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Old 10-12-2007, 10:58 PM   #1
V-10 Killer
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Default Would you put up plastic under ceiling drywall?

Seeing as how the weather isn't cooperating with my epoxying plans, I think I'm going to hang the drywall on my ceiling Monday. But there's something I was wondering about that. Would you put up a plastic vapor barrier up there?
I imagine it would help insulate, and protect the drywall from any moisture spots, but is there a risk? Example, you get some water up there, and instead of staining the drywall, it sits in your cellulose insulation and messes up your trusses. Also, I know if you put plastic up, you can't use any drywall adhesive(s).

So, is it worth the headaches (I'd be putting this plastic up myself)?
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Old 10-12-2007, 11:10 PM   #2
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Default Re: Would you put up plastic under ceiling drywall?

I wouldn't forego the adhesive for a membrain in the ceiling.

If you're going with celulose, the membrain might offer more benefit above the celulose.
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Old 10-13-2007, 07:22 AM   #3
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Default Re: Would you put up plastic under ceiling drywall?

I put 6 mill plastic between the fiber glass insulation and the sheet rock as a vapor barrier. Then I got the idea to put 1" Styrofoam onto the studs, and fur that ceiling with 1" x 2" wooden strapping. Then the sheet rock was applied. That is the warmest room of the house. As for water leaking from the roof, I don't see that as a problem for at least 20 years, if you have properly installed the shingles.
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Old 10-13-2007, 11:10 AM   #4
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Default Re: Would you put up plastic under ceiling drywall?

They also make a sheetrock that has plastic instead of paper on one side I think.
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Old 10-13-2007, 11:26 AM   #5
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Default Re: Would you put up plastic under ceiling drywall?

If you are going to heat the garage, and not the attiic above, the answer is:
NO>>>NO>>>NO>>
You put a vapor barrier on the walls, but not the celing.
You want the vapor to work its way up into the attic space and get vented through the vents you should have under the eves and along the ridge line.
Go to the insulation mfgs websites. They all have install instructions.
That sheetrock with th plastic side is meant for walls only.
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Old 10-13-2007, 01:08 PM   #6
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Default Re: Would you put up plastic under ceiling drywall?

I disagree with all install instructions then! Why?? Well lets see here, when I put up drywall with the old man as well my next shop, all seams are filled in and sanded smooth, now tell me how does vapor work its way up?

I say plastic sheet between the rafters and drywall. Staple the membrane up and use drywall screws to fasten the drywall sheets up, the longer the better.
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Old 10-13-2007, 02:00 PM   #7
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Default Re: Would you put up plastic under ceiling drywall?

Let me give you some ideas to ponder and then you decide.

First, poly is a moisture barrier not truly a vapor barrier. The distinction can be illustrated by wrapping an onion in poly and noting that the smell of the onion permeates thru the poly. Do the same using Dow Saran or some other vapor barrier film and you will not detect the onion smell once wrapped. Poly is good at keeping/preventing water transfer/migration from occurring.

Insulation is most effective if it is kept dry. I assume most know that wet insulation is a much better thermo conductor than dry insulation. A moisture barrier is employed to keep the insulation dry and therefore most effective in saving your heating dollars. I'm addressing heating but for those interested in cooling the principles are the same, only the direction of the "delta T" (the driver behind heat transfer) is reversed.

Where does the moisture come from that is desirable to keep out of the insulation? For this exercise I'll assume that the roof is really a roof and by definition keeps the rain out, further, that the roof doesn't leak. So the moisture that needs to be garded against is in the air. You ask (or should be asking) in the air inside the garage or in the air outside of the garage? Allow me to jump ahead of the example about seeing your breath as a fog on a cold day etc. and simply say that the higher concentration of moisture is in the warmer air. So if you are heating your garage, the air inside will have more water (moisture) in it, than the cooler air outside. Okay so far?

Remember now that the objective is to keep the insulation dry - most effective. People will argue this next point on the particular placement of the moisture barrier (i.e., put it on the inside vs. outside, warm side vs. cold side) but if you followed the reasoning so far and agree that it is worthwhile to spend a little for the poly to keep the insulation dry then its placement will be just as obvious. It goes on the warm side so as to keep the moisture from getting to the insulation. This is one place where doing it wrong is not the same as not doing it at all. Putting the poly on the cold side will trap and keep the moisture in the insulation greatly reducing its effectiveness.

Construction detail; If you are interested in gluing drywall to reduce finish work and midspan nail pops maybe this will work for you. The way all of the ceilings were drywalled in my house is first poly, then one layer of 3/8" nailed and a second layer of 3/8" glued with no mid span nailing/screwing. I bought the house that way and I'm not sure I would have gone to that much trouble but I'm happy the PO did in 1961.

One final point because you mentioned "risk". This is an extreme case but it did happen and has been in my data bank since witnessing it. Once upon a time in a garage far away where a moisture barrier was used on the warm side of walls and ceiling but not under the floor and a dryer vent was directed into said garage the sheetrock on the ceiling got wet enough that it was pulling from the nails. Re-routing the dryer vent and re-nailing the ceiling solved the problem. I don't park cars in my garage or shop but have wondered if enough ice and snow got brought in on cars (day after day), into a heated space if this extreme condition might be duplicated. I don't know, maybe someone here does.
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Last edited by Steve in Mi; 10-13-2007 at 02:13 PM.
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Old 10-13-2007, 07:30 PM   #8
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Default Re: Would you put up plastic under ceiling drywall?

never put a vapor barrier on ceiling, the water vapor will be trap on underside of barrier and that where the drywall is. it will destroy the drywall within six weeks.
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Old 10-13-2007, 07:44 PM   #9
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Default Re: Would you put up plastic under ceiling drywall?

Quote:
Originally Posted by brownbagg View Post
never put a vapor barrier on ceiling, the water vapor will be trap on underside of barrier and that where the drywall is. it will destroy the drywall within six weeks.
As I said, I have 6 mil poly under the fiberglass insulation, and that is covered with 1" tongue and groove Dow blue insulation board. The the entire ceiling is strapped with 1" X 2" furring strips. So far, this year it will be 25 years, and by my estimation, that is 1300 weeks and the drywall hasn't been destroyed yet. I run a humidifier on the hot air furnace all winter long, and keep the humidity about 35% / 40%, depending on outside temperature.
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Old 10-13-2007, 07:49 PM   #10
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Default Re: Would you put up plastic under ceiling drywall?

Just check the mfg websites.
There is a differance between a vapor barrior and a moistsure barrior.
Where you want them and why.
It explains it all.
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Old 10-13-2007, 08:50 PM   #11
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Default Re: Would you put up plastic under ceiling drywall?

Quote:
Originally Posted by kbs2244 View Post
If you are going to heat the garage, and not the attiic above, the answer is:
NO>>>NO>>>NO>>
You put a vapor barrier on the walls, but not the celing.
You want the vapor to work its way up into the attic space and get vented through the vents you should have under the eves and along the ridge line.
Go to the insulation mfgs websites. They all have install instructions.
That sheetrock with th plastic side is meant for walls only.
The problem I see with this is that the moisture in the warmer air will evidentially reach a place where it cools enough that the moisture drops (actually drips) out rendering the insulation much less effective or it freezes. It is hard to to get "vapor to work its way up into the attic space" with a sheet of ice in the way.

Would you name just one "insulation mfgs websites" where it is stated as you say - I would take the time to read that.

Thanks Junkman for some actual humidity data where the drywall survives. I really appreciate it when folks state their actual experience and not some misinformation they read on the web.
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Old 10-13-2007, 09:00 PM   #12
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Default Re: Would you put up plastic under ceiling drywall?

The only barrier that I can think of that goes on the cold side of the wall is Tyvek, the building wrap. It is designed to allow moisture to travel through it on the way out of the building, and to keep air infiltration from getting into the building. How it accomplishes this, I have no idea, but I do know that homes that are wrapped with Tyvek home wrap have less and in many cases, no cold draft problems when the wind blows. I only wish that it was available back in 1982 when I built my home.
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Old 10-13-2007, 09:20 PM   #13
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Default Re: Would you put up plastic under ceiling drywall?

Quote:
Originally Posted by Junkman View Post
The only barrier that I can think of that goes on the cold side of the wall is Tyvek, the building wrap. It is designed to allow moisture to travel through it on the way out of the building, and to keep air infiltration from getting into the building. How it accomplishes this, I have no idea, but I do know that homes that are wrapped with Tyvek home wrap have less and in many cases, no cold draft problems when the wind blows. I only wish that it was available back in 1982 when I built my home.
I have said the same with regards to my shop build in 1981 but I was too early for Tyvek. If I were to build another house or shop I would use Tyvek under the roof shingles as well. I haven't seen this done but I would gladly be first to give it a shot. It seems to me it would be a very cost effective replacement for the rubber sheet used over soffet areas where ice dams occur.
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Old 10-13-2007, 09:29 PM   #14
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Default Re: Would you put up plastic under ceiling drywall?

I don't believe that it will work under shingles. The rubber membrane that is used under the shingles acts as a gasket to keep the water that backs up under the shingles when an ice dam forms, from getting to the sheathing and dripping into the home. I dams happen when the warm air in the attic melts the snow, and that water moves to the edge of the roof and freezes, since the edge of the roof is colder than the surfaces above it. The only way to keep the snow from melting is to keep the attic at the same outside temperature, which is very difficult to do. If you have a ice dam problem at your home, the best thing is to install heat tape at the edge to keep the ice melted and to allow the water a passage to the gutters. My North facing roof has always had this problem. This roof is very long, and it is a 4" in 12" pitch, and it does get the morning sun. This melts the snow, and when it gets to the eves, it always freezes. I have put heat tape cables on it a few years ago, and no more problem. I turn them on in the morning when the snow is melting, and off in the afternoon, when the temperature is dropping, and the snow is no longer melting. Snow on a roof always melts from the bottom up, so ice dams are just an unavoidable inconvenience....
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Old 10-13-2007, 10:04 PM   #15
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Default Re: Would you put up plastic under ceiling drywall?

I don't have any ice dams, haven't had any since I moved in (78') but code required that I put the rubber sheet down when I re-roofed and I needed two coarses as my overhangs are more than 3' deep on the horizontal. Stick built 2' X 6" rafters give ample room over the top plate to insulate well and still allow my full length soffet vent plenty of air circulation up to the peak vent. Water runs off Tyvek and it stays tight around nails so it gets my vote for keeping water out should there be a leak in the roof system plus it still breaths so the decking shouldn't suffer from its presence. I plan to build a hut/playhouse for the grandchildren soon and perhaps can test my theory on it. I can test it for water shed wthout shingles first. Maybe rain pounding on it by itself will push water thru - don't know.
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Old 10-13-2007, 10:22 PM   #16
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Default Re: Would you put up plastic under ceiling drywall?

Well I guess my opinion only would matter in my areas and like dryer climates. For my dads garage where its done like I said it was done, humidity doesnt last long anyways.
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Old 10-14-2007, 11:27 AM   #17
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Default Re: Would you put up plastic under ceiling drywall?

Go here:


http://www.certainteed.com/NR/rdonly...NAIMABI402.pdf

16 pages of standard pratices and why they are standard pratices.
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Old 10-14-2007, 12:27 PM   #18
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Default Re: Would you put up plastic under ceiling drywall?

Quote:
Originally Posted by kbs2244 View Post
If you are going to heat the garage, and not the attiic above, the answer is:
NO>>>NO>>>NO>>
You put a vapor barrier on the walls, but not the celing.
You want the vapor to work its way up into the attic space and get vented through the vents you should have under the eves and along the ridge line.
Go to the insulation mfgs websites. They all have install instructions.
That sheetrock with th plastic side is meant for walls only.
First, the link you provide is not that of an insulation manufacturer, at least not the pdf.

Second, the only question about use of a moisture barrier that you may have misconstrued, is mention of following local code. It does not support your claim.

However it does support a couple of points I made that are contrary to what you said, Namely;

Page 1

Moisture Control
A vapor-resistant membrane (commonly
called a vapor retarder) attached to batt or roll
insulation, or installed separately, decreases the
possibility of moisture vapor condensing to
water within the structure.


Page 11

Batts and rolls may be installed one on top of the
other in ceilings where there is adequate space.
In most cases, only the bottom layer should have
a vapor retarder, which should face down toward
the space that is warm in winter. Additional layers
normally should be unfaced. If unfaced insulation
is not available, use the faced type but
remove the facing completely before installation.


Did you even read what you linked? It is not a very good source IMHO but it does not support your claim.
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Old 10-14-2007, 02:36 PM   #19
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Default Re: Would you put up plastic under ceiling drywall?

the vapor barrier wants to be on the heated side of the insulation. and above the insulation, you want Ventilation.
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Old 10-14-2007, 02:40 PM   #20
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Default Re: Would you put up plastic under ceiling drywall?

Quote:
Originally Posted by Steve in Mi View Post
... would use Tyvek under the roof shingles ... I haven't seen this done ...
http://www2.dupont.com/Tyvek_Constru...omeowners.html

there's a good reason: it isn't marketed for roof installations.

use ice & water shield to keep out, well, "ice & water". most of the roofing companies have got on the wagon and brand their own product.

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