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Kraft & poly mixed

Posted by The Insulator on March 10th, 2001 10:40 PM
In reply to double vapor barriers by Bill Winters on March 10th, 2001 11:02 AM [Go to top of thread]

This would not create any reason for concern. The situation commonly refered to as a "double vapor barrier" is created when there is when vapor barriers are installed on both sides of the insulation. In such a situation, the cold side vapor barrier acts s a condensation point and the barriers on both side them trap the moisture with no where to go. With kraft covered by poly, they are both on the warm side of the fiberglass, ruffle at the same temperature (thus one is not a cold condensation area, which would create a mechanism to cause condensation. Also, since a vapor barrier needs to be in contact ith the drywall or other barriers to do the function of a vapor barrier, the kraft set back is really not functioning as one.{a vapor barrier is to stop water vapors from passing THROUGH the wall/ceiling materials or air tight barrier. poly. therefore, it needs to be in contact with the material, otherwise the air just convects around the vapor barrier.}).

Though the air is in the space is an insulator, it is not significant enough to cause any moisture issues. However, the insulation should not be installed that way, as the air pocket can, and does induce convection, thereby lessening the effectiveness (R-Value). No matter how good you do the kraft facing, it is not an effective air (Convection) barrier! Once you create convection, the air movement takes the warm air right around the fiberglass (in wall cavities, some test show that leaving even the edges of the fiberglass pushed back can reduce the R-valu by as much as 50%).

If you use kraft fiberglass, cover it w/ poly &keep it right against your wall/ceiling material. Or just use unfaced & poly & save a few cents.

I hope this isn't oo confusing
The Insulator

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