Re: Vapor Barrier Confusion
There is plenty of confusion as to the function of the vapor barrier.
The main role of this barrier is more in preventing warm moist [B]air[/B] from passing through into the exterior wall cavity and mixing with the colder [B]air[/B].... which would condense and result in moisture. In other words think of this as more of an air barrier..... which also aids in stopping drafts.
The plastic is of low air & moisture permeability ...pretty much zero .... meaning it doesn't allow moisture laden air to penetrate ... and typically 6 mil is usually recommended because it's thicker and won't puncture as easily than 4 mil ... which in some areas is the minimum code.
Now having said that .... it's important to install this barrier with as few seams as possible and those that are present should be well sealed .... forming a continuous monolithic envelope. This can be done using a house wrap tape.
If this step isn't properly done then the idea of this barrier is greatly reduced , negating any advantage it offers.
One additional step that can be done to ensure a premium seal is to use a caulking like acoustical sealant along the top and bottom sills and each stud with a continuous bead. This will seal any punctures from the fasteners and maintain that monolithic envelope.
Hope this helps.:)