My Research on Vapor Barriers Before Meeting a Crawlspace Contractor

Dirt crawlspace under a single family house.
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Now that standing has been eliminated from belly height dirt crawlspace, I can confirm the dirt is only moist and damp, enough so that if I were to crawl around, my clothes would get all muddy. Could that wet dirt be contributing to higher humidity in the main floor of the home? And if the humidity is too high, can there be mold and mildew in the crawlspace? How about other insects or rodents that are feeding off the water? I wanted to get a evaluation and quote from the vapor barrier installers on their suggestions on how to resolve the issue.

But before they arrive, I wanted to do a bit of research on the topic so I have some knowledge before they arrive to make a informed decision. Below is a summary of the research of done on vapor barriers and dirt crawlspaces. There are quite a few YouTube videos along with some links to websites. Hats of too Crawl Space Ninja's, who have a ton of informative videos on YouTube. Unfortunately, they only operate out of Tennessee and Georgia, not the San Francisco Bay are in California.

How much will a vapor barrier reduce humidity?

  • According to Crawl Space Ninja (this video), vapor barriers will slow down the escape of humidity, but it won't stop 100% of the moisture which can still get through. They recommend also installing a dehumidifier.
  • Tanner Flowers (his comments from his video) also confirms that a vapor barrier works better in conjunction with a dehumidifier. The same is vice versa (a dehumidifier works better when the crawlspace as a vapor barrier).
  • Dr Crawlspace (in Should You Vent Your Crawl Space?) also confirms a vapor barrier with dehumidifier is best.

How much of the crawlspace to encapsulate?

You want to cover as much as the dirt as possible. Concrete, wood, and dirt can all hold moisture. But the dirt holds the most moisture, so the goal of the vapor barrier is to prevent as much of the dirt moisture to evaporate into the crawlspace. However, you never want to encapsulate any wood or pipes in the crawlspace.

  • Never cover wood such as floor joists in the crawlspace with a vapor barrier. Condensation may build up and pool up behind the barrier resulting in rotted wood. Covering the wood may increase the chance of the wood rotting, since any moisture that does get behind the vapor barrier will also have a more difficult time getting out. There's also a chance if your home needs a termite inspection, the wood won't be accessible and that section of the vapor barrier will need to be removed.
  • Never cover any pipes (water or sewage). If there's a leak, the water will pool up behind the vapor barrier.
  • Do cover the entire dirt floor.

How do I know if the a vapor barrier is correctly installed?

My dirt crawlspace is very short. There's only enough height to belly crawl. There's not enough height to crawl on my knees of stand anywhere in my crawlspace. So how will I know if the vapor barrier is correctly installed in my crawlspace? If it's too difficult for you to access it yourself, the best way is for the vapor barrier contractors to shoot a quick video of the entire space.

Confirm that the concrete walls and other concrete footers are all secure taped up and glued 3 inches from the top (or based on your regions building code). It should be attached to the top of the concrete foundation. The vapor barrier should also be overlapped (ideally around 12 inches), with the overlapped sections completely glued together with construction adhesive.

What are different types of vapor barrier?

Another informative video by Crawl Space Ninja that outlines different types of vapor barriers. There's also a sales video by IDI distributor (so he's a salesman selling Viper product) that talks about his products. Most importantly, ask for a sample of the vapor barrier during the evaluation and quote. There are pros and cons to different types of vapor barriers.

  • Thickness? A 6 mil vapor barrier from hardware stores like Home Depot may start to degrade (thin out) over 6-7 years.
  • Glue up wall? You'll want a thicker vapor barrier.
  • Reinforced vapor barrier? There are cords sandwiched between two layers of vapor barrier that provide extra strength. But over time, if the reinforced vapor barrier comes apart, the cord gets exposed can start off-gassing smells described as similar to cat urine.
  • How they measure thickness? If the vapor barrier is reinforced, is it measured on the cord thickness (which will be thicker) or the plastic only thickness (which IMHO is more accurate)? What's the true thickness?
  • What color? White, black, or clear? Depending on your building code, you might be required to go with a certain color.
  • Virgin Sourced Poly? Recycled poly breaks down easier.
  • Puncture resistance?
  • Tear resistance?
  • Weight? Important if this is a DIY project.
  • Permeance Rating? This is the amount of vapor that can pass through (one sqare foot per hour)

12 mil or 20 mil vapor barrier?

Crawl Space Ninja (https://www.youtube.com/watch?v=nu57G1Jnm_U) only installs 12 mil vapor barriers because when it reaches 20 mil, it becomes less flexible and more difficult to handle. Now there's 20 mil reinforced vapor barriers, which is basically two 10 mil joined together, but that's not recommended.

What is a reinforced vapor barrier? Is it worth it?

A reinforced vapor barrier is two sheets of vapor barrier joined together. Reinforced vapor barrier suffer a similar problem like laminate flooring. If any liquid gets between the layers, its not only difficult to detect, but even more difficult to remove. In the case of reinforced vapor barrier, if the liquid is urine, the source of the smell is difficult to detect.

  • Crawl Space Ninja (this video) doesn't recommend reinforced vapor barrier, but instead to go with a single layer solid vapor barrier.
  • Though many installers push from this reinforced vapor barrier, the glue it uses to connect the two may separate. It may also give off a smell, similar to cat urine. Ask your installer what brand of vapor barrier is used. See from Crawl Space Ninja DIY store.

How does the crawlspace need to be prepared before laying down a vapor barrier?

Installing a vapor barrier covers the entire crawlspace. So if any prep work is required, you better have it done before the vapor barrier is set down.

  • Removing debris such as wooden board, leftover buckets, etc.
  • Leveling the dirt
  • Installing a french drain into a sump basin with a sump pump.
  • Dehumidifier with proper drainage
  • Add a soil gas dispersion fabric (though I haven't found this anywhere)
  • Repair any exterior entryways so rainwater won't flow into the crawlspace.

How much does a vapor barrier cost to install?

Costs vary more than just total square feet, as I'd assume the height of the crawlspace will factor into the final cost.

  • Tanner Flowers (his comments from his video) who is based in Tennessee states in a comment that the particular video (with fairly high crawlspace height, but large square footage done several years ago, maybe 2015) costed over $10,000. In a separate comment, he states in 2018 that jobs start at $7,000 and up. A 2,000 sq/ft house would be more than $7,000.

How long does a vapor barrier last?

Since a crawlspace vapor barrier isn't cheap, knowing how long it will last is important.

  • Crawl Space Ninja (this video) warranties their 10 mil vapor barrier for 15 years and their 12 mil vapor barrier for 20 years.

What happens if water gets on top of a vapor barrier?

If water gets on top of a vapor barrier because of a pipe leak, then there's no easy way for the water to evaporate or dry out. One expensive way is to leave the work to a dehumidifier. This would cost lots of money to literally evaporate all the water. Another way is to have a sump pump installed which would pump out most of the water. But this would need to happen before a leak. Also, if the sump pump isn't installed on a UPS (universal power supply), then a cut in power during a storm can cause flooding in the crawlspace.

Vapor barriers in in a vented crawlspace?

My crawlspace is vented, which means there are vents to the outside. So theoretically, this means the outside humidity will always want to equalize with the humidity in the crawlspace. So if I install a vapor barrier over the dirt, but leave these vents open and exposed, will there be any change to the humidity in the crawlspace? According to a post called "Ground vapor barriers in vented crawl spaces: do they work?" at Basement Systems USA in Maryland, a vented crawlspace with a vapor barrier can still cause problems.

Other Helpful YouTube Videos

Ever since we moved into our home in Millbrae, our guests have commented our home is notably chilly. We felt it too, but wasn't sure what to do about it. Here's are the projects I've done to improve the temperatures inside my home. For those not familiar with Millbrae, it is a city in the SF Bay Area. The weather rarely gets to freezing temperatures in the winter and rarely gets to 100F in the summer. We never get snow.
  1. Results of Re-Insulating Attic in SF Bay Area
  2. How to Seal Power Outlets and Switch Airleaks
  3. Better Home Insulation by Sealing Gaps
  4. My Dog Door for Sliding Patio Door Is Drafty? Seal it with Insulation!
  5. Solving Water Intrusion into VERY SHORT Dirt Crawlspace
  6. Alternative to Messy "Great Stuff"
  7. How to air seal recessed lights on a budget?
  8. My Research on Vapor Barriers Before Meeting a Crawlspace Contractor
  9. How to Stop Cold Air from a Unused Fireplace
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