Crawl Space Encapsulation Vapor Barrier

A professional grade 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 Matters

A typical painters plastic is 6-mil or less, and is usually only 0.5 mil. This type of plastic 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:

Perm Classification of Vapor Barriers

  • 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 Barriers and Cat Pee Odors

I know what you are thinking: What do cat pee odors have to do with vapor barriers and crawl space encapsulation?

Cause of the Ammonia Smell in Vapor Barriers

We discovered over the years that some encapsulation vapor barriers can actually give off gas that creates 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.

Misconceptions by Manufacturers and Resellers

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.

Why Should a Vapor Barrier be Reinforced?

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.

Fix Your Crawl Space!

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.

Contact Crawl Space Ninja for Basement Waterproofing and Encapsulation

Please contact us to schedule your assessment to fix your crawl space, basement issue. 


Do you need help with mold removal, crawl space encapsulation, crawl space insulation, vapor barrier, waterproofing, foundation repair, basement waterproofing, or controlling humidity in your crawl space?

If you live in Georgia, Delaware, North Carolina, South Carolina, Tennessee, or Kentucky, Crawl Space Ninja can help!


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12 thoughts on “Crawl Space Encapsulation Vapor Barrier”

  1. 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,

    1. 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.

  2. Linda I McCormick

    Which is best for crawlspace encapsulation (walls) … closed cell spray foam or a poly vapor barrier (100ml)?

    1. We prefer a foamboard insulation and 12-mil vapor barrier or you can do an insul-barrier type material on the walls. We will use closed cell foam on brick, stone but normally not on concrete or block. We find it is difficult to ensure a consistent r-value and it tends to be extremely messy with overspray. Other companies really like it though. Hope that helps.

  3. We have blocked our air vents, turned off the AC but still have a smell that has appeared in the last 3 months. Not to mention the company but we have had a barrier laid during this time. It is put down with 5 inch nails. In the past we have had our barrier tarred to our block. This company who is also I pest control said they will not do a barrier applied to our block. We are trying to figure this smell. Not sure if all this is related…..

  4. Hi there, we have a dirt basement that is having high humidity problems (~70%). We live in Atlanta, GA. Currently have fans and a dehumidifier, but looking for longer-term solutions (finishing the basement isn’t in our budget right now).

    What mil thickness should we use for a vapor barrier/should we not use a vapor barrier over the dirt floors? I read that the Insulation Institute recommends a Class II or III vapor retarder, but I’m having trouble understanding if this means we can’t use a traditional poly barrier.

    1. Hi Nikki, class 2 and class 3 vapor barriers mean they allow more moisture to pass through than a class 1 vapor barrier. I recommend a standard class 1 vapor barrier like ours which is 12-mil thick. If you are planning to walk on it or store things, an underlayment will make it more puncture resistant. Here are links to both if you’d like to take a look. I hope that helps.
      Vapor barrier –
      Underlayment –

  5. Hi Michael, I just got a quote on replacing my 6 mil poly with a 10 mil poly in my crawlspace. I live in Virginia (unfortunately you do not cover this area) and I do not intend to close off the air vents. The crawlspace quote I have includes running the poly up the walls and also wrapping the piers. Should I wrap them and also run the poly up the wall? The company I have chosen, I have used previously and is a reputable local family owned business. Thanks Michael!


    1. Although you are not sealing the vents, I feel this is a good install in case you take on standing water or decide to seal vents in the future and it fits in your budget. But technically with a vented crawl space, all that is required is a loose laid vapor barrier. I hope that helps.

  6. Hi Michael,

    First I’d like to wish you a happy Easter Holiday!!! Thank you for sharing your skills and knowledge via your very informative youtube videos. I plan to DIY my crawl space by purchasing needed products from your company. I live in Maryland, the area where you don’t have service yet. My crawl space now has a 6 mil black loosely laid barrier. All vents are sealed off. No humidifier installed yet. Humidity is tested regularly in summer and it reads the normal range. What products do you recommend I purchase for my DIY project? My one-story home is about 2000 sq ft. Thank you in advance for your advice!!!

    1. My apologies for the late response and I want to wish you a Happy Easter as well. I would love to answer your questions but I have more questions for you and probably the best thing to do would be to speak to one of our DIY professionals to make sure that we get you the right products and only the products that you need for your crawl space. If you could give them a call at 865-659-0390, that would be great and thank you so much. I really appreciate you saying such nice things.

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