Member “otinkyad”—let’s call him Oti—has been reading about managing moisture and insulating in basements, and he and has some questions, which is posed in this Q&A post. He thinks he understands the premise of “How to Insulate a Basement Wall” i.e., polyethylene shouldn’t be used as a vapor retarder anywhere in a basement.

But then Oti read a more recent article, “Vapor Retarders for Basements and Crawlspaces,” which recommends installing poly over uninsulated, vapor-porous slabs that are often found in older homes. He’d like someone to explain why a vapor retarder is recommended for a basement slab but not for basement walls.

Commenters point out the difference between a wall assembly’s ability to dry to the outside vs. to the inside, and the influence a vapor retarder such as poly would have on those processes in a concrete basement. DCcontrarian explains, “Moisture moves from warm to cold and from wet to dry… To the extent a basement wall can dry, it’s to the interior only, so it has to be vapor-open to the interior. At the same time, it needs to be vapor-closed to the concrete, otherwise you’re trying to dry all the moisture in the soil through your walls.”

He continues, “Placed on the wrong side of the wall assembly, poly can trap vapor, where it can condense on the cool concrete wall surface.”

There is some discussion, too, between how to handle vapor retarders in basements vs. crawlspaces. Typically, the latter is at least partially above grade, and therefore is likely to have fewer moisture issues due to greater air circulation and drying.

That said, the question for this Spotlight is: Why is it okay to install a vapor retarder over a basement slab, but not on…