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The Skinny on Vapor Barriers

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I was at a meeting recently when the discussion turned to wall assemblies. One of the speakers then asked what we should do about vapor barriers. It’s what everyone wants to know, right? Should I use a vapor barrier? Should I install it on the inside or the outside? But I had a different question for the speaker.

In the world of building science, we spend an inordinate amount of time talking about vapor diffusion. In reality, though, it’s responsible for only a tiny, tiny fraction of all the cases of moisture damage. I’ve written about how the story of peeling paint put us on this misguided path of vapor barriers.

What IS a vapor barrier?

But wait! When I hear that term come up, I always have to ask a simple question:

What is a vapor barrier?

When you use that term, are you talking about 6 mil polyethylene? Are you talking about asphalt-impregnated kraft paper? Are you talking about house wrap?

Unfortunately, people use the term “vapor barrier” to mean all of those things and more. And that means I often don’t know what the heck you mean if you use that term.

If instead you mention a Class 1 vapor retarder, I know exactly what you mean. You’re talking about a material with a water vapor permeance of 0.1 or less. This is what a lot of people mean when they talk about vapor barriers, but there’s enough confusion that I still have to ask.

And if I have to ask, that means the term “vapor barrier” doesn’t help the conversation. Here’s what I’d rather hear when you talk about a material’s ability to deal with water vapor:

Impermeable – a Class 1 vapor retarder (≤ 0.1 perm)

Semi-impermeable – a Class 2 vapor retarder (between 0.1 and 1 perm)

Semi-permeable – a Class 3 vapor retarder (between 1 and 10 perms)

Permeable – not a vapor retarder (> 10 perms)

Let’s just drop the term “vapor barrier” completely and use words that everyone can understand.

When do you need a Class 1 vapor retarder?

OK, now that we’ve got that cleared up, when do you need a Class 1 vapor retarder? Turns out that there are only a few places you need ‘em in a house. Here’s my short list:

  • Under a slab
  • On the ground in a crawl space
  • Surrounding the insulation on ducts in unconditioned space

If we take the term “vapor barrier” to mean a Class 1 vapor retarder, there aren’t many materials that qualify. The main ones used in buildings are polyethylene (6 mil poly) and nonperforated aluminum (as you see in the foil facing of some rigid foam insulation).

So you can put this stuff under a slab, in a crawl space, or on your duct insulation. You might get away with putting it other places, but the lesson of peeling paint I mentioned above is that exterior water management solves most moisture problems. We know now that it’s more important to have assemblies that can dry than it is to stop vapor diffusion.

Vapor barriers, vapor retarders, and vapor diffusion are responsible for a whole lot of confusion among the trades who work on buildings. Understanding the basic principles here will help you sort through that confusion.


Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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