# Insulation in plastic: Should I remove?



## gacole2000 (May 25, 2006)

I am working on my dedicated theater today. The insulation I got is wrapped in plastic. The R-38 for the floor joists has little holes in it but the R-13 for the walls does not. Should I remove the plastic for better absorbtion behind the drywall (double layer w/ green glue and ISO clips on a common wall and the ceiling)?

Thanks- Greg


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## tonyvdb (Sep 5, 2007)

Removing the plastic will not improve the absorption of the insulation once it has been placed behind the drywall so just leave it particularly if the is an exterior wall as you must have a vapor barrier between it and the drywall.


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## gacole2000 (May 25, 2006)

Thanks!


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## Ted White (May 4, 2009)

I agree with Tony. The absorption, especially with mid and lower frequencies will not be negatively affected with the plastic.

The ceiling would be fine with R19, also. If you have the R38 already installed, that's fine.


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## Space (Jul 18, 2007)

A vapor barrier(plastic) should be one, not multiple layers as this will develop a dual barrier and essentially lock moisture inside the two.


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## Ted White (May 4, 2009)

Curious if that would be the case if the plastic bag remained sealed?


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## Space (Jul 18, 2007)

It is running a risk, in my experience. 

But everything matters, and I know very little of the build perimeters as of this post.

Just throwing this out as there is often confusion on what a barrier is, do I REALLY need one, where it should go, do I need more then one, etc.


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## Ted White (May 4, 2009)

Got it. Thanks


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## gacole2000 (May 25, 2006)

I have those same questions! This is a basement build. 2 new walls w/ ISOMAX clips, double drywall and greenglue (ceiling also). The exterior foundation wall I treated with heavy Drylock (wrapped corner). The other foundation wall has the garage above it. Never had any water issues. I plan on using the R13 left in the plastic, solid side facing the wall. Yes, it will be a tight fit, but I can make it work. Any advice is much appreciated. Thanks!


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## Ted White (May 4, 2009)

Are your two new walls up next to foundation walls or shared walls with other rooms?

I assume the bagged insulation is rated for exterior walls, though I've never inquired.


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## gacole2000 (May 25, 2006)

New walls go to common areas. It is rated for exterior walls. It will not be sealed as it comes in a roll and I cut it to length so the top and bottom are open.


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## Ted White (May 4, 2009)

Had you considered staggered or double stud walls? Great low frequency performance. Might save a few $. Or use the clips on the ceiling, maybe.

Just a thought


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## gacole2000 (May 25, 2006)

Considered it, but I needed to decouple the header from the floor joist, so I went with the clips and will be using them on the ceiling also.


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## Ted White (May 4, 2009)

Obviously that's fine. You'll love it.

For others that may be reading, and have interest in a double wall, you can also decouple the top plate of the inner wall from the joists with a different type of clip.


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## gacole2000 (May 25, 2006)

I took out some beams so that wall became load bearing. I'm turning my riser into a bass trap and will put treatments on the walls. My weak link is probably my return air vent. The HVAC guys didn't catch my concept. I did line it with insulation and a layer of drywall (used flex ducts w/ 2 90 degree angles).


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## Ted White (May 4, 2009)

Thanks for that clarification. So a fixed single load bearing wall. Installing a 2x4 wall in front of that would eat up too much room space at this point.

Clips decouple and are designed for such instances where you can't conveniently decouple with staggered or double stud.


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## Ted White (May 4, 2009)

Curious, are the air ducts in a soffit or ceiling?


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## gacole2000 (May 25, 2006)

Ceiling.


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## Ted White (May 4, 2009)

I see. Had you thought about placing the duct in a soffit? Better isolation is why I ask.


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## gacole2000 (May 25, 2006)

Too low of a ceiling and space constraints. I wish I had more room, but I am working with what I had.

Thanks- Greg


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## Ted White (May 4, 2009)

We all have had to marry practicality with performance and aesthetics. You'll love your room.


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## Space (Jul 18, 2007)

gacole2000 said:


> I have those same questions! This is a basement build. 2 new walls w/ ISOMAX clips, double drywall and greenglue (ceiling also). The exterior foundation wall I treated with heavy Drylock (wrapped corner). The other foundation wall has the garage above it. Never had any water issues. I plan on using the R13 left in the plastic, solid side facing the wall. Yes, it will be a tight fit, but I can make it work. Any advice is much appreciated. Thanks!


If the basement is underground you wouldn't need or want a vapor barrier.


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## Ted White (May 4, 2009)

I see this asked a lot and different replies. A search will turn up different answers also.

So Brien you subscribe to the no-need school of thought? Even if there are bare concrete walls and fiberglass insulated stud walls?

Thanks for the insight.


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## Space (Jul 18, 2007)

If the basement is underground, what would be the need for either a barrier or insulation 

Essentially you would trap the moisture in a dual barrier which would not allow escape in either direction and allow the moisture to condense on framing members and the added insulation. 

If it is above ground then there are different schools of thought, but it is a simple process. In a simple form(I'm a simple guy) vapor moves from the warm(high concentration of moisture) side of a barrier to the cold(low concentration of moisture) side.


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## Ted White (May 4, 2009)

Well, that's a good point about insulating (for thermal reasons) in the first place.

So what's your take if a person is installing insulation for soundproofing? Vapor barrier? (I'm thinking you yourself would not?)


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## tonyvdb (Sep 5, 2007)

Space said:


> If the basement is underground, what would be the need for either a barrier or insulation


It totally depends on what part of the country you live in. In the Northern US and all of Canada you are required to have a vapor barrier on all exterior walls including basement walls if you have any sort of insulation and drywall over top of it due to the cold in the winter. For example we have one wall in our basement that is just bare concrete because its just a storage room but the walls are wet with condensation all winter long because the outside temperature is so cold even the ground down to about 4' is frozen solid.


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## Space (Jul 18, 2007)

My only interest in the OP's questions where that he may be duplicating a vapor barrier.

I agree with much of what the Industry suggests, being in the construction trade myself. Pretty much anything that goes on in these kinds of builds, home theater, recording studio, etc, are making use of known practices, it isn't new stuff as you most likely know.

So if it works, I agree with it


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## Space (Jul 18, 2007)

tonyvdb said:


> It totally depends on what part of the country you live in. In the Northern US and all of Canada you are required to have a vapor barrier on all exterior walls including basement walls if you have any sort of insulation and drywall over top of it due to the cold in the winter. For example we have one wall in our basement that is just bare concrete because its just a storage room but the walls are wet with condensation all winter long because the outside temperature is so cold even the ground down to about 4' is frozen solid.


I would think a sump pump and a dehumidifier would be more in line to help with extraction.
Is the wall underground? Does this area only receive intermittent heating or none at all?


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## tonyvdb (Sep 5, 2007)

Space said:


> I would think a sump pump and a dehumidifier would be more in line to help with extraction.
> Is the wall underground? Does this area only receive intermittent heating or none at all?


Are you asking me personally or just making a general statement?

If your asking me, no a sump pump or dehumidifier will not solve the problem as the inside temperature is around 72 F all year round but outside the temperature fluctuates depending on the time of year in the winters it can get as cold as -30 F and in the summer it gets up to 90 F
The basement walls are well into the ground (about 7')


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## Space (Jul 18, 2007)

Yes, I was ask you personally. 

I am thinking that an underground wall that is constantly sweating may have an outside waterproofing issue, but that too, could be incorrect.

As they say, I stand corrected on the vapor barrier on an underground wall system.


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## gacole2000 (May 25, 2006)

Yes, my insulation soley for sound absorbtion. If I don't need a barrier, I would love to split the R13 in half (thickness) and save some hassle (very tight fit w/ studs on side) and cost. Thanks- Greg

The walls are completely underground and I live in SW Missouri.


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## gacole2000 (May 25, 2006)

I'm a little confused now. Should I use a moisture barrier or not for the foundation walls that are underground?

Thanks for the help!


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## Ted White (May 4, 2009)

gacole2000 said:


> Yes, my insulation soley for sound absorbtion. If I don't need a barrier, I would love to split the R13 in half (thickness) and save some hassle (very tight fit w/ studs on side) and cost. Thanks- Greg
> 
> The walls are completely underground and I live in SW Missouri.


Maybe you could clarify this for my benefit. You have below grade stud walls in the basement. I believe you could install the vapor barrier next to the foundation. Then you should have plenty of space for the R13, right? Just one layer in the stud walls. You moved the framed wall an inch or so away from the foundation?



gacole2000 said:


> I'm a little confused now. Should I use a moisture barrier or not for the foundation walls that are underground?
> 
> Thanks for the help!


I believe you could. I didn't personally, but it seems most do.


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## Space (Jul 18, 2007)

Greg,

Please forgive my mutilation of this thread It was not my intention. I was thinking that you already had an interior vapor barrier, and did not want to see you add an additional one.

That said, in areas like yours that stay cold at a higher percentage then are warm, the vapor barrier is needed. The problem with moisture in this type of climate, and Canada as well, the moisture is inside your home trying to get out. Warm moving to the cold side.

It can be compounded by materials that are porous, like concrete block, that tend to retain moisture, the same moisture that is trying to get out of your home.

So if you are installing plastic covered insulation which is in essence, a vapor barrier, then that is all you should need. It is the vapor barrier, I believe the company sells it with this purpose.

As Ted alluded to, you don't want the barrier to come in contact with the exterior wall.

Or maybe it was low frequency he was talking about?

Anyway, hope that helps you, and gets me out of the dog house....


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## gacole2000 (May 25, 2006)

No worries. I appreciate the help! I'm not sure how I could do it without the insulation (in the plastic bag) touching the exterior facing concrete wall. I had access to free 2x4s so I turned them on their side as firing strips. Therefore I have an 1 1/2" of area 16" OC and the R13 will be pretty compacted and obviously touch the foundation.


Thanks again- Greg


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## tonyvdb (Sep 5, 2007)

Doesn't the plastic around the insulation have holes in it? I would think the plastic wrapped around it is just to aid with installing it but has no vapor barrier properties.


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## MatrixDweller (Jul 24, 2007)

To back up Tony...

There is a big debate over how basements should be insulated. The common and most used method is to stud the foundation and put insulation between them, then a vapor barrier over the whole stud surface. The sill plate is gasketted with vapor barrier also so that it doesn't wick up any moisture from the floor. Floors are typically covered with vapor barrier and insulated or another type of membrane is applied (like Drycore).

The vapor barrier always goes on the warm side. The foundation will always be cooler then the inside air (unless it is mostly above ground and/or exposed to warm air and/or sunlight as in a walkout basement). Water vapor comes through the cement as cement is porous. The vapor is invisible and is not the same as a leaky foundation that is wet. 

Typically the water vapor in an unfinished basement is drawn into the basement and gets mixed with the home's air. The moisture coming through the walls is not great but is augmented by humid air from the rest of the home (ie: an open window, having a shower without using an exhaust fan, etc) since water vapor is heavy and will go to low spots.

In a finished basement, with insulation and vapor barrier, the moisture from the walls stops at the vapor barrier. Since the barrier is on the warm side it does not condense and stays as a vapor. If the barrier were to be on the cool side the vapor would condense into liquid. In the absence of a vapor barrier the vapor would penetrate the insulation and drywall and enter the basement air. 

The opposite flow is also true where the air from the home goes through the drywall but stops at the barrier. It does not condense. If the barrier were on the cool side or there wasn't any insulation or both it would condense on the cool surface just like a it does on a cold glass of beer on a humid day. The condensation could then pool on surfaces that have the potential to grow mold and rot.

It is a very bad idea to put a barrier between floors of a house, on the cool side of a wall, or without insulation. If in between floors you risk destoying the joists to moisture damage not to mention the toxic release of spores. If you've been in quite a few finished basements your nose can usually tell which ones were done properly.


*In regards to your insulation that is surrounded in plastic...are you sure that's not just the over wrap. If you were to remove it there would be more than one bale inside. Either that or it's there to stop the insulation from getting all over the place (itchy) or it's loose stuff that is meant to be blown in. That's the only time I have ever seen plastic around insulation. How about you take a picture of one and show us?*


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## Ted White (May 4, 2009)

Probably the best summary I have read to date on the topic. Thanks for the visual description.


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## Space (Jul 18, 2007)

Indeed. I think we are all in support as it applies to a cold or frigid climate.

Are these comments at odds with each other?
"Floors are typically covered with vapor barrier and insulated or another type of membrane is applied (like Drycore)."

"It is a very bad idea to put a barrier between floors of a house,"


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## MatrixDweller (Jul 24, 2007)

Space said:


> Are these comments at odds with each other?
> "Floors are typically covered with vapor barrier and insulated or another type of membrane is applied (like Drycore)."
> 
> "It is a very bad idea to put a barrier between floors of a house,"




Just the slab. 

Between floors meaning basement to first level...first level to second level...etc


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## Space (Jul 18, 2007)

In my area it is common practice to install the vapor barrier before the concrete is ever poured. In any area I have ever been in really. But I haven't been to Canada


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## tonyvdb (Sep 5, 2007)

Space said:


> In my area it is common practice to install the vapor barrier before the concrete is ever poured. In any area I have ever been in really. But I haven't been to Canada


Im a little confused by what you say here, I think your referring to the water proofing done to the exterior of the basement. A totally different thing. 
The vapor barrier stops condensation. Think of a pop can you have just taken out of the fridge on a hot day, condensation builds up almost immediately on the outside of the can (think of the can as your exterior walls). If you were to place the can inside a Ziploc bag immediately after removing from the fridge you reduce or eliminate the condensation all together as you remove the ability of the can to contact the outside air. 
An air conditioned house can also cause condensation buildup if the cold air hits the warm exterior walls so this also is a concern for hot climates.


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## Space (Jul 18, 2007)

No sir, I'm not confused even a little

He typed slab. That means it is on the ground.


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## MatrixDweller (Jul 24, 2007)

You are right about poly going under the slab, but it's still code to have sill gasket and no materials should stay in contact on top of the slab without a moisture barrier (ie: sleepers for subfloor, carpet, etc). 

If you go down to your basement and tape a peice of plastic to the concrete floor and leave it there for a week, you should see a small amount of condensation on the inside of the plastic. Same reason why a cardboard box left on the floor for a long time will start to deteriorate.

One practice that is starting to catch on is to insulate the slab from underneath. For the minimal added cost it pays for itself in a matter of years with savings on heating. You can also insulate the foundation walls from the outside. Good construction would do that along with a dimpled membrane for water abatement.


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## Space (Jul 18, 2007)

Thanks.
As you were saying this(digitally speaking) I was already reading about it here:http://www.buildingscience.com/documents/profiles/designs-that-work-cold-climate-minneapolis-profile/?searchterm=moisture%20control%20vapor%20barrier

I see how this would be a far better remedy then simple solutions that tend to break down over time and create costly re-habs.


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## gacole2000 (May 25, 2006)

I would agree that the plastic on the insulation is most likely to aid in keeping it from getting all over everything. One side is solid and the other has holes, but most likely not designed for this purpose. Therefore, should I go buy some plastic and string it up against the concrete or over the insulation before the drywall? Is the wall with drylock OK or should I do it also? What about the floor...will the carpet and pad be OK? I hope to really crank this project up this weekend and start hanging drywall so please let me know what is best because I want to make sure I get this right!

Thanks for the help- Greg


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## MatrixDweller (Jul 24, 2007)

tonyvdb said:


> An air conditioned house can also cause condensation buildup if the cold air hits the warm exterior walls so this also is a concern for hot climates.


The problem is when the vapor comes into contact with a surface that's temperature is_ below the dew point_. In an air conditioned house, the vapor from inside the house (minimal because the A/C removes a lot) hits the barrier and doesn't condense (unless you live in a freezer). The vapor outside the house shouldn't condense either unless the temperature of the barrier is as low as the dew point. In most cases that would mean below 65°F (18°C). The HVAC ducts and cold water plumbing usually is but the inside of the wall should not be.

If it is exteremly humid outside then it is most likely very hot also. If it's 110°F and humid outside and you have the A/C at >75°F then you could have some problems (not just your electric bill). 

I live in Southern Ontario Canada and the weather is very cold in the winter (although not as cold as Alberta) and very hot and humid in the summer. The houses around here do not have mold problems in the walls.


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## MatrixDweller (Jul 24, 2007)

gacole2000 said:


> I would agree that the plastic on the insulation is most likely to aid in keeping it from getting all over everything. One side is solid and the other has holes, but most likely not designed for this purpose. Therefore, should I go buy some plastic and string it up against the concrete or over the insulation before the drywall? Is the wall with drylock OK or should I do it also? What about the floor...will the carpet and pad be OK? I hope to really crank this project up this weekend and start hanging drywall so please let me know what is best because I want to make sure I get this right!
> 
> Thanks for the help- Greg


Put the insulation between the studs then staple Poly6 over them and seal any seams with tuc tape. Leave a few inches below the wall so that it can meet up with the floor. Throw up your drywall and finish it (finish before carpet is down for easier cleanup). Then continue the vapor barrier over the entire floor and tape the seams and where it meets the wall. Now you can put down the underpad and carpet on top of the poly. That is unless your underpad is meant for basement application and already has a vapor barrier attached to it.

Drylok is a moisture barrier not a vapor barrier (ie: It's water proof, but gas can still pass through it).


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## Space (Jul 18, 2007)

Greg,

I think you need to identify the type of insulation you have from reading the data that is printed either on the packaging or directly on the batts. I still am thinking you have one of the vapor barrier insultion packages, but you have not mentioned otherwise. The holes on the outer side(placed facing away from the warm side) do have a function. To not be a dual barrier  and to protect persons from air borne particles...maybe even as a means to allow the vapor a chance to get closer to the vapor barrier temperature.

Where I am, typical kraft paper face(retarder) type insulation is, well, typical. But I am certain that you can get this type where you are.

That said, it could very well be the packaging that you are talking about and only you can clarify that for us.


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## Space (Jul 18, 2007)

oops, sorry Matrix. Didn't see the post.

It's probably more helpful then mine overall anyway.


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## Space (Jul 18, 2007)

Did I apologize for being an idiot yet?


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## gacole2000 (May 25, 2006)

I have an extra roll of Tyvek, could I use this as a moisture retarder and be OK?


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## gacole2000 (May 25, 2006)

I think it is only for comfort from what the label says but has a retarder.

Thanks- Greg


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## MatrixDweller (Jul 24, 2007)

Tyvek is water proof but not a vapor barrier. It's meant for the exterior walls and applied to the studs before the siding, stucco, bricks, etc. 

The wall sandwich would be brick - Tyvek - sheathing - studs and insulation - vapor barrier - drywall.

http://www.turnertimberframes.co.uk/resources/Standard.jpg

You could use it on the foundation wall...but it's not nessessary. The studs should not be touching the foundation and any water abatement _should _be taken care of from the outside.


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## gacole2000 (May 25, 2006)

Pic


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## MatrixDweller (Jul 24, 2007)

I think that length is meant for attic or crawlspace installation. You would end up having to cut it to fit in the walls. Remember no vapor barrier between floors.

Here's the product page
http://www.jmhomeowner.com/products/product.asp?category=FiberGlass&Product=ComfortTherm

See the Installation tips

Batts would be much easier, especially in basement walls since they are not the sandard 96" high usually. Batts in celings work well too. Take a look at Roxul if you can for celings as it provides excellent accoustical properties and is fire proof.


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## Space (Jul 18, 2007)

Actually at 32 feet long per roll, cut in 8 feet lengths, it should be fine for a standard eight foot tall wall.

A bird in the hand and all that.


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## gacole2000 (May 25, 2006)

So should I be OK to use this product, retarder facing the drwall?


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## Ted White (May 4, 2009)

Great thread. Love the details.

The thought of having a continuous piece of poly from the wall then across the floor has me worried about flanking sound under the wall. Fine to have poly under the stud wall (bottom plate) but that front edge needs to be sealed. Similarly after the drywall is installed, fill that crack under the drywall with sealant.

Then continue with the poly on the floor if desired.


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## gacole2000 (May 25, 2006)

I already have my studs up, so I have to work around what I may have already screwed up. I think I am going to DryLock this wall also and put the insulation in the bag with the retarder against the drywall. The ground above is covered with a three car garage and I have a pretty good HVAC system in place. Do you think this will be OK?

Ted. I have LOTS of sealant to use on all joints!


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## Ted White (May 4, 2009)

With the studs turned sideways, it appears as though the cavity is too shallow for insulation. Are the studs contacting the walls?

Generally the studs are turned 90 degrees so the 1.5" face is ready for drywall attachment. This also gives you the space needed for R13 or so.

There is a garage under the room? Is there a slab in between? Slab = your floor and garage ceiling?


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## gacole2000 (May 25, 2006)

It's a tight fit, no doubt, but I need something in there and already have it on site. I guess I could use ridge, but I thought that was not good for absorbtion. On the other side and up is the garage. the are glued and RamXed to the wall as firring strips.


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## Ted White (May 4, 2009)

So the studs are attached to the concrete.

Are they, in fact sideways?

I'm thinking you have less than 2" of air cavity depth. This is a problem in and of itself. It would be made worse by installing insulation and compressing.


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## gacole2000 (May 25, 2006)

Yes
Yes
1.5"

I can get some 1" ridgid foam board from another site if needed.


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## Ted White (May 4, 2009)

Well, what you need is to start over.

Your air cavity is too small. This will cause a higher frequency resonance than we want. I can forward some data on that if interested.

Your walls should be decoupled from the concrete, but they're rigidly connected.

Is that possible?


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## Ted White (May 4, 2009)

Rigid board will simply reduce the effective air cavity volume.


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## gacole2000 (May 25, 2006)

No, starting over is not an option. The studs don't connect to the mudsill, only 10" concrete walls. I will be using double drywall and LOTS of GG. A compromise on costs.

I'm just trying to figure out what I should do for a vapor barrier.

None
Plastic
Compressed R13 insulation w/ retarder
Drylock (must buy)
Faced ridgid foam board
Tyvek
Or a combination of the above???

These are options I have available from left over jobs and thus can afford (family deal).

From what I have read, I do not want to leave the cavity empty and obviously don't want a mold problem...


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## Ted White (May 4, 2009)

Best you can do at this point:

Open the plastic covered batts.
Tease the fiberglass in half
Apply the 1/2 thickness insulation into the cavity
Apply vapor barrier (plastic)
Drywall

Are the two other "non-foundation" walls decoupled?

Will the ceiling be decoupled?


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## gacole2000 (May 25, 2006)

Yes, two others and ceiling use ISOMAX clips. GG.


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## gacole2000 (May 25, 2006)

Is it OK for the insulation to be touching the wall? I'll put plastic up right before the drywall...right?

Thanks- Greg


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## Ted White (May 4, 2009)

I believe the insulation is OK for light contact against foundation. Install plastic just before installing drywall.

Clips are great on ceiling, but you'd do better if you convert the existing single stud walls to staggered instead of using clips: http://www.soundproofingcompany.com/upload/SPC-Staggered-Stud-Wall-Guide.pdf

Saves money, works better


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## gacole2000 (May 25, 2006)

Thanks Ted, I'll wait to get some others to weigh in to make sure we have a consensus solution.

As mentioned, it is a load bearing wall and I used the clips to decouple the wall from the header and thus the floor joists.


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## Ted White (May 4, 2009)

gacole2000 said:


> As mentioned, it is a load bearing wall and I used the clips to decouple the wall from the header and thus the floor joists.


Please excuse me. Of course you described this earlier. I am on 5 forums this AM and it can get confusing.

Having said that, I would still have you consider the wall conversion. No reason it can't be done. As I said, the staggered wall is a better option than clips.

But either way is fine.


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## Space (Jul 18, 2007)

gacole2000 said:


> No, starting over is not an option. The studs don't connect to the mudsill, only 10" concrete walls. I will be using double drywall and LOTS of GG. A compromise on costs.
> 
> I'm just trying to figure out what I should do for a vapor barrier.
> 
> ...



You will help your case a little if you can install another 2X4 flat directly unto the ones you attached to the basement. Thing is, it is not going to be a mass-spring-mass because of the connection with the fasteners. It is already short circuited.

Never-mind that pressure treated lumber should have been used. 

Crushing insulation down defeats the ability of this product to do it's primary job, warm up vapor, and secondary job to convert frequency to heat.

That said, on that one specific wall you showed, leave the plastic retarder on the insulation and use it if you must. Ridgid insulation would be my material of choice, in this instance, due to the shallow cavity and the direct connection with the source of potential exterior seepage.

The drylok as mentioned earlier is simply another retarder, about the same process as painting the interior side of a wall. Install the plastic on the wall like Matrix has mentioned.

I haven't warmed up to a vapor barrier on the concrete floor yet either, but for different reasons. I think it was mentioned to place a 12 inch square of poly taped to this basement slab floor to detect if moisture is present in the floor. This I would do first before committing to what could be an additional barrier.

Drylok is fine, the retarder faced insulation is OK, and the poly barrier is fine as well.

Tyvek is not going to do anything but create more work. By design, it allows vapor to pass thru to allow a wall assembly to dry to the outside and protect the same assembly from moisture entry from the outside.


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## Ted White (May 4, 2009)

I'm wondering if this might actually be a situation where foam should be used tightly... to eliminate the air cavity and therefore cavity resonance altogether.


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## MatrixDweller (Jul 24, 2007)

Ted White said:


> I'm wondering if this might actually be a situation where foam should be used tightly... to eliminate the air cavity and therefore cavity resonance altogether.


That depends on the type of foam. Closed cell foam (styrofoam, cyanurate, etc) is not a very good absorber of sound energy. It's also pretty flammable and needs to be covered. It can also be more expensive and harder to work with. I would choose rockboard over anything foam.

You can always stuff the cavity completely with fiberglass or mineral wool batts shaved in half (or boards). Stuffing with that will actually make the cavity into a trap.

The studs touching the wall is a bad thing though and will eventually cause them to rot unless they are pressure treated or have a gasket between them and the foundation. There is normally an inch or so gap between the studs and the foundation. I would rip them off and redo the wall now rather than 5 years from now when you start smelling a musty smell coming from the wall. After it's sealed up there will be a lot less airflow inside the wall cavity which will accelerate the wood's decomposition since there will be less evaporation. 2x4's are cheap now with the house market crashed.

I wouldn't worry about staggering the studs. That's good if you're attaching drywall on either side so that the transmission from room to room is less. As long as the cavity is filled sufficiently you should be good. One area that could benefit from extra soundproofing would be where the studs meet the ceiling and floor. Decouple that and there will be less bass traveling up the wall and into the joists above it.

I personally would not even mention Drylok any more. In my opinion, if it's not used on both sides of the wall it will cause the concrete to retain moisture and cause it's premature deterioration. If the wall is extra dry on both sides the paint would do no harm, but would just be a waste of time and money (since you wouldn't see it).


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## Ted White (May 4, 2009)

When we're dealing with air cavities, we're looking for how much volume is in the cavity. If you're at 3" or more, you're in good shape. Less than that, and you're going to have resonance issues. 

So there are times when it's best to have no air cavity at all. That was my point. Filling that small cavity with foam would essentially take the scenario from a libelous 1.5" to a less harmful 0".

Filling with compressed fiberglass or mineral wool isn't at all the answer in this case. Small air cavities are a problem, as I've stated earlier.

Regarding the other 2 non-foundation walls, there would be an enormous benefit to staggering them. Foundation walls with a 1" space away from the foundation are already decoupled and therefore would not derive benefit from staggering, as they are already decoupled.

That make sense?


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