Is it ok to put plastic poly sheet over kraft faced insulation to keep additional drafts out?????…outside is foamboard over tyvek.
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Only if you're sure the walls are dry before installation. You could be creating a space for condensation to be trapped and then you'd have a mold and rot issue. I'd tape the seams of the kraft paper and foam any utility intrusions in your walls and plates. That should be sufficient. Then you'll never lay awake wondering if you created a problem.
NCtim
Thanks...thats what I thought!!!
Use the plastic but turn it into a true air barrier with good sealing techniques. This will be better than taping the seams of the paper faced batts.
None of these discussions should occur and no advise should be given without at least knowing where the building in question is located. C'mon Ya'll. If this house is in the gulf states, poly is a huge mistake. It's not a good idea in most of the country. It is a terrible idea in the deep south. Even if this building is in an extreme cold climate, it is still a dangerous practice to have discussions of vapor barrier placement in the absence of climate data.
Yes, Ray, it is a bad idea to use a vapour barrier in the humid southern and gulf states, especially if the house has air conditioning. If he is in the south, he should take the kraft vapour barrier off the batts also or remove them entirely.
Since richie737 was talking about stopping drafts- something we do a lot of up here- I just figured that he must be in a cold location.
Richie, where are you?
Couldn't he use Tyvek on the inside instead of poly, which would eliminate any discussion of a double vapor barrier, cost issues aside?
No!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I disagree that it should be done this way. Whenever you put a vapor barrier over another vapor barrier (and this is a classic example), moisture will eventually get trapped in between. When this happens, it will create a mold problem. You risk ruining all the insulation you've installed.
I've worked on remodeling jobs where every exterior wall had to be re-rocked for this very reason. Use plastic over unfaced insulation only.
-T
Quote: "(and this is a classic example)"
The classic theory of moisture problems in insulation has changed since the 1940-60 period that people are still quoting from today!! In another thread last week, I quoted some sentences from a 1980's report to the NY State Energy Commission (this is all cold climate stuff). Although this stuff is dealing with attics, changes have been made in understqanding how walls get wet and how to stop it also.
Found some of the papers mentioned in a post. One is titled Attic Ventilation and Airsealing: A Technical Review of the Issues and may still be avilable from the NY State Energy Research and Development Authority, Technical Publications Dept, Two Rockefeller Plaza, Albany, NY, 12223. This is a mid-late 80's pub so much may have changed in the gov't departments since then. This is one of the better intros to the change in understanding of insulation, moisture, venting, etc by looking at buildings through the eyes of a building scientist rather than a tradesman. I only have the first of a two part series but it described the historical research done and how those working in the construction field did not pay attention!!
Here are a few choice statements:
From the background intro to the article-
"The traditional approach assumes the energy problem in attics is conductive heat loss and the solution is insulation. Further, diffusion of moisture into attics leads to rot and the solution to this problem is ventilation.
Unfortunately, the real world has proven to be more complex. Evaluations have indicated disappointing energy savings and ventilation standards/approaches are questionable. Some attics have no problems even when "undervented" while others develop problems with presumably adequate venting. Some concerned practitioners have suggested that added venting may actually worsen attic moisture problems and increase energy use in some cases."
"In 1981, ASHRAE, a major building science organization, altered its description of venting standards from "Recommended Good Practice" to "Past Practice"......and has made no new recommendations since then."
From section titled Ventilation Effectiveness-
"In fact, there is no guarantee that providing a specified area of passive ventilation openings will yield some desired level of ventilation"
From section titled House-to-Attic air Leakage-
"The moisture and, to some extent, thermal implications of house-to-attic air leakage were first noted by FB Rowley of the U. of Minnesota in the late 1940's. In the early 1960's, other Canadian and American resarchers indicated that air leakage is indeed more important than vapour diffusion but few practitioners paid attention."
-----end-----
So you can see how things have changed. Others question whether we need vapour barriers at all. I have insulated hundreds of older houses without putting vapour barriers in them and there have been no problems as long as the house does not have other moisture problems. As you can see in post #4, I suggest turning the VB into an AVB as the AVB with attack 100% of the potential for moisture condensation in walls and not just 1% as a simple VB will.
It is true that we will have a double vapour barrier in this wall but both are together and, even with the classical theory, no moisture will condense/collect here as they are both fully on the warm side of the insulation- they always are going to be warm and at the same temperature. This double VB situation will not trap moisture in normal situations!!
If you found moisture here, something else is going on!! Were these walls in a warm humid climate with an air conditioned building? If so, there should be no vapour barrier at all, just a very good air barrier, preferably on the exterior to stop warm, moist air from getting to the cool, interior drywall or interior finish.