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Old 01-24-2011, 10:15 PM   #16
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One more thing: You could help by painting your ceiling below the attic with low-permeability paint.


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Old 01-24-2011, 10:20 PM   #17
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Kraft paper, laminated w/ asphalt, has a perm rating of 0.3, but I think that is the material, and does not take into account its edge gaps, ie, "as installed". Here is a quote from another forum: Since 2007, the IECC has required (in section 402.5) that walls in climate zones 5 (e.g., Nevada, Ohio, Massachusetts), 6 (e.g., Vermont, Montana), 7 (e.g., northern Minnesota), 8 (e.g., northern Alaska), and marine zone 4 (Western Washington and Oregon) have a Class I or Class II vapor retarder in other words, kraft facing or polyethylene.

Class I Vapor Retarder (vapor barrier): less than or equal to 0.1 perms
Class II Vapor Retarder: less than or equal to 1 perm and greater than 0.1 perms
Vapor III Vapor Retarder: greater than 1 perm but less than 10 perms

From another site: Later primer paint, P = 8.6. Latex vap retard paint, 0.45.

As noted above, a VR must be within 1/3 of the R value of the wall in cold places; not sure about SC, but I'd read where I suggested before I did anything. You may well be in a place that suggests a Class II on the outside, esp if you run air conditioning. Good luck; this is tricky stuff in your climate.
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Old 01-25-2011, 09:11 AM   #18
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Thanks guys. That is a great website. It seems to have no end to useful articles.

Yeah, South Carolina is hot and humid. Not as hot and humid as New Orleans, but much more humid than most. You know, old houses don't have vapor retarders, and rarely have mold issues. It seems like no matter how you do it, if you live somewhere where it's hot and humid in the summer and you use AC, and you have a vapor retarder, then you're going to get condensation (either in the winter or the summer, depending on where you put it.

It seems that there's a cost to saving energy in this regard.

I think maybe I'll inspect the existing insulation to see if there is any mold issue, and if not, then I'm not going to worry about it, because it's been that way a long time.


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