Plastic Vapor Barrier
Reinhard Zellmann
| Posted in Energy, Heating & Insulation on
Recently my son had a final plumbing and electrical rough-in inspection in a Chicago suburb.With insulation and drywall next, the inspector warned him that if he used a plastic vapor barrier he would make him rip out the plastic and drywall and start over. I have been using plastic sheeting over my insulation for years so now I’m worried.What’s going on here?
Replies
Vapor barrier wars. The need for a VB is iffy in Chicago, but it's probably not harmful. Where do you live?
Is the BO implying that a vapor retarder is not required ... in fact is not allowed in that jurisdiction? Seems strange to me. I'd ask for the code section that states it isn't alowed. Maybe Chicago is different than other areas. He should respectfully ask for clarification/reason so that he can understand as the issue may affect some other decisions. Personally I have to know why they want me to do something rather than blindly comply, particularly when the 'requirement' tends to be against common practice in many builder's eyes.
To me it's
To me it's insulation-dependent.
With fiberglass insulation I'd want a vapor barrier.
With dense-packed cells I could see not using one.
With 2-lb foam you don't need one.
Mongo, why a VB with FG batts but none with dense packed cellulose? Both are very porous to water vapor diffusion. With either, I would prefer to see a 1 perm VR, not a VB like a poly sheet. The VR can be done with a vapor retarding primer on the drywall. It really slows down diffusion of water vapor, while allowing bidirectional drying at different times of the year.
FG can't handle water vapor
FG can't handle water vapor very well at all. Cells can hold it, then dry during a drying cycle.
Poly can also help a bit with pressure driven movement during the heating season.
DP cells also resist air infiltration much better than FG batts.
"DP cells also resist air infiltration much better than FG batts."
But that may have little to do with the migration of moisture through the material. While air movement can/will carry moisture, you don't need convective air movement for moisture to move through a material.
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