A vapor barrier is a low-permeance layer that slows moisture diffusion through walls, floors, and slabs to prevent condensation, rot, and mold.
For the full breakdown, see our best 10 Mil Polyethylene Vapor Barrier guide.
Understanding what a vapor barrier is in construction starts with one idea: it manages moisture in vapor form, not liquid water. A vapor barrier is a low-permeance layer — commonly polyethylene sheeting or foil-faced board — placed inside a wall, roof, floor, or slab assembly to slow water vapor from migrating somewhere cold enough to condense. Put it on the wrong side, though, and it can trap moisture instead of stopping it.
What Does a Vapor Barrier Actually Do?
A vapor barrier slows the diffusion of water vapor through solid building materials — nothing more and nothing less. It does not stop air leakage, and it does not stop liquid water. That distinction drives every decision about where and when to use one.
Moisture reaches building assemblies two main ways. Air leaks and bulk water carry far more moisture, and they are handled with drainage, flashing, and air sealing. Vapor diffusion, by contrast, is a slow movement of moisture through the assembly itself, and that is the job a vapor barrier does. Building Science, a leading building research firm, describes a vapor barrier as the most restrictive class of vapor retarder — a Class I rating. In the framework the building codes use, a vapor retarder is any material with a permeance of 1.0 perm or less under the standard test method, and a vapor barrier sits at the most restrictive end of that scale.
The word “barrier” itself is part of the problem. Several technical sources note that “vapor retarder” is the more accurate term, because no material forms a perfect seal against vapor. Building Science’s guide to understanding vapor barriers explains the physics behind these classes and why the distinction matters in real assemblies.
Vapor Retarder Classes and Where They Go
A vapor retarder’s class is set by its permeance, measured in perms, and the class you need depends on your climate, your assembly, and which way the wall can dry. Class I is the most impermeable, and it is what most people mean by vapor barrier.
| Retarder Class | Permeance | Typical Materials |
|---|---|---|
| Class I | 0
 Send us your feedback Tell us what you thought about this post — we read every note. |