonable that it will work
jjslegacy's Avatar jjslegacy 09:59 AM 09-15-2013
if you are putting in the investment to do everything is correct I wouldn't skimp on the drywall - I think it's something like 30% lighter isn't it? That would be less mass at each layer. Do it once and do it right
LeBon's Avatar LeBon 11:45 AM 09-15-2013
If I recall correctly, STC only measures down to 125 Hz. Home theatres have (or should have) significant sound energy below that down to 30 Hz and lower. And that is where the additional mass makes the big difference, at the low frequencies. So STC is not the number you should be using here.

Send back the lite drywall, and get the 5/8" fire-rated (heavy) drywall.
cgott42's Avatar cgott42 12:31 PM 09-15-2013
Ok I'm convinced thanks
HopefulFred's Avatar HopefulFred 02:29 PM 09-16-2013
Careful. Sheetrock brand does have firecode X 5/8" drywall in an ultralight variety. It weighs just a hair more than traditional 1/2"
BIGmouthinDC's Avatar BIGmouthinDC 02:43 PM 09-16-2013
Just had a conversation today with my local drywall supplier. They can't can't get anything other than lightweight 1/2 now. But their 5/8 is still heavy.
HopefulFred's Avatar HopefulFred 06:34 PM 09-16-2013
I figured that would happen. Ted was seriously disappointed when I didn't return my shipment of ultralight, but I decided to make do (especially since I was/am building with two layers plus OSB) The website says "up to 15% lighter" than traditional firecode X. 5/8 times 85% gives just over .5, so it's still about 6% heavier than traditional 1/2, and for 99.99% of their customers, the light weight is a great selling point - so why would they keep making the old stuff unless the costs were seriously different (and they're not).
BIGmouthinDC's Avatar BIGmouthinDC 06:46 PM 09-16-2013
Lighter means less shipping costs, so we get less for a premium price and we are being told it's better.
cgott42's Avatar cgott42 11:14 PM 09-17-2013
Just finished taking down old wall. Tomorrow putting up regular 5/8 inch
snowblitzing's Avatar snowblitzing 09:11 PM 08-07-2014
I have a local company that sells both types of drywall. For the same price. I initially told them I want the old heavy drywall. The sales guy who seems really knowledgable and helpful told me that they have found that the new lite rock board while not as heavy is more dense when it comes to sound transmission. He explained it by talking about how it is a synthetic by product of gypsum. Any thoughts before I pull the trigger.
BIGmouthinDC's Avatar BIGmouthinDC 09:41 PM 08-07-2014
Ask to see a published impartial test result. If they can't produce one, walk away from the lightweight. I'm open minded but "show me the data" not just a salesman's word on it.
HopefulFred's Avatar HopefulFred 01:08 PM 08-09-2014
^Agreed, but I still would encourage use of the heavy stuff. As LeBon mentioned in post 5 above, STC (and almost all other lab tests) don't test sound transmission at low bass frequencies. The extra mass in an isolated damped partition functions to push down the resonant frequency. Below the resonant frequency, isolation efforts are much less effective; so it's important to keep that resonance as low as possible.
JakeRobb's Avatar JakeRobb 01:12 PM 08-09-2014
Quote:
Originally Posted by snowblitzing View Post
they have found that the new lite rock board while not as heavy is more dense
Given boards of the same physical dimensions, this is literally impossible. Anyone who claims that doesn't know what density is.
Johnsteph10's Avatar Johnsteph10 03:56 PM 08-09-2014
Quote:
Originally Posted by JakeRobb View Post
Given boards of the same physical dimensions, this is literally impossible. Anyone who claims that doesn't know what density is.
Exactly.

Density = Mass / Volume

Assuming volume stays the same for the drywall (LxWxD), and the mass is less (it is lighter!), then there is no way density can be equal or greater than the old school drywall.

I think of the lightweight drywall like the "whipped" butter, peanut butter, cream cheese, etc. spreads = add air, etc. to reduce the actual amount material contained in it, and sell it for the similar cost.

No thanks.
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