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southern california bathroom vapor barrier before drywall or not?

southern california bathroom vapor barrier before drywall or not?

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Old 04-20-13, 01:42 PM
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southern california bathroom vapor barrier before drywall or not?

Two sources say not to use vapor barrier under drywall, what's the real truth?



However, you cannot install a vapor barrier behind a drywall product, including green board. If you were to install a vapor barrier behind it, water vapor would build up between the vapor barrier and the drywall. It would be just a matter of time before your walls began to crumble.

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07-13-2008, 06:40 AM
Not sure I agree 100% with All Pro, but definitely.....

NO PLASTIC VAPOR BARRIER.

ANYWHERE.

ANYTIME.

Plastic VBs may have a place in extremely cold climates, but for most climates they are not only unnecessary, they are a valuable building block in constructing a mold factory, by trapping moisture in wall cavities.

Current thinking is vapor RETARDER that has low permeability, but has SOME permeability, so wall cavities can dry in both directions.

See *****************

The kraft paper on your FG is fine. Now FG itself has deficiences as an insulator, but I digress......
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Last edited by Shadeladie; 04-20-13 at 01:59 PM. Reason: Sorry, too many links and some not allowed
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Old 04-20-13, 02:18 PM
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Welcome to the forums! With hot and cold forces colliding inside your walls, you want to stop condensation at some point before it gets into your room. The best place is inside the wall where it has a better chance of dissipating. Thus the vapor barrier. A permeable membrane will not stop the moisture and once it gets to the surface of your drywall, bingo, mold. I don't believe they use plastic VB in Florida, but they don't have cold.....not even cool...not even warm....just hot.
 
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Old 04-20-13, 02:58 PM
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Larry, it's been 20+ yrs but all the new homes I painted in fla had a plastic vapor barrier underneath the drywall on all exterior walls - it was code.
 
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Old 04-20-13, 04:31 PM
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So what is it they don't use, WRB on the outside? I guess, obviously, since many are cinder block buildings, right??
 
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Old 04-20-13, 05:47 PM
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Hi questionss and welcome to the forum.
Since you are in s cal I will assume you air condition and rarely heat. As stated in the second suggestion, extremely cold climates should still use a VB on the inside. Extremely hot climates might consider one, but if the building has air conditioning, it would go on the outside, just under the sheathing. But caution needs to be used to not use vinyl wall paper or vapor retarder paints on the inside.

So, what happens if you do not use a vapor barrier? Diffusion has been shown to not transport enough moisture to be a problem. Air leakage is the number one source of moisture to create mold. Air seal 100% and allow the moisture that enters through the materials to exit the same way it came in.

It will take me a bit to dig up the reference, but if you need it I can.

Bud
 
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Old 04-21-13, 04:20 AM
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So what is it they don't use, WRB on the outside?
Most of the houses are block and just get painted, some stucco'd. The wood frame houses got a house wrap over the plywood before the siding is nailed up. A few would get plywood, tar paper, lath and stucco. I'm not sure what the houses that just used T-111 or RB&B used - maybe nothing.

Don't forget I left Fla in 1991 and don't know what changes in construction methods they've had since then
 
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Old 04-21-13, 04:40 PM
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1) I'm talking about in the interior of the room. I'm not doing anything to the exterior of the house.

2) I'm using plastic vinyl vapor barrier, not semi permiable, not tar paper.

3) We do NOT live in Florida, we live in California, we do not use air conditioning because we live near the beach and its usually about 65-75F year round. We do heat sometimes. Florida is NOT like California regarding weather.

4) This is for a bathroom with 1 wall on the outside of the house. The ceiling faces an attick crawl space. We have treated bluejean/canvas insulation in all walls and ceiling.

The two sources I posted said NOT to use vapor barier on the inside under the drywall because it will cause it to crumble. I'd like to verify if this is true or not.


I'd like to know if I put up the plastic vinyl under the drywall on interior of the outside facing wall or not? It's a simple yes or no question.
 
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Old 04-21-13, 04:57 PM
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Wow! Since you're looking for FREE advice, you might want to keep your tone down a bit or no one will want to answer you!
Everyone volunteers their time here, so you might want to pay someone if you want to shout and lecture like that.
 
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Old 04-21-13, 05:25 PM
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YES..................have a great day.
 
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Old 04-22-13, 01:09 PM
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I just think this topic is comical. I'm not a contractor but I have purchased 2 reputable books on the subject and none of them recommend a vapor barrier except for the shower enclosure. I have also found more than 2 reputable sources (linked below) that state "no vapor/moisture barrier" behind drywall. There are even other posts on this site where one "expert" states yes and another states "no" on the same job.

My conclusion is that no one knows what they are talking about and they just go off of what they have used in the past.

It's a bathroom remodel and all that was on the walls for the last 25 years (all 4 walls and ceiling) prior was tar/felt roofing paper and some cedar wood planks. There is no moisture damage or mold anywhere in our house (recently had high-tech testing done) so I assume that it really isn't a big deal since it's just steam from a shower and the factual evidence that a bathroom with no vapor barrier for 25+ years in the southern california beach climate does just fine. We recently remodeled the bathroom in the center of our house too (no external facing walls) and it didn't have any vapor barrier in it either and there is no mold (per testing and visual inspection.)

The logic behind this is that you would need them where it gets very cold and the cold will cause the moisture to condense out of a gaseous state, but this condensation will still happen due to the temperature even with an airtight/water tight barrier because it's due to the factor of temperature. They key is to keep the temperature higher than the condensation point inside the structure of the house to prevent mold by using insulation and air/moisture barriers on the outside of the house and using ventilation in the shower/steam areas with internal vapor moisture partial-semi/barrier like a sealant paint. But as the links below state, not behind the drywall (in front of it.)

Based on this I think that most people/contractors are don't really know and just do what they want/think based on their whims and experience. Luckily I studied Chemistry in college.

correction: one of the books says to put plastic vapor barrier on top of the insulation and wood stud framing on all walls that face unheated spaces (exterior or attick space i am assuming)... and behind the sheetrock in the shower enclosures... however this contradicts the two source links below.
 
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Old 04-22-13, 01:19 PM
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The tar paper on the exterior wall acted as a vapor barrier. Albeit on the outside rather than the inside. Today's methods move that vapor barrier under the sheetrock and a breathable weather resistive barrier is installed on the outside under your siding. It keeps your interior moisture at bay as well as moisture from the outside from moving into your room.

You DID have a vapor barrier, it was just in a different place. Not too sure what Chemistry in college has to do with Physics in real life, but hey, I'm just a naildriver that does this stuff every day for a living.
 
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Old 04-22-13, 01:27 PM
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the tar paper was right behind the interior wood paneling (not outside like you stated)... they had wood paneling with big knots and gaps in it (very amateur job - found clods of dirt and grass on the inside of the wood wall panels when tearing it down) for the style the previous owner liked so they needed the tar paper so there wouldn't be gaping holes directly into the inside of the wall cavity, instead of drywall...

The books says to put a plastic vapor barrier on all walls the face a non heated area, overlap seems and leave a lot of extra plastic around the permiteter of the wall. Which still contradicts the 2 links below.

I'm wondering why there is such a contradiction?
 
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Old 04-22-13, 01:52 PM
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There are some differences because of climate. I suspect Larry's experience like mine is mostly in the southeast. Building or remodeling a house in our region without installing a vapor barrier would result in a rejection check off on the permit.

There are many things in life that don't have a definitive answers but when it comes to local code - we must abide by their mandate.
 
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Old 04-22-13, 10:37 PM
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Two sources say not to use vapor barrier under drywall, what's the real truth?
The real truth is that all codes is local. No advice you find online or in any number of books matters. What matters is what your local jurisdiction will approve.

After all, they know what works best there, far better than any of the rest of us can.
 
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Old 04-23-13, 02:18 PM
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I've checked local code. No call for internal vapor barrier and it was present in the other bathrooms.
 
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Old 04-23-13, 04:53 PM
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I want to do what is right not just what is code. I appreciate all the help and don't mean to seem rude but it's amazing how difficult it is to get the defenitive "right" answer on this.
 
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Old 04-23-13, 05:19 PM
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This is as simple an example as I can furnish. I really don't think you want to deal with a vapor barrier, hence the resistance. I would not build a house or remodel one without it. Localities have different "code" requirements. It doesn't matter if you want or don't want code....it's there.

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Old 04-24-13, 07:52 AM
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it's amazing how difficult it is to get the defenitive "right" answer on this.
IMO, it's not amazing - this is an area of building science that is catching up with changing construction methods. It used to be that no VB was needed because homes were not insulated and air moved freely all around. Now that we've tightened them up, we're discovering new issues as a result. Best example I can think of is all the mold problems found in 90's era stucco homes.
 
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Old 04-24-13, 03:27 PM
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it's amazing how difficult it is to get the defenitive "right" answer on this.
That's because the best solution for that wall in your house might not be the best solution for your next-door neighbor's house. It depends on the specific situation that's being addressed.

As I said earlier, ask the inspectors for your jurisdiction. They have the definitive answer to what they like to see, and are willing to approve.
 
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