Crawl Space Encapsulation Vapor Barrier

Encapsulation vapor barrier is typically not available at major retail stores. Most of the vapor barrier plastics found at Wal-Mart, Amazon and Home Depot are 6-mil or less. Using a cheaper thinner vapor barrier for crawl space encapsulation has some challenges that we will discuss in this article.

Encapsulation Vapor Barrier Thickness

Painters plastic is 6-mil or less and is usually 0.5 mil. It is used to create barriers between rooms. Water restoration companies will use 0.5 mil but many times use a 6-mil plastic to create containment areas.

Crawl space encapsulation vapor barrier is usually at the lowest a 10-mil and some get well over 20-mil in some cases. Why does the crawl space industry choose a thicker mil plastic than other industries?

Vapor Barrier Perm Rating

The perm rating is the measuring unit used to describe the mass rate of water vapor flowing through one square foot of material. Many people are under the assumption vapor barrier does not allow moisture to pass through it. The fact is all encapsulation vapor barrier will allow moisture through it. Some just allow less than others.

A perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury. The higher the perm rating, the more water vapor can travel through the material.

Class 1, 6-mil polyethelene sheeting found at hardware stores has a perm rating of about 0.06. This vapor barrier is the lowest mil thickness accepted as a ground cover in building code for crawl spaces.

Make sure you check out Crawl Space Encapsulation Seam Tape

Image of Crawl Space Vapor Barrier

Class 1 Encapsulation Vapor Barrier

Class 1 vapor barrier, also called a vapor retarder, is the only type acceptable by building code to use in crawl spaces. The class is defined using the desiccant method of ASTM E 96:

  • Class 1 Vapor Barrier - 0.1 perm or less.
  • Class 2 Vapor Barrier - 0.1 perm to 1.0 perm.
  • Class 3 Vapor Barrier - 1.0 perm to 10 perm.

The desicant testing method is done at 25% relative humidity in order to get the perm values listed above.

Examples of Class 2 vapor barriers are unfaced expanded polystyrene and kraft faced fiberglass batt insulation. Class 2 vapor retarders are considered semi-impermeable.

Class 3 vapor barrier examples are latex paints over drywall (never use paint in crawl spaces - check out this video), #30 building paper and some plywood. Class 3 vapor retarders are considered semi-permeable.

Vapor Barrier and Cat Pee Odors

I know what your thinking, what is this about? We discovered over the years that some encapsulation vapor barriers can actually off gas an ammonia smell that is similar to a cat urine odor. This is usually found in cheaper plastics that use a nylon cord for reinforcement.

I know crawl space vapor barrier manufacturers and resellers try to blame soil gasses. I'm not saying soil gases are not a problem. In fact we are one of the only crawl space contractors that address soil gasses as part of our encapsulation process. This is done according to the EPA standards. But I don't know of any soil gases that smell like cat urine, do you?

Reinforced Vapor Barrier

The question I always ask is why does the encapsulation vapor barrier need to be reinforced? The reinforcement is great if you are pulling on it but that is not a required quality in a crawl space. Laying the plastic on the ground requires encapsulation vapor barrier to be more puncture resistant than tear proof.

If you want to add puncture resistance to vapor barrier we recommend using a felt or dimple underlayment to keep rocks and other debris from creating small punctures. Preventing small punctures will in turn keep humidity lower when using a crawl space dehumidifier.

We have found reinforced vapor barrier used in crawl space encapsulation is no more puncture resistant when compared to a non-reinforced product of similar thickness. If it is no better for puncture resistance and has a risk of smelling like cat pee, why use it? The fact is, we don't install, recommend, or sell reinforced encapsulation vapor barrier.

Are you a DIYer and want to fix your own crawl space? Check out the Crawl Space Ninja DIY Store for all the products you need to perform crawl space encapsulation.

Vapor Barrier for Your Crawl Space

When choosing a vapor barrier for your crawl space we hope the information above has been helpful. Most homeowners like a bright white plastic while others prefer a clear or even black vapor barrier.

Remember investing in a good vapor barrier is just one part of the puzzle. Make sure you have a great seam tape along with humidity control in order to prevent mold in the crawl space. You may also need to do some proper yard drainage or crawl space waterproofing if you are experiencing standing water problems.

Correcting all of these issues can preserve the foundation of your home and improve indoor air quality. We hope you like this information. Please leave us a comment below and check out our YouTube Channel as well. I hope you make it a happy and blessed day!

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About Michael Church

Michael Church has been involved with indoor air quality since 2005 and feels the unhealthy crawl space is one of the major problems causing poor indoor air quality.


  1. Meliza on July 5, 2021 at 8:57 am

    Hi there, my property is a duplex style condo and the next door has water pipe leak under their crawlspace which flooded and my crawlspace ended up having standing water under the vapor barrier. Months later, there’s still standing water on 1 part of the crawlspace.. is that ok, or do i need to resolve it? It is under the vapor barrier,

    • Michael Church on July 5, 2021 at 1:37 pm

      Hi Meliza, if the standing water is affecting humidity or has the chance to stagnate and smell bad, I would pump it out. Hope that helps. Thanks for reading our articles.

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