Friday, April 17, 2009

Crawlspace Vapor Barrier Mythbusters

I am going to outline 4 methods I have seen used in installing a vapor barrier. If you talk to 10 different builders, you usually will end up with many different methods of installing a vapor barrier in a crawlspace. Here are the most common:

1. Pea Gravel on top of plastic vapor barrier – This has to be the all time dumbest thing I have ever seen, and yet it is probably one of the most common. I have had builders tell me that the plastic on the ground KEEPS ground water and moisture from coming up from the ground. In never never land, this might actually happen but not in any crawlspace I have seen. So here is the idea – ground floor (bottom) – plastic (middle) – 4” pea gravel fill (top). Once the crawl foundation is built, builders install a 4 – 6 mil plastic on the ground and dump about 4” – 6” of pea gravel on top of the plastic. Eventually, what always happens is that water comes in from the walls and the ground floor and ends up on top of the plastic. So what you end up with is a swimming pool liner that holds water in the gravel for prolonged periods of time. Nearly all the water and moisture in the gravel backfill has to evaporate into the structure. Another example of building practices and science turning a blind eye to crawlspaces for decades.

2. Vapor barrier on top of ground floor – By far the most common practice for installing a vapor barrier. A 6 mil polyethylene vapor barrier is placed over any ground floor. Here is the idea – ground floor (bottom) – plastic (top). The ground floor could be river rock, pea gravel, dirt floor, sand, etc. The seams are typically overlapped 6” – 12” and almost never taped. While this will temporarily stop some moisture evaporation, it does not seal out moisture from the internal perimeter wall where most water penetration occurs. Also moisture can come up from the seams, and the plastic is not durable enough to crawl on. It nearly always ends up with many punctures and holes in it.

3. Vapor barrier on bottom of floor joists – This is rarely seen, and usually only home owners attempt this. This is probably the method that accounts for more wood rot than any other method. If you are even thinking of doing this, quit thinking and call a professional to fix your crawlspace. Most crawlspaces are vented and the cooler surfaces such as duct work, pipes, and the floor will condensate in the summer. The plastic will trap the condensation up against the floor structure and mold and wood rot will occur. Good intentions do not always produce good results.

4. Vapor barrier fastened to sill plate – There is a new industry in crawlspace repair that encapsulates crawl spaces. The process of encapsulation is to install a heavy think plastic liner on the floor and up the foundation walls. The plastic liner is fastened and sealed to the foundation wall and all the overlapped seams are taped. Every potential gap or seam in the liner is meticulously sealed to prevent any moisture from evaporating. Then the vents are sealed in the encapsulation system to prevent hot humid air from entering in the summer. There is another system being sold and installed out there being represented as an encapsulation system, but is far from it. This “other” system is a 6 mil plastic that is ran up the foundation walls and stapled to the sill plate. None of the overlapped seams are taped. It is basically a glorified vapor barrier on top of the ground floor being ran up the walls and stapled to the wood. They seal the vents without properly sealing the moisture from the ground floor or foundation walls. The problem with fastening plastic to the sill plate is that moisture will “wick” up the foundation wall, and moisture will absorb into the sill plate and floor joists. They are giving free access to all the moisture under the liner to rot the sill plates and floor joists. Not to mention that all the moisture will evaporate up through all of the seams that aren’t taped and the plastic liner is only 6 mil and eventually will puncture and tear. Be very careful in the contractor you choose to properly encapsulate your crawlspace.

If you are in need of a new crawlspace vapor barrier or encapsulation system, give us a call @ 1-877-409-2837 or SIGN UP ONLINE for a FREE CONSULTATION.
Also, visit our website to see more about our crawlspace vapor barriers.

Thanks for reading the rambling thoughts of a crawlspace inspector,
Larry Ralph Jr.


  1. Hello,
    I am currently experiencing a funny smell only in one area of the crawlspace. Had 3 estimates. 1st suggested interior drain installation, dehumidifiers for water removal - $1949, 2nd said everything looks good, no mold, no water, maybe a dead animal, 3rd said fix vapor barrier by sealing edges and fog for odor - $3660. Smell is located in cabinet under stove top. All said floor insulation missing in that particular area. It's hard to know who to trust. Thanks for listening. A.R. Moore

  2. I would have to know more information than that to help you. If you want, email our office at and I will correspond via email.

    First, have you ever seen water in your crawlspace? Two, how often do you go down there? Three, is the smell only under one cabinet, and what does it smell like? Four, does the smell under the cabinet smell like your crawlspace? How does your floor insulation look? Is this area next to an outside vent? It might be best if you email me.

    I had a customer call us out because of a smell under one sink, and the drain for the sink was vented into the cabinet causing mold to grow and a strong sewage smell.

    Larry Ralph Jr.

  3. I have a radon ventalation system installed plus crawl space fan and yet still getting the smell of dusk or animal farm type. What's your though?

  4. okay, we purchased our 1998 manufactured home 5 years ago and had an inspection at that time and had no problem. Now, we are in the process of selling our home and have ran into a few things such as...
    1. it was not anchored down
    2. we had " drystacked" blocks
    3. there is gravel on top of the vapor barrier

    So we payed to have it anchored, got cement bonding to go on the block, now they are saying the vapor barrier is not correct. It's there, we scraped some of the gravel away and took pictures.
    Can this not still pass an inspection?

  5. Probably not, the vapor barrier should be on top of the gravel. They may want you to remove it from under the gravel, but unless water is standing in the crawl, it should not be necessary. Install a new vapor barrier on top of the gravel and you should pass the inspection.

  6. Hello, I have a similiar situation going on. I have gravel over top of a plastic vapor barrier. The house is a tri-level house built in the '70's. The crawl space is accessable through the closet in the laundry room. Our home has an incredible musty smell especially after a few days of rain. I've seen some standing water in some places. Can I cover the existing vapor barrier and gravel with another vapor barrier or do I have to pull up the old plastic?

  7. It depends if there is water standing in the crawlspace or not. If there is a water intrusion problem, then water will sit on top of the plastic below the gravel. This could be resolved be stopping the water intrusion or installing a internal perimeter drain to a sump pump.

    If there is NO water intrusion, then it shouldn't be a problem to cover the gravel with another vapor barrier.

    If your problem is a musty smell, then I doubt another plastic cover on the ground will help that much. You should probably look at having a encapsulation system installed. See our website at

  8. Hello, we had a pool installed about 4 years ago and the poor drainage has caused problems in our crawl space. We've fixed the drainage problem and have been told we should have our crawl space sealed since the presence of brown spores. One company said we only need to seal the space with 6 mil vapor barrier and that the low humidity will kill any existing spores and fungus in the crawl space (cost $600). Another company states we need to spray with Timbor, seal with 20 mil vapor barrier and have a dehumidifier costing over $7000! The moisture readings were 14% to 16% before the drainage problems were rectified. The third company suggests a 10 mil vapor barrier and then fog the space with fungus/spore killer at a cost of $1500. The crawl space is not used for storage and the only people that enter it is for inspections of termite and moisture. We are confused and don't know what to do.

  9. The thickness of the plastic installed in a crawlspace is more about the durability and longevity of the product than about how much moisture it will seal out. The amount of moisture a plastic vapor barrier seals out is related to the installation process. The proper way (encapsulation system) is to fasten and seal the liner at the top of the foundation (not the sill plate), and seal every seam with tape.

    As far as what they are telling you, It is unrealistic that a 6 mil vapor barrier can guarantee you a dry enough crawlspace to prevent future mold growth. Low humidity does not kill mold growth, it puts the mold spores in a dormant state. Once the relative humidity spikes above 65 - 70 percent, the mold can potentially start growing again. This is why it is so important to prevent high humidity levels in a crawl space.

    I would only choose the contractor that is installing the encapsulation system properly. A 10 mil liner can be used, if they are sealing it properly.

    I would try and find another bid for a 20 mil encapsulation system. That seems a bit high, but I do not know the size of the crawlspace.

  10. Our crawl (house built 1974) does not have a plastic moisture barrier. We have moisture coming up by walls in our living room above crawl. Starts to dry in cooler weather, more in hot moist weather. The sub-floor and rafters look in good shape. Crawl is dirt and some stone. We are going to put a 6 mil plastic barrier on floor. Do we have to seal the plastic to the wall area? Do we have to tape seams? Money is a problem... This is something we have to do ourselves.

  11. Just a comment for others information. Do not put your absolute trust in installers from places like Lowes or Home Depot. I had 600 sq ft of Pergo installed by one of these companies on a wood subfloor. The Pergo had its own attached foam backing. However, he placed 4 mil plastic under the Pergo directly on my wood subfloor. Less that 8 months later I had 600 sq ft of mold growing underneath as a result of the moisture from crawl space coming up and being trapped beneath the plastic next to my wood subfloor. This man was supposed to be a flooring "professional".

  12. So I have problem #1...Now what do I do???

  13. Depends on if there is a water problem in scenario #1. If there is a water standing, an internal perimeter drainage system needs to be installed to prevent water from standing on top of the plastic.

    If there is no water, the crawlspace can be encapsulated over the gravel.

    Good Luck.

  14. Our house was built in 1994 and has a cement floor crawl space. We have a drainage easement in our backyard about 15 feet away, which holds about 3-4 ft. deep of water even days after heavy rains. The soil seems to be alot of sand. There is farmland across the street from us. Our house does sit alot higher than our neighbors, but we keep getting water in our crawl space after lots of rain. We can tell that some of the water is coming from small cracks in the floor, but when the house was built they glued styrofoam to the walls and we can't tell if the walls are cracked. They did not tell us we were in a high water table, nor did they install a sump pump. So my husband has to go down and manually pump the water out with a utility pump and a wet/dry vac. We are getting the musty stagnant water smell in our home now. What would you recommend for our situation?

  15. There are some great discussions.

    I have a 1890's farm house in Virginia. I currently have large pieces of plastic throughout the dirt floor crawlspace. I have no problems with smell or moisture, however I have no insulation in the crawlspace either. 1)I have some wood supports that I'm assuming are untreated lumber since they look a lot like tree stumps. I don't want to seal moister around them. How should I handle these when installing a vapor barrier. 2) I am considering insulating the walls of the crawlspace based on many discussions and my crawlspace is only about 18" from dirt to bottom of joist, on average. I like the space between the joists for turning over and I also have pipes that don't exactly hug the frame, these might be difficult to insulate with the floor. My concern with insulation is termites.

    Any comments or suggestions are welcome.

  16. We had our crawlspace encapsulated and a dehumidifier installed a month and a half ago, the smell has dissipated some but I still smell it. When if ever will the smell go away?

  17. Every situation is a little different so I'm not sure what is still causing the smell. I've seen dead mice cause a smell, a sewage pipe vent in a crawl, etc. If it is a musty earth odor, a few customers that continue to have this odor install a radon mitigation system under the liner, and it seems to do the trick. You may want to condition the crawl space with the heating and cooling system. If you do, make sure there is a return air duct in the crawl.

    A environmental consultant should be able to pinpoint the odor if the company who installed the encapsulation system cannot find the cause.

  18. I am planing to put up insulation Lowes has this one that is wraped in plastic I was going to put up into the floor joist first then use an unfaced for the rest of the 9 1/4" held up with wire. Do you see a problem with this I think the plastic up to the floor plywood would be ok what do you think

  19. I will refer you to these pages for info on Crawl Space Insulation :


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