We Still Need to Talk About Vapor Barriers – GreenBuildingAdvisor

By Martin Holladay | September 5, 2025

Maybe you’ve read so many articles on vapor barriers that you are sick of the topic. If so, I sympathize. (Feel free to search for another online article to read.)

But just when I think that there’s no further need to educate anyone about vapor barriers, I encounter an example of online ignorance that proves me wrong. Recently, for example, I came across a web page for a Texas building material company called Polyguard. A heading on one of Polyguard’s web pages reads, “Signs That Your Building or Home Needs a Vapor Barrier.” The article continues:

Several signs can indicate the need for a vapor barrier:

• • Water stains on the ceiling, foundation walls, or carpeting
  • Mold on the walls or ceiling …
  • High indoor humidity levels …

Is this useful information? No. It’s utter nonsense.

Unraveling several misunderstandings

The web page quoted above is spreading a muddled understanding of vapor barriers and vapor retarders. I’ll try to untangle a few threads.

First of all, it’s important for homeowners and builders to know that the typical house has several types of “barriers.” For example, most U.S. homes include a water-resistive barrier (WRB) like Tyvek or Typar in the wall; it’s generally installed between the siding (brick veneer, vinyl, or wood siding, for instance) and the wall sheathing (OSB or plywood).

Tyvek and Typar are not vapor barriers. They are barriers to liquid water (wind-driven rain), and they’re designed to allow water vapor to travel right through them. In other words, they are vapor-permeable.

Some plastic barriers (like Tyvek) allow water vapor to pass through; others (like polyethylene) don’t. The type of barrier that doesn’t allow water vapor to pass through is called a vapor barrier.

A third type of barrier is called…