rc channel? Homosote?

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rc channel? Homosote?

Postby archive » Wed Mar 31, 2004 2:45 am

From: dude@c...
Date: Sat Jan 13, 2001 10:06 am
Subject: rc channel? Homosote?

I've seen several references to rc channel being used between two
layers of drywall. Doesn't it reduce the effectiveness of the rc
channell to lay it ontop of drywall. Isn't it supposed to float on the
studs so the drywall ontop of the rc has minimal contact with
underlayers?

Also, how do you lay the ceiling? I can't imagine hanging sheetrock
overhead hanging by rc channell. Seems unsafe.

finally, I have an already built room I'd like to obtain STL of about
55 db. I still haven't figured what materials to use that are most cost
effective. If I stuff stud cavities with r12 paperless..where do I go
from there with layers of whatever to obtain the 55 db loss...gybpsum,
felt, foam, homosote, rc channel, etc...

BTW, what is homosote? That cardboard soundboard stuff?

Thanks.
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Postby archive » Wed Mar 31, 2004 2:58 am

From: SRF7@a...
Date: Mon Jan 15, 2001 4:46 pm
Subject: Re: [acoustics] rc channel? Homosote?

In a message dated 1/13/01 2:06:38 AM Pacific Standard Time, dude@c...
writes:

> I've seen several references to rc channel being used between two
> layers of drywall. Doesn't it reduce the effectiveness of the rc
> channell to lay it ontop of drywall. Isn't it supposed to float on the
> studs so the drywall ontop of the rc has minimal contact with
> underlayers?
>

Run the RC perpendicular to the wall studs... laying drywall over the studs
prior to running RC enhances the STC (more layers more mass).

> Also, how do you lay the ceiling? I can't imagine hanging sheetrock
> overhead hanging by rc channell. Seems unsafe.

Yeah ... we scratched our heads on that one, but its tried and tested,
manufacturer approved (and it works ... you're soaking in it right now ... no
wait, I'm typing under it ... yeah that's it).

>
> finally, I have an already built room I'd like to obtain STL of about
> 55 db. I still haven't figured what materials to use that are most cost
> effective. If I stuff stud cavities with r12 paperless..where do I go
> from there with layers of whatever to obtain the 55 db loss...gybpsum,
> felt, foam, homosote, rc channel, etc...
>

An STC of 55 aint too ambitous and can be done pretty easily without
resorting to masonry. That said, masonry is a great way to go if feasible.

I did a very simple wall between my bathroom / vocal booth and my HVAC closet
with an approximate goal of STC 50 as follows, 2x6 top and bottom plates,
double 2x4 studs (offset so that no stud is common to both sides of the
wall), continuous R-11 in the cavity (no gaps .. it actually doubles up in
some places), one layer 5/8's drywall on one side, two layers other side.
Caulked all seams and gaps. Nonthing fancy, but you can't hear the AC unit
... and that was the goal.

The above design with only one layer of 5/8's and one layer 1/2" drywall on
opposite sides is rated at STC 42 with no fiberglass, and from 46 to 52 with
fiberglass ... depending on how much fiberglass you stuff in there. I used
R-11, and doubled the 5/8's on one side for the same dis-similar effect but
slightly more massive.

I reckon I ended up with about a little over STC 50 on this wall .. probably
not quite 55. To improve, to your standard, I'd add RC between the double
drywall layer ... this should hit betwenn 55 and 60. Adding a fourth drywall
layer, and RC to both sides would probably yield around 60 (I'm extrapolating
from Everest's Studio Construction on a budget .. you should buy this book
... and others). To get up over 60 you could add an additional massive
dis-similar layer to one side (1/2" MDF or OSB)... this will add mass and
diverge the range of sympathetic resonance of the two sides of the wall away
from each other. A layer of foam board on the studs before the first layer
seems to help a lot and should also be considered. To get much over 60 you
will have to build two seperate stud walls with an air space between (or go
to a grout filled block wall and furr off that with RC).
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Postby archive » Wed Mar 31, 2004 3:00 am

From: steve <whitaker@p...>
Date: Mon Jan 15, 2001 6:20 pm
Subject: Re:rc channel? Homosote?

hello,

>I've seen several references to rc channel being used between two
>layers of drywall. Doesn't it reduce the effectiveness of the rc
>channell to lay it ontop of drywall.

Yes. could even make the transmission worse than if only 1 layer of
drywall had been used.

> Isn't it supposed to float on the studs so the drywall ontop of the rc
has >minimal contact with underlayers?

yes

steve
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Postby archive » Wed Mar 31, 2004 3:22 am

From: SRF7@a...
Date: Tue Jan 16, 2001 6:32 pm
Subject: Re: [acoustics] Re:rc channel? Homosote?

In a message dated 1/15/01 10:38:21 AM Pacific Standard Time,
whitaker@p... writes:

> >I've seen several references to rc channel being used between two
> >layers of drywall. Doesn't it reduce the effectiveness of the rc
> >channell to lay it ontop of drywall.
>
> Yes. could even make the transmission worse than if only 1 layer of
> drywall had been used.
>

I beg to differ ... for example, refer to any of Everest's books on studio
construction and start looking at the wall cross-sections and listed STC
ratings. Many examples include drywall/RC/drywall elements. (see page
page 186 of his "Sound Studio Construction On A Budget" for example, <A
HREF="http://www.amazon.com/exec/obidos/ASIN/0070213828/o/qid=979667241/sr=8-2/r\
ef=aps_sr_b_1_2/002-2903776-3178444">ISBN
0-07-021382-8</A>)

Further, I can't see how installing RC on wall sheathing on studs is going
allow a higher transfer of energy between the finish sheathing and the stud
than attaching the RC to the stud directly (no intermittant layer of
sheathing). Seems to me that the transfer of energy would be lessened by the
mass of the intermittant sheathing and by the dis-similar modal patterns of
intermittant sheathing on vertical studs vs. the finish sheathing on
horizontal RC.

Further, I believe this approach provides the means to achieve useful overall
mass while maintaining a differential density for each diaphragm without
having to get a large variety of materials on the job ... for example using a
common drywall thickness one could array drywall/drywall/stud-with
insulation/OSB/RC/drywall and all three diaphragms are of widely varying
density. If the studs are doubled and offset you get a pretty awesome wall
with standard materials and techniques without a double top and bottom plate
(provided you minimize wall penetrations and use lots of caulk).

It's true that standard cross-sections rarely include the third layer of
drywall ... it is more common to see iterations of drywall/stud-with
insulation/RC/drywall, with permutations on the drywall thickness, insulation
of the cavity and stud spacing ... but these wall sections are typically
examples of design for use in condos, offices, hospitals, schools, etcetra
... and usually top out at about 45 STC. I see the drywall/RC/sheathing
technique as one that is only pertinent for high need applications like sound
studios.

My $0.02

Scott R. Foster
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Postby archive » Wed Mar 31, 2004 3:23 am

From: "Sjoerd Koppert" <sjoerd@n...>
Date: Tue Jan 16, 2001 10:52 pm
Subject: Re:rc channel? Homosote?

OOOOOOPPPSS
Almost forgot - when using RC between 2 layers of drywall, whenever
possible use 2 different thicknesses of drywall. The best kind is the
high density one - which is the one with the highest fire rating.
Amen again
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Postby archive » Wed Mar 31, 2004 3:23 am

From: "Sjoerd Koppert" <sjoerd@n...>
Date: Tue Jan 16, 2001 10:49 pm
Subject: Re:rc channel? Homosote?

--- In acoustics@e????ups.com, SRF7@a... wrote:
> In a message dated 1/15/01 10:38:21 AM Pacific Standard Time,
> whitaker@p... writes:
>
>
> > >I've seen several references to rc channel being used between
two
> > >layers of drywall. Doesn't it reduce the effectiveness of the rc
> > >channell to lay it ontop of drywall.
> >
> > Yes. could even make the transmission worse than if only 1 layer
of
> > drywall had been used.

????? In my 30 something years I've never encountered a situation
where RC made things worse, even when it could have been installed
better. A few tips for the best results:
1. Know where your studs are.
2. Avoid screwing into the studs.
3. I always use contact cement to place the RC channel first - then
screw it.
4. NEVER let the inner layer of drywal touch anything (floor, ceiling,
side walls), always leave a small gap.
5. Mud all the joints carefully.
6. DON'T use mud to fill the floor / ceiling / side wall gaps. Use a
good make of chalk for this.
Amen
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Postby archive » Wed Mar 31, 2004 3:24 am

From: steve <whitaker@p...>
Date: Tue Jan 16, 2001 11:20 pm
Subject: Re: rc channel? Homosote?

Hi Scott,

>In a message dated 1/15/01 10:38:21 AM Pacific Standard Time,
>whitaker@p... writes:
>
>
>> >I've seen several references to rc channel being used between two
>> >layers of drywall. Doesn't it reduce the effectiveness of the rc
>> >channell to lay it ontop of drywall.
>>
>> Yes. could even make the transmission worse than if only 1 layer of
>> drywall had been used.
>>
>
>I beg to differ ... for example, refer to any of Everest's books on studio
>construction and start looking at the wall cross-sections and listed STC
>ratings. Many examples include drywall/RC/drywall elements. (see page
>page 186 of his "Sound Studio Construction On A Budget" for example, <A
HREF="http://www.amazon.com/exec/obidos/ASIN/0070213828/o/qid=97966724
>
>Further, I can't see how installing RC on wall sheathing on studs is going
>allow a higher transfer of energy between the finish sheathing and the stud
>than attaching the RC to the stud directly (no intermittant layer of
>sheathing). Seems to me that the transfer of energy would be lessened by
the
>mass of the intermittant sheathing and by the dis-similar modal patterns of
>intermittant sheathing on vertical studs vs. the finish sheathing on
>horizontal RC.

Ok, i'll try to explain my thoughts here.

I was speaking of a sound barrier. One made from the concept of
mass - air - mass

XX XX
XX XX
XX XX
XX XX
XX XX

I'm sure we can agree on the effectiveness of the concept

then there is the RC currently being discused:

XX X-X
XX X-X
XX X-X
XX X-X
XX X-X

the - being the RC.
there is now a small air space between the two masses.
there will be flexing of each of the sheets of drywall.
The flexing will allow sound transmission through the two sheets in a
greater amount than if the two sheets were together because together
there is both more stiffness and more mass of the diaphram.

I hope this clarifies a bit.

>Further, I believe this approach provides the means to achieve useful
overall
>mass while maintaining a differential density for each diaphragm without
>having to get a large variety of materials on the job ... for example
using a
>common drywall thickness one could array drywall/drywall/stud-with
>insulation/OSB/RC/drywall and all three diaphragms are of widely varying
>density.

Yes, but there is flexing because of the RC, as opposed to DW/OSB/DW
which has total surface contact between the DW and OSB and is therefore
incredibly stiff.
Stiffness being *aproximately* equal to the square of the thickness.
(and yes, there are many other variables that i'm not geting into here).

>If the studs are doubled and offset you get a pretty awesome wall
>with standard materials and techniques without a double top and bottom plate
>(provided you minimize wall penetrations and use lots of caulk).

I won't argue against the staggered or separate studs as they help.
as does adding an extra inch or two (where feasible) between the
two wall halves.

>It's true that standard cross-sections rarely include the third layer of
>drywall ... it is more common to see iterations of drywall/stud-with
>insulation/RC/drywall, with permutations on the drywall thickness,
insulation
>of the cavity and stud spacing ...

There is a reason for the Mass-Air-Mass, as opposed to
Mass-Air-Mass-Air-Mass. The latter has a reduced capability.

> but these wall sections are typically examples of design for use in
>condos, offices, hospitals, schools, etcetra
>... and usually top out at about 45 STC.

two layers DW with staggered studs should yield STC 48-53
using double studs should yield STC 51-56
adding fiberglass batting inside just about any wall design will improve
the STC by about 3

no accounting is made for carpentry skills

>I see the drywall/RC/sheathing technique as one that is only
>pertinent for high need applications like sound studios.

Bottom line is not what works, but to make the customer happy and give him
what he'll pay for.

If you like it, i guess that's what matters most.

Steve
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Postby archive » Wed Mar 31, 2004 3:39 am

From: David Chertock <ap412@l...>
Date: Tue Jan 16, 2001 9:28 pm
Subject: Re: rc channel? Homosote?

Hi all.
When this list was young we reviewed my studio design. Its still at the
studiotips website at http://www.studiotips.com/files/chertock.jpg .
Now with a few delays and changes, I'm preparing to build. I'm going to
put insulation and two layers of drywall over the existing garage wall.
Then inside I am going to frame up a room within a room type design, but
I am confused about the inner walls and ceiling construction. This
current thread echoes the contradictory opinions I have received. Just
today I spoke with a "known designer", as well as a very experienced
contractor, and they totally disagree. I've scanned my books (Everett,
Newell, Cooper) and I'm still lost. I know this has probably been
discussed ad nauseum, but I've been away. Please revisit this once more
and save my ass.

I can focus my question quite narrowly; I am most concerned about low
frequency leakage out of my garage. What I really need to know is which
of the following 4 options will best block the transmission of bass and
drums to the outside world?

An inner floating room with walls built of :

1. stud/drywall/drywall

2. stud/resilient channel/drywall/drywall

3. stud/drywall/resilient channel/drywall

4. stud/drywall/soundboard/drywall

FWIW, I have made 3 changes to my studio design. The sound lock and iso
booth are going to be bigger (9'6" in length), the splayed walls are now
going to have sliding glass doors with thick drapes, and the angle of
these glass door walls is going to be a few degrees wider than drawn (so
no reflections from the monitors bounce from glass door to the sweet spot).

Thanks! Your help is very appreciated.

David
Venice, CA
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Postby archive » Wed Mar 31, 2004 3:40 am

From: SRF7@a...
Date: Wed Jan 17, 2001 6:21 am
Subject: Re: [acoustics] Re:rc channel? Homosote?

In a message dated 1/16/01 3:24:02 PM Pacific Standard Time,
sjoerd@n... writes:

> >
> >
> > > >I've seen several references to rc channel being used between
> two
> > > >layers of drywall. Doesn't it reduce the effectiveness of the rc
> > > >channell to lay it ontop of drywall.
> > >
> > > Yes. could even make the transmission worse than if only 1 layer
> of
> > > drywall had been used.
>
> ????? In my 30 something years I've never encountered a situation
> where RC made things worse, even when it could have been installed
> better.

I think the was referring to my suggestion that the studs could be sheathed
before the RC went up, then do the final sheathing ... and was suggesting
that the inner layer of sheathing might somehow keep the RC from working.

As you point out, getting the most out RC is a matter of simply allowing the
installation of the final layer to remain free-floating (don't rest it on the
floor, jam it to the ceiling, or screw it to the inner wall).

>
A few tips for the best results:

> 1. Know where your studs are.
> 2. Avoid screwing into the studs.
> 3. I always use contact cement to place the RC channel first - then
> screw it.
> 4. NEVER let the inner layer of drywal touch anything (floor, ceiling,
> side walls), always leave a small gap.
> 5. Mud all the joints carefully.
> 6. DON'T use mud to fill the floor / ceiling / side wall gaps. Use a
> good make of chalk for this.
> Amen
>

Using the right length screws is also important.

Something no-one has reported using on this list yet, but is out there, are
RC strips for plaster. I wonder how well that works out? You could do some
cool shapes with plaster if you had a plaster guy who knew what he was doing.

Scott R. Foster
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Postby archive » Wed Mar 31, 2004 3:40 am

From: steve <whitaker@p...>
Date: Wed Jan 17, 2001 8:18 am
Subject: Re: rc channel? Homosote?

Hello,

You may get as many answers as there are options :-(

>An inner floating room with walls built of :
>
>1. stud/drywall/drywall

a good sound barrier

>2. stud/resilient channel/drywall/drywall

a good sound barrier + slight movement of the wall, adding a tad of absorbtion

>3. stud/drywall/resilient channel/drywall

barrier, and diaphram. I would think less of this, but i've seen others
swear by it.

>4. stud/drywall/soundboard/drywall

stiffened barrier. also excellent
Personally, i'd recommend this one.
but what others will say remains to be seen.

>FWIW, I have made 3 changes to my studio design. The sound lock and iso
>booth are going to be bigger (9'6" in length), the splayed walls are now
>going to have sliding glass doors with thick drapes, and the angle of
>these glass door walls is going to be a few degrees wider than drawn (so
>no reflections from the monitors bounce from glass door to the sweet spot).
>

wishing you the sweet sound of success :-)

Steve
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Postby archive » Wed Mar 31, 2004 3:41 am

From: SRF7@a...
Date: Wed Jan 17, 2001 8:16 am
Subject: Re: [acoustics] Re: rc channel? Homosote?

Howdy Steve:

In a message dated 1/16/01 3:49:25 PM Pacific Standard Time,
whitaker@p... writes:

> Ok, i'll try to explain my thoughts here.
>
> I was speaking of a sound barrier. One made from the concept of
> mass - air - mass
>
> XX XX
> XX XX
> XX XX
> XX XX
> XX XX
>
> I'm sure we can agree on the effectiveness of the concept
>
> then there is the RC currently being discused:
>
> XX X-X
> XX X-X
> XX X-X
> XX X-X
> XX X-X
>
> the - being the RC.

Exactly ... um ... 'cept different.

Given (rough scale):

X = 5/8's drywall;

------------- = a 2x4 stud;

RR = RC

what we're talking about is the difference between either adding a sheathing
layer before the RC ... or not.

Thus:

X-------------RRX, versus

X-------------XRRX

or drywall/stud/RC/drywall vs. drywall/stud/drywall/RC/drywall

The tried and true mass-air-mass concept shows up in both designs ... both
have this attribute (albeit in slightly different degree ... as the top
version, and the bottom version differ in width of the air mass by the
thickness of the RC ... nonetheless, since both "dead air" columns exceed 3"
of depth, both can be expected to perform similarly ... for example 5"s of
dead air between studio window panes does little better than a 3" gap).

There is a refinement ... namely decoupling (the RC) extant in both proposed
designs. I do not agree with the suggestion that mounting RC on sheathing
will diminish it effectiveness. RC is designed to be firmly mounted on both
edges ... the decoupling comes from allowing the RC (and therefore the
interior layer) to flex, and thereby decouple the interior air mass from the
wall system). This is achieved by taking care not to couple the RC mounted
layer to the remainder of the wall system. If properly installed, RC will
work great in either design ... if the final layer is free-floted it will
decouple the air mass of the room from the wall system to the full extent of
its ability ... on the other hand both designs could be screwed up ... for
instance ... if 2" screws are used on the final layer (RC1 + 5/8 drywall =
1- 5/8") the interior layer will get coupled (mechanically connected) to the
balance of the wall system by the screws.

>> there is now a small air space between the two masses.
> there will be flexing of each of the sheets of drywall.
> The flexing will allow sound transmission through the two sheets in a
> greater amount than if the two sheets were together because together
> there is both more stiffness and more mass of the diaphram.

Huh? ... you mean three masses don't you? ... I think we have a semantics /
apples vs. oranges problem here. In other owrds ... I agree with with you
right up to the point I feel like we're changing the subject ... maybe we are
discussing different design choices.That said, I am sure we agree that on the
interior side of either design we have a decoupling layer, and I'm sure we
agree that drywall/RC/drywall is similarly massive, but much less stiff than
drywall/drywall. All I'm trying to point out is that on the interior side
... adding mass/decoupler/mass is better than just adding decoupler then mass
.. if you are saying that RC won't work when mounted on sheathing, then I
disagree.

I also hope this clarifys a bit.

snip

> Yes, but there is flexing because of the RC, as opposed to DW/OSB/DW
> which has total surface contact between the DW and OSB and is therefore
> incredibly stiff.

Naaaaaa .... I'm losing you. What I'm talking about is adding a layer
before the RC (drywall, OSB or otherwise), and I'm saying that the addition
third layer is worth the $ and effort.

>
> There is a reason for the Mass-Air-Mass, as opposed to
> Mass-Air-Mass-Air-Mass. The latter has a reduced capability.
>

Nope.

Mass-Air-Mass-Air-Mass would generally work better. This is why Vacuum
flasks with a plastic coated styrofoam sheath work better. More layers ...
more attenuation. But, given the design goal (STC-55), it would hardly be
worth the effort ($) to construct an additional structural frame.

Moreover ... I feel there is a bigger fish to fry here, that being False
premise ... false conclusion.

The premise is not as you state ... it is rather one of
mass-air-mass-decoupled mass vs. mass-air-decoupled mass.

>
> two layers DW with staggered studs should yield STC 48-53

Bingo ... now I feel we are both on a congruent topic.

This thread goes back to a guy who wanted STC 55 wall.

5/8ths drywall both sides, standard studs, no penetrations, no gaps, no
insulation = STC-36.

Add one layer of 5/8ths outside (double it up .. directly to existing) and
mount RC and then 5/8ths (properly floated) to the inside and Bob's your
Uncle ... STC-56. No penetratrations ... no carpentry holidays ... plenty of
caulk ... and you are there.

Want to do a bit better ... maybe account for reality .... or a bit of wiring
... like I done said: add just a few bucks to your design costs by adding
staggered studs, a bit of cavity insulation, use OSB vs. drywall on the
interior 1st layer (differential density - higher mass) and you'll probaly
actually get close to a 55 dB drop out curve on the system.

snip

> Bottom line is not what works, but to make the customer happy and give him
> what he'll pay for.

Screw 'em. Average retail customer aint got the sense God gave a bucket of
hair, and the way of he world is: Eat the stupid.

But that aint what this list is for ... if I were trying to bullshit you, by
my lights, you'd have been right in the first place, regardless (we'd just
have had run a few Monster Cables to get the speaker wire "bias" in tune with
the moon).

>
> If you like it, i guess that's what matters most.
>
> Steve
>
>

Ya .. like Ginger Baker said ..... Do what you like.

Take Care,

Scott
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Postby archive » Wed Mar 31, 2004 3:41 am

From: SRF7@a...
Date: Wed Jan 17, 2001 8:46 am
Subject: Re: [acoustics] Re: rc channel? Homosote?

In a message dated 1/16/01 10:56:56 PM Pacific Standard Time, ap412@l...
writes:

> I can focus my question quite narrowly; I am most concerned about low
> frequency leakage out of my garage.

Hey Dave:

I think I remember your design .. upside down "V" shape ... vocal booth left,
sound lock on the right???

FWIW I'd skip the conclusions (if any) of the thread in question as it
pertains to a built from scratch wall system. If I understand correctly you
have an existing common wall and a reasonable budget. The best solution for
you would start with examining what you have.

What is the common wall made of? Insulated? How?

Are you willing to build a new wall frame? (do you have room ?)

What's the target STC? or maybe better asked, does a mean ole Momma sleep
with her head against the opposite side of the wall, or are we just trying to
knock things down a bit so somebody can watch TV in the den?

What about overhead (or other flanking paths)? Stone cold dead common walls
and a thin leaky ceiling into a common attic won't do you much good. What
about the slab? Floating Floor? What are the existing ceilings like (1/2"
drywall on roof trusses?) ... this could well be the main transmission path.
How high is the existing garage ceiling, how much room do you have to work
with there?

Scott R. Foster
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Postby archive » Wed Mar 31, 2004 3:41 am

From: dude <dude@c...>
Date: Thu Jan 18, 2001 7:47 am
Subject: orrect rc channel installation?

So can someone describe the right way to install rc channel? If layed on studs,
it screws into the studs, then the sheathing screws into the rc channel, right?
Leave the drywall floating and caulk the seams. Ok. Now if you sheath directly
to
the studs, then lay rd, what do you screw the rc to and what do you screw the
final sheathing layer to? Sounds like 2 layers of drywall or osb then drywall is
much easier and yields same stc, perhaps better on low end...
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Postby archive » Wed Mar 31, 2004 3:45 am

From: SRF7@a...
Date: Thu Jan 18, 2001 5:39 pm
Subject: Re: [acoustics] orrect rc channel installation?

In a message dated 1/17/01 11:48:40 PM Pacific Standard Time, dude@c...
writes:

> So can someone describe the right way to install rc channel? If layed on
> studs,
> it screws into the studs, then the sheathing screws into the rc channel,
> right?
> Leave the drywall floating and caulk the seams. Ok. Now if you sheath
> directly to
> the studs, then lay rd, what do you screw the rc to and what do you screw
> the
> final sheathing layer to? Sounds like 2 layers of drywall or osb then
> drywall is
> much easier and yields same stc, perhaps better on low end...
>

Try this URL:

<A
HREF="http://www.national-gypsum.com/assets/34-2171.pdf">http://www.national-gyp\
sum.com/assets/34-2171.pdf</A>

If the above comes out garbled, go to www.national-gypsum.com and look for
product literature, then click on the option to have the manual displayed by
Section. There is a Section on RC that covers most of your questions.

Generally (from what I gather from various listed STC ratings) adding RC and
an additional layers of 5/8ths drywall to a wall is roughly equivilent to
going from a single row of studs to staggered 2x4 studs inside 2x6 top and
bottom plates. Both of these techniques reduce the coupling between the two
sides of the wall. Both serve to increase the STC and better than just
adding a layer of drywall, or adding R-11 to the cavity.

Obviously RC is the way to go if you are trying to beef up an existing wall
(since re-studding the wall would require demolition). In a new wall
consider doing both as the extra studs add little in cost and add a real
boost to the decoupling of the two sides (the top and bottom plates are the
only common connection between the two sides.

As to RC being the same as just adding an additional layer of drywall, I
don't think this is true. Say you have a unfinished common wall between your
house and your garage .... 1/2" drywall on the inside the house side, with
R-11 between studs, and no sheathing on the garage side. Given the choice
... say between three layers of 5/8's on the garage side vs. two layers of
5/8's with RC in the middle, I'd opt for the later. RC is a lot a lot easier
to put up than a third layer of drywall ... its just a matter of metal strips
horizotally attached 6" from the floor and ceiling ... and no more then 24"
apart in between. There is no finishing (mud-tape-caulk work), and the
finished STC is probably higher (I can't find a direct comparison of three
layers vs, two layers with RC, but the wall ratings that include RC jump up
there pretty high.

Also consider that using RC as a decoupling layer also serve increase the
differentiation of resonances between the layers, which would flatten out the
STC curve (I would guess that a free floated panel on horizontal RC will have
very different phase and resonances than the panel firmly mounted to vertical
studs ... with different spacings ... even when the same material is used.

Bottom line ... RC is a proven performer ... its easy to install too.

Scott R. Foster
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