In a typical decoupled framing assembly, this would be what your wall section will look like...please imagine that the insulation is already in the stud cavities on both frames:)
The vapor barrier/retarder may not be doing what you are thinking it is doing. Warm air has more moisture than cold and the process is in simple terms, the warm air moves to the cold side. So when this warm air starts moving from the warm side (usually in a cold environment this will be from the inside of the structure) to the cold side (in a cold environment this will be the outside) the warm vapor encounters this "barrier" and condenses on it.
Like taking a cold aluminum can directly from the refrigerator into a warmer atmosphere, the transfer of "from the warm side to the cold side" is seen visibly on the exterior of the can, since it is the warmer side of this transaction.
Your HVAC is left with the service of extracting the condensation, the wetting of the assembly from the interior side.
Exterior wall concept
Originally posted at johnlsayers.com, topic 15096.
From what i understand the building wrap breathes outward only in order to let moisture in the wall out. The vapor barrier goes on the hot side of the wall (inside) and any moisture goes out the exterior. I have been asking around about this, what it seems to come down to is if drywall is considered a vapor barrier or not since it will be caulked tight it may be an issue. It is common practice to put the drywall on the outside of the shear membrane for fire guard purposes but it isn't to seal it up like we are discussing. **********Quick edit: sorry i did not see the above post before i posted this....why would there be condensation in that space because you add drywall? you don't have air flow through there now? most construction uses an exterior vapor retarder to seal the building. then again, if you need to ventilate that space you'll have to consider some form of baffled air flow to ensure isolation.
Ok makes sense. What about the moisture inside the cavity, what happens to that?In a typical decoupled framing assembly, this would be what your wall section will look like...please imagine that the insulation is already in the stud cavities on both frames:) The vapor barrier/retarder may not be doing what you are thinking it is doing. Warm air has more moisture than cold and the process is in simple terms, the warm air moves to the cold side. So when this warm air starts moving from the warm side (usually in a cold environment this will be from the inside of the structure) to the cold side (in a cold environment this will be the outside) the warm vapor encounters this "barrier" and condenses on it. Like taking a cold aluminum can directly from the refrigerator into a warmer atmosphere, the transfer of "from the warm side to the cold side" is seen visibly on the exterior of the can, since it is the warmer side of this transaction. Your HVAC is left with the service of extracting the condensation, the wetting of the assembly from the interior side.
one other thing, i have searched and searched and not found a definitive answer............
Do you need to mud and tape the first layer of drywall if using green glue between layers???
Just seal the seams?
Even out the taper?
What is the minimum?
Oh, 2 -29 ounce tubes per 4x8 sheet btw.
Well, what makes you think there is moisture in the the interior of the wall?
Moisture is a product of air infiltration which is why it is such a demanding aspect of building to stop air from moving into or out of an assembly.
Assuming your house is adequately prepared for the climate you exist in, do you think there is moisture in the cavity existing between interior sheathing and exterior sheathing of this home?
Most likely not.
The goal for engineers when making up the assemblies that will be used in different climates is to have a balance of materials that allows the wall to dry to the exterior and to the interior. Seems simple enough.
Vapor is not moisture...vapor is classified as a gaseous material. Moisture is the accumulation of vapor.
But direct to your question, what does happen to the moisture inside the cavity? If it exists then it has opportunity to collect and do one of several things. One would be to dry up, the other being to begin the rotting process. But this is moisture..not a hose full of water running for seven hours...the difference in the two can take decades to produce any noticeable depreciation in the materials that may be involved or that will be come into contact with by this moisture.
"From what i understand the building wrap breathes outward only in order to let moisture in the wall out. The vapor barrier goes on the hot side of the wall (inside) and any moisture goes out the exterior.
I have been asking around about this, what it seems to come down to is if drywall is considered a vapor barrier or not since it will be caulked tight it may be an issue. It is common practice to put the drywall on the outside of the shear membrane for fire guard purposes but it isn't to seal it up like we are discussing."
Building wrap also called a drain plane breaths, it is vapor permeable....period...inside outside...it don't care what side:) It is a retarder by difinition but is in fact a part of the assembly that allows condensation to be shed from the structure...the condensation that happens on the exterior side of the building due to environmental climate changes. E.G. The siding gets sun and heats up, we already know that warm air has more moisture than cold air, so the back side of the siding will sweat and this barrier, the drainage plane, allows this condensation to run off or exist and not be an issue or have a direct negative impact on the structure sheathing.
I already explained the vapor barrier/retarder/vdr, so your assessment of how it fits into the scheme and to where what is actually going should have already been corrected, hopefully.
Sheetrock or most any building material is not now nor never was considered a vapor barrier. A class one vapor barrier of which typical visqueen plastic makes that grade, is the number two on the list. A class one vapor barrier is foil...we already discussed the aluminum can, so that should be fresh in your mind and be a credible example of why this is why it is.
But let's entertain the notion that perhaps there is moisture in this cavity. We still have "high pressure to low pressure" or warm side to the cold side working in our favor since we can all agree that this cavity that we use with double thicknesses of insulation, will be hot in deed. But this cavity is not a vacuum...it is an enclosed air space capable of being manipulated by either one, hot or cold depending on the season and geographical location of the assembly.
"It is common practice to put the drywall on the outside of the shear membrane for fire guard purposes but it isn't to seal it up like we are discussing."
Correct, it is to make a fire rated wall of "X" hours based on depth of stud, type of stud and thickness of drywall and how many layers, which is event specific.
Ok, lets say this is accepted as cool(and dry :wink: )
Rather than cut strips of drywall and add to the inside of the studs, how about just layering drywall on the outside over the osb(shear membrane) and putting the building wrap on that. It would be a hell of a lot easier and faster. Caulk it up and off we go.
No?
Are you building this...and are you a qualified builder or are you just asking questions?
You have options in this build if it is not already built. But installing drywall to the exterior of the sheathing is not the answer. You can install heaver gauge OSB/sheathing and multiple layers...and exterior gypsum sheathing in a configuration that should get you the mass you require rather than installing strips in stud bays.
Do you have any prints for this build...floor plans?
It's to here already.....
Obviously i don't want to pull the insulation out and add strips of drywall inside the studs.
The osb on the exterior is one layer of 1/2" at this point.
Have a look at page 3 of this thread for a rough outline of the interior. I have revised it a bit but basically it's the same. download/file.php?id=36835&mode=viewDo you have any prints for this build...floor plans?
maybe cement board would suffice? it should be safe to use (or a type should be) on the exterior and will add the mass required. however it is likely to be more costly.
I need to cover roughly 1750 sq ft. 1 layer 1/2" cement board =$2643 1 layer 3/4" osb =$1015 1 layer 5/8" Drywall = $881 I'm looking into TL stats of the cement board.........maybe cement board would suffice? it should be safe to use (or a type should be) on the exterior and will add the mass required. however it is likely to be more costly.
You don't need to, since you won't be using it on its own. All that really matters is the mass. Cement board is about 3 or 4 times the density of OSB, and twice the density of drywall, roughly. Therefore a 1/2" of cement board is roughly the same mass as two inches of OSB or one inch of drywall. Do the math: Cement board is actually your cheapest option! You might even be able to get thicker cement board, and really beef up your isolation. Or put two layers of it on there. It all depends on how much isolation you need: What is your number on that? What total TL are you aiming for? - Stuart -I'm looking into TL stats of the cement board
In my research I am getting 2.4(Durock 1/2")- 2.6 (Canwell 1/2")psf of the cement board and 2.3psf of the 5/8ths drywall. ??? This product http://www.certainteed.com/resources/CT ... _FINAL.pdf is showing 2.4psf and is safe outside, easy to put up, no special screws etc and runs me $1795 per layer.You don't need to, since you won't be using it on its own. All that really matters is the mass. Cement board is about 3 or 4 times the density of OSB, and twice the density of drywall, roughly. Therefore a 1/2" of cement board is roughly the same mass as two inches of OSB or one inch of drywall. Do the math: Cement board is actually your cheapest option! - Stuart -I'm looking into TL stats of the cement board
It has been hard to get a non windy, non snowy, non work, non minus frikkin' too much celcius day to do that test. Although i do very much plan to do it. Meanwhile i am researching options for methods by which the sound proofing can be done and planning on an stc 60-70 system.It all depends on how much isolation you need: What is your number on that? What total TL are you aiming for? - Stuart -I'm looking into TL stats of the cement board
You need to mud tape to stop air penetration and to maintain the fire rating of the wall. Some have caulked the edges/seams, but the cost and ease of mud and tape versus acoustic caulking is easier to my thinking.one other thing, i have searched and searched and not found a definitive answer............ Do you need to mud and tape the first layer of drywall if using green glue between layers??? Just seal the seams? Even out the taper? What is the minimum? Oh, 2 -29 ounce tubes per 4x8 sheet btw.
ThanksYou need to mud tape to stop air penetration and to maintain the fire rating of the wall. Some have caulked the edges/seams, but the cost and ease of mud and tape versus acoustic caulking is easier to my thinking.one other thing, i have searched and searched and not found a definitive answer............ Do you need to mud and tape the first layer of drywall if using green glue between layers??? Just seal the seams? Even out the taper? What is the minimum? Oh, 2 -29 ounce tubes per 4x8 sheet btw.
"In my research I am getting 2.4(Durock 1/2")"
I wonder if you want to look into a James Hardy product, a cement based exterior siding. Durock and the like are going to be labor intensive in the installation of these smaller square footage panels, require specific screws and the warranty is most likely void in this application.
An exterior gypsum based 1/2" sheathing plus a James Hardy 32 sq. ft. per panel siding should get you where you are going with the existing 1/2" OSB sheathing.
I think Canwell and Hardie have merged now. Cement board siding crossed my mind and is an option for the exterior. I wonder about the fact that it isn't a totally sealed surface though. The panel siding looks like it needs a 1/4" air gap left on horizontal connections. But the Lap siding does look pretty tight as you can caulk the edges."In my research I am getting 2.4(Durock 1/2")" I wonder if you want to look into a James Hardy product, a cement based exterior siding. Durock and the like are going to be labor intensive in the installation of these smaller square footage panels, require specific screws and the warranty is most likely void in this application. An exterior gypsum based 1/2" sheathing plus a James Hardy 32 sq. ft. per panel siding should get you where you are going with the existing 1/2" OSB sheathing.
On the ceiling. I talked with the rep at Kinetic sound control today.
With these clips http://www.kineticsnoise.com/arch/wave/index.aspx and hat channel and 2 layers of 5/8" drywall on the trusses, he claims an stc 60 rating. That seems a bit much. But i plan to use green glue between the layers.
Any thoughts on this approach?
If you plan to build interior decoupled frames then you will have a ceiling framed on top of this framing and that would negate the need for clips and hat track.
Otherwise...your going to have to do this procedure to the entire build with a resultant lower TL
I want to avoid another framed ceiling, i would lose too much height. If the ceiling is stc 60 and the walls are stc 60 does it matter what the method is?If you plan to build interior decoupled frames then you will have a ceiling framed on top of this framing and that would negate the need for clips and hat track. Otherwise...your going to have to do this procedure to the entire build with a resultant lower TL
If an arbitrary number, or a number assigned to a specific assembly rather, is assigned then it is assigned only to that assembly.
In your case the ceiling/roof has been given a number based on someones educated estimate.
The walls have been given this same number?
In order for this to be truly the...well lets forget stc and focus on the isolation gauge of transmission loss, complete environment you are wanting, and you have spent much money on this project, you have to keep in mind the mass-air-mass, which is the decoupled room-in-a-room environment, so many westerners attempt to gain.
Doing one thing with the ceiling and another with the walls and assuming that somehow they have no relation to one another, is missing the big picture of this much over stressed always over used room in a room environment, that what you seem to be suggesting, is not.
Huh? :shock: :?: Sorry, but that makes no sense at all! The final ceiling will be at exactly the same height, no matter how you support it. The air gap and mass both MUST be the same size, regardless of the way you support it, if you want to get the same TL. It's a simple mathematical equation, based mostly on mass and air gap. You cannot beat the math and the physics by resorting to a different method. What defines the TL is mass and air gap. Therefore the final ceiling for any given TL number will ALWAYS end up at the exact same height. Like Brien said: it makes much more sense (and will most likely be cheaper) to just put joists across the top of your inner-leaf walls, and hang your ceiling from that. - Stuart -I want to avoid another framed ceiling, i would lose too much height.
Well, I didn't say it makes more sense, it will mostly inject structural complications since this is a large room. But the simple things we work with in this ride down the avenue for isolation are always the same.