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Tips for keeping floor insulation dry

What kind of vapor barrier is most effective?

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Q: I have 2-by-12 floor joists, and I plan to use R-38 batts to insulate my floor. Is it OK to go with un-faced batts, or do I need faced insulation with the Kraft paper face-up towards the floor? I do have 6-millimeter black plastic on the ground. --Rod G. A: The purpose of the vapor barrier is to prevent moisture from inside the house from migrating through the floor and into the insulation, where it can reduce the R-value (the measure of resistance to heat flow) of the insulation and, in extreme cases, cause structural damage. Today's plywood subfloors utilize exterior-grade resins in their construction that will act as a vapor barrier in most normal vapor situations. So if you have a subfloor made from plywood, oriented strand board (OSB, also called waferboard), or other sheet materials, you should be fine with un-faced insulation. If your subfloor is individual boards of any kind, I would recommend that you use insulation with a vapor barrier face such as Kraft paper, and install...