[Editor's note: When this article first appeared in print, the title was offered tongue-in-cheek; we didn't really expect it would be the last word. Fast forward 25 years and we are still getting the same type of questions from readers. Over that time period, the building code has evolved, although it still has a ways to go on the topic of vapor retarders (see "Avoiding Wet Walls," May/2017). The biggest change in the code is that air barriers are now required, and in most places in the U.S., the air barrier must be verified with blower door testing. In this article we have made a few changes to reflect current code. What hasn't changed, and remains in this article, is the basic science of how water vapor moves through building assemblies and the guidance, expressed in as straight-forward a way possible, on how to manage it.]

We receive more questions each year about vapor barriers and their close cousins, air barriers, than any other single topic. In an effort to clear up any lingering confusion about these pesky membranes, we’ve assembled this list of the most common queries along with the clearest answers we could provide.

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