Pergunte a si mesmo: "Se eu instalar uma barreira de vapor, onde está a água que ela pára de coletar?"
Se você colocar uma barreira de vapor de polietileno atrás de uma parede de alvenaria com revestimento de parede de alvenaria, a resposta é que a água que condensa no poli cairá sobre a placa de madeira, formando mofo e eventualmente apodrecendo.
As barreiras de vapor dedicadas não têm lugar na parede do porão. Se a parede do porão é isolada do lado de fora, então a barreira de vapor é supérflua; Caso contrário, é perigoso, pois irá capturar a umidade contra um elemento sensível à umidade, como eu indiquei.
Aqui estão muitas evidências, já que essa perspectiva parece ser controversa. De link
Should I include a polyethylene vapor barrier?
No. Basement wall systems should never include any polyethylene. You don’t want poly between the concrete and the insulation; nor do you want poly between gypsum drywall and the insulation. You don’t want poly anywhere. Paul Ellringer, an energy and mold consultant in Saint Paul, Minnesota, has a collection of slides showing moldy basement insulation. In most cases, these basement walls were insulated with fiberglass batts, and included two layers of polyethylene — one on each side of the studs. Ellringer calls this a “diaper wall,” and reports that most of them are a mess. “Fibrous insulation and poly are inherently problematic, and should not be used in below-grade walls,” says Ellringer. “Sometimes when you open it up, the fiberglass is soaking wet. If the house is two to four years old, the studs are often beginning to rot.”
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No interior vapor barriers should be installed in order to permit inward drying.
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Unfortunately, locating insulation layers on the interior often conflicts with the traditional approach of foundation water control – namely inward drying. Constructing frame walls, insulating the resulting cavity and covering with an interior plastic vapor barrier is common (Photograph 2) and often leads to odor, mold, decay and corrosion problems (Fugler, 2002; Ellringer, 2002). Also common, and prone to similar problems, is the use of “blanket insulation” often derisively referred to as “the diaper” for the odor problems associated with the approach (Photograph 3).
Se você ainda não construiu seu paredão contra a parede do porão existente, leia esses links para descobrir como fazê-lo de maneira segura e adequada.