vapor/moisture barrier. Needed???

frontMay 6, 2007

Does a house with plywood siding need a vapor/moisture barrier? All the houses with this type of siding in my neighborhood do not have any barrier. I guess the plywood acts as the barrier since it is not permeable. I can understand using applying it to sheathing before one uses vinyl siding, but what about this type of siding. I mean all of the joints are sealed with caulk. The only reason I ask is that the lumber yard parts guy said I needed it. I have never seen anybody apply this stuff directly to the studs (framing). It has always been applied to sheathing with a permeable siding placed on top. Any info would be greatly appreciated.

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I think you're confusing moisture barriers, which go on the inside of the house, under the drywall, with products such as Tyvek which go under the siding and serve different purposes. I don't know whether you need one, but thought it might help to clarify that.

    Bookmark   May 7, 2007 at 6:39AM
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OK... I guess I am confused. I have never seen anybody put moisture barrier up in Florida (inside house). I guess I was using generic terminology when I was referring to moisture barrier. The guy at the lumber yard was also. I am referring to the exterior _________ that goes over the sheathing. It is mostly sold as private label I'm guessing, since it always has 84 lumber, Lowes, etc... written all over it.

    Bookmark   May 7, 2007 at 8:34AM
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I would want a barrier over the plywood. Siding will not keep out water. The barrier could be as simple as tar paper. I would not put siding up against bare plywood.

    Bookmark   May 7, 2007 at 8:53AM
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I think you misunderstood what I was referring to. The plywood is the siding. There is no sheathing. The sell similar siding at Home Depot. It is pine plywood with grooves cut into it. The plywood goes directly onto the studs. I'm guessing since this type of siding is not permeable and is completely sealed that it acts as a barrier.

    Bookmark   May 7, 2007 at 9:28AM
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I did some research and now I am more confused. I have read not to use tar paper/ asphalt paper because it will block vapors. Basically, a house needs to breath. The house wrap (tyvek) retards moisture, but it is vapor permeable. This will cut down on mold issues. The house that I'm referring to didn't have house wrap applied. I don't know if this was because it wasn't available back in the 80's or it was not needed. I also just watched a video where they put house wrap directly onto the stud framing. I have never seen this before, but this could be due to the types of siding in my area. Well, the studs are clean and dry. I'm thinking it is best to leave it unwrapped. Any opinions...

    Bookmark   May 7, 2007 at 9:56AM
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Interior vapor barriers are dependent where you live, but i know of no area where a house wrap,felt paper.or some type of underlayment shouldn't be used under sidings.Before housewraps, in the north east, they would cut felt strips and run the corners and around openings and run sidings directly over the studs. Now housewrap is the norm.Depending on the siding, a lot of builds have plywood sheating(shaer panels), over the entire wall surfaces,housewrapped, then sided. Since your siding is the shaeathing, i would recommend at least the use of 15 or 30# felt before you run your plywood siding.Running siding and relying on caulk doesn't cut it, and you are not just concerned with rain water, but condensation/moisture build up behind the siding that the felt or housewrap acts as a barrier to keep it from getting in the stud cavaties.

    Bookmark   May 7, 2007 at 11:37AM
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Moisture is generally driven into wall systems by moving from the warm side of the wall to the cooler side of the wall.

In northern or heating climates, vapor barriers are put on the interior side of the wall, normally right behind the drywall. The goal is to prevent interior moisture vapor from entering into the stud cavity and in cold weather, condensing within the wall.

In northern climes housewraps, such as Tyvek, which are designed to shed liquid moisture but allow moisture vapor to pass through, are commonly installed on the exterior of the wall, OVER the sheathing, but UNDER the siding.

Tyvek allows moisture tha has gotten into the wall to dry to the outside, yet it prevents liquid moisture (wind-driven rain, solar driven moisture) that gets behind the siding from getting into the wall system.

In southern or cooling climates, the vapor barrier is normally put on the exterior side of the house. This prevents outside warm humid air from being driven into the wall system toward the interior cooler air conditioned space. WEre humid air allowed into the wall system, as it cooled due to the cool indoor air conditioned air, moisture vapor could condense within the wall's stud cavities.

Or something like that!

In Florida, when plywood sheathing (or something like T111 siding which it sounds like you are using) is being used on the outside of the wall, an additional vapor retarder/barrier is generally not required.

Some southern builders will still use Tyvek/Typar or some other type of housewrap between the framing and the siding, but remember, these housewraps are NOT vapor barriers. They are vapor permeable, but will still shed water droplets.


    Bookmark   May 7, 2007 at 12:43PM
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[quote]Running siding and relying on caulk doesn't cut it, and you are not just concerned with rain water, but condensation/moisture build up behind the siding that the felt or housewrap acts as a barrier to keep it from getting in the stud cavaties.[/quote]

I live in a neighborhood of 200 plus homes that use just the plywood and caulk (T111). What do you mean it doesn't cut it? I just pulled three sheets of siding down and the studs are dry and in perfect condition. Keep in mind this house is 25 plus years old. The only damage was to the siding, which a barrier underneath would not have protected. I think contruction techniques differ by region. I'm actually more concerned about mold down here in Florida. I would rather let the house breathe. I also read on wikipedia that felt/asphalt paper should not be used. This is because it doesn't allow for vapor to pass which could lead to mold.


Thanks for your reply. I really don't see the need to add any kind of barrier. The siding isn't as bad as I thought. I will probably only replace sections of the siding.

    Bookmark   May 7, 2007 at 3:15PM
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Thanks for the correction,front, as im only slightly familiar with n.w.florida practices in the mid eighties where they always used an underlayment before siding.Things change and maybe underlayments are not the ticket. What i meant by not cutting it as i would be a little nervous solely relying on caulk, but if your project is 25 plus years with dry cavaties, thats proof enough.You are right to say building practices vary greatly by region. What works in one area, doesn't mean it will for others. Best of luck with your porject.

    Bookmark   May 7, 2007 at 4:55PM
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Check with your local building offices. "Best practices" have changed a lot since there have been so much hurricane damage in your state. Local building codes may mandate some form of moisture barrier, and then again, they may not. But, it's best to be informed by the local authorities what is considered necessary. If another storm occurs and creates a moisture damage issue to you, and your replacement was not up to code, then your insurance company will have an issue paying for that claim.

    Bookmark   May 7, 2007 at 6:41PM
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i'm remodling a bathroom in a basement in Atlanta GA area. the exterior wall has ComfortTherm by John Mansfield instulation. Do I need to install 4mil vapor barrier over this especially if the insulation is ripped? Should I only install it on the exterior wall?

    Bookmark   October 14, 2008 at 4:41PM
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Mine is another, similar question. We're converting an already-begun garage in NW Florida, have discovered, when pulling back the insulation, water seepage indications on the east siding that was placed directly on the studs. Looks like it's wicking from the concrete? The building sits high enough- no water collects. My son's only idea is a bead along the wall outside. What's a good fix, before we put that insulation back up? Also, the north wall siding has mold/fungus growing on the inside. What can we do about That?

    Bookmark   May 29, 2014 at 10:45PM
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You should start your own thread and post photos.

    Bookmark   May 30, 2014 at 2:23PM
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Thanks, Renovator8; I'm great at misposting.

    Bookmark   May 30, 2014 at 2:53PM
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