Why not use poly below grade?
Chris writes from Milwaukee, Wisconsin: “Why should I not use polyethylene on any below-grade walls over Kraft faced insulation? I have read that faced insulation is only a vapor retarder not a barrier and that a barrier should be used. But I have also read that I should not use a vapor barrier. Which one is correct?” Answer: We do not recommend a plastic vapor retarder in below-grade or basement applications. The moisture content is generally high in basements and a plastic vapor retarder is too strong for those conditions. You will want to use a weaker vapor barrier such as Kraft paper so the cavity can breathe somewhat. We use the term “retarder” because the facing on our FIBERGLAS™ insulation actually slows the movement of air or moisture-containing vapor. From our perspective, “barrier” would indicate that all moisture is blocked and that is not the case with our Kraft facing.
