Never Use 6 Mil Vapor Barrier in Crawl Space (Here's Why It Fails)

If a contractor tells you 6 mil plastic is good enough for your crawl space vapor barrier, they're either lying or don't know what they're talking about. In this video, I'll show you exactly why 6 mil vapor barriers fail, what it's costing you, and what you should use instead.

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⚠️ THE 4 MAJOR PROBLEMS WITH 6 MIL VAPOR BARRIERS:

1️⃣ TOO THIN - Tears easily with foot traffic, HVAC contractors, plumbers, and electricians going through your crawl space. 70-80% are damaged within 2 years.

2️⃣ PEST DAMAGE - Rodents, mice, rats, and possums tear through 6 mil plastic like tissue paper. They live underneath it, creating moisture, odors, and serious health hazards.

3️⃣ POOR SEALING - Most contractors just overlap the seams without taping. Moisture vapor still gets through unsealed seams, making it almost completely ineffective.

4️⃣ MOISTURE FAILURE - 6 mil is designed for vapor, not standing water. After heavy rains, it floats around, exposing dirt and allowing moisture to evaporate into your floor joists, causing mold and wood rot.

💰 HIDDEN COSTS OF 6 MIL PLASTIC (10-Year Total):
• Replacement every 2-3 years: $1,500-$3,000
• Increased energy bills: $3,600-$9,000
• Mold remediation: $3,000-$15,000
• Pest control & cleanup: $500-$3,000
• Structural repairs: $5,000-$25,000
• Insulation replacement: $2,000-$5,000
TOTAL: $15,000-$50,000+

Compare that to a proper 12 mil encapsulation system: $4,000-$8,000 upfront, lasts 20-25 years with minimal maintenance. That's only $300/year vs $667/year+ for 6 mil.

✅ WHAT YOU SHOULD USE INSTEAD:
• 12 mil solid vapor barrier OR 12 mil woven vapor barrier
• Proper vapor barrier seam tape (not duct tape)
• Mechanical fasteners (Christmas tree fasteners or Hilti nails)
• Double-sided butyl tape for wall attachment
• Crawl space dehumidifier (essential - plastic alone isn't enough)

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**Disclaimer:** This information is for educational purposes only. Consult professionals for specific advice. Some links may be affiliate links, which means we may earn a small commission at no extra cost to you.
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Transcript

Why 6 Mil Fails

SPEAKER_00 0:00

If a contractor tells you that a 6mm plastic is good enough for your crawl space vapor barrier, they're either lying to you or they don't know what they're talking about. And I'm going to prove it to you in this video. Over the past 20 years, I've personally removed tens of thousands of square feet of failed six mil vapor barrier, and I'm going to show you exactly why it fails, what it's costing you, and what you should be using instead. But stay tuned all the way to the end because I'm going to share with you the only time you should ever use six mil plastic in your crawl space, and it's probably not what you think. If you're new to Crawl Space Ninja, we talk about everything related to crawlspace encapsulation, basement waterproofing, vapor barriers, moisture control, mold remediation, and indoor air quality. So if all of those things are important to you, I suggest that you subscribe to our channel and ring that notifications bell. So let's start with something that most contractors use as their excuse for installing six mil plastic, building code.

Code Minimums Aren’t Best Practice

SPEAKER_00 1:01

According to the International Residential Building Code, the minimum requirement for a vapor barrier in a vented crawl space is 6mm polyethylene plastic. That's it. 6 mil. Now here's what contractors want, Tilly. Minimum means minimum. It's the absolute bare minimum that you can get away with and still meet code. It doesn't mean it's good. It doesn't mean it's going to last. It doesn't mean it's in your best interest as a homeowner. Think about it this way. Building code also says you can build a house with 2x4 studs. But most builders who believe in quality use 2x6s because it provides space for better insulation, for improved comfort, and soundproofing. It's stronger, and according to the internet, R21 insulation in a 2x6 versus R13 insulation in a 2x4 may reduce utility bills by up to 57%. It's just a better house. Same thing with vapor barriers. The building code for vapor barriers was written decades ago, and it hasn't kept up with what we know about moisture control, building science, and long-term durability. The code is really designed to keep you from building something dangerous. It's not designed to give you the best possible solution. And I can tell you from 20 years of experience in thousands of crawl spaces across seven states, six mil plastic is not a long-term solution. It's barely a short-term solution. And I'm going to show you exactly why. Now let's talk about the number one problem with six mil

Real-World Damage And Traffic

SPEAKER_00 2:28

plastic. It's too thin, period. Six mil means six thousandths of an inch. To put that in perspective, that's about the thickness of a cheap trash bag. Actually, it's thinner than most contractor grade trash bags. You can easily tear it with your bare hands. No tools, no sharped objects, just your hands. Now, do you really want something this fragile as the only barrier between your home and ground moisture? Here's what happens in the real world. Your crawl space isn't some sealed-off area that nobody ever goes into, although you may not personally go in there. You've got the HVAC contractors going down there to service your heating and cooling system. You've got plumbers going down there to fix leaks. You've got electricians running wires. You've got pest control technicians. You've got home inspectors during real estate transactions. You may even have a bear. Who knows? Because you never go down there. Every single time someone or something crawls across that six mil plastic, they're creating stress points. They're creating tears. They're creating punctures. And once you get a little tear, it just keeps getting bigger and bigger. I've seen six mil vapor barriers that look perfect when they were installed, and six months later, they've got tears. One year later, they look like Swiss cheese, and two years later, they're completely destroyed. And this isn't an isolated problem. This is the norm. This is what happens with six mil plastic and moderate to high traffic crawl spaces, which by the way is pretty much every crawl space. In fact, I'd estimate that 70 to 80% of the 6mm vapor barriers I inspect are damaged within the first two years. That's not a long-term solution. That's a temporary band-aid that's costing you money. Now the durability problem is bad enough on its own, but there's something even worse that happens when the 6mm plastic starts to fail. And this is the part that's going to affect your family's health and your home's air quality. But before I get into that, let me share with you what happens when pests discover just how easy it is to get through 6mm plastic.

Pest Intrusions Under Plastic

SPEAKER_00 4:34

Okay, so this is one of the most common problems I see with 6mm plastic, and it's something that contractors almost never warn homeowners about. Rodents, mice, rats, possums, even raccoons, they can tear through 6mm plastic like it's tissue paper. And here's what happens: they'll find one little edge, one little seam, one little existing tear, and they'll just rip it wide open. Then they crawl underneath the vapor barrier, and now you've got rodents living between your vapor barrier and your dirt floor. And when they're down there, they're leaving droppings everywhere, they're urinating, they're bringing in nesting materials, they're dying down there. And all of that is creating moisture, it's creating odors, and it's creating serious health hazards for your family. Many years ago, I found a Mama Possum with her babies living right underneath the six mil vapor barrier of a crawl space I was inspecting. The homeowner had no idea. They called me out because they kept smelling this terrible odor and couldn't figure out where it was coming from. Turns out Mama Possum had set up shop right under their kitchen. Now compare that to a thicker vapor barrier like the Ninja Brand 12mm solid or 12mm woven material found on our DIY store. By the way, link in the description. I'm not gonna lie and tell you pests can't get through it, but they cannot get through it as easily as the Builder Grade 6mm you have in your crawl space. You see, pests are pretty lazy. So many times they give up when they encounter thick vapor barriers and the crawl space is well sealed, well insulated, and moisture is being controlled. I'll bet they'll mosey over to your neighbors instead. Maybe you should share this video with them. Now, if you're in an area where you've got any kind of pest problem or infestation, and let's be honest, that's pretty much everywhere from Delaware to Georgia where we fix crawl spaces, basements, and attics, six mil plastic is just not going to protect your crawl space long term. But here's where it gets even more frustrating for homeowners.

Bad Seams And Poor Sealing

SPEAKER_00 6:39

Even if you somehow manage to keep pests out and avoid tearing the plastic with foot traffic, there's a fundamental problem with how six mil vapor barriers are typically installed that makes them almost completely ineffective for moisture control. And this is the part that really gets me fired up. Alright, so here's the thing that drives me absolutely crazy about most 6mm vapor barrier installations. And this is something I see probably 80 to 90 percent of the time. The plastic itself is only part of the system. What really matters is how it's sealed. And most contractors installing six mil plastic don't seal it properly. What I see all the time is contractors will roll out the six mil plastic, they'll overlap the seams maybe one to two inches, if at all, and then just leave it. No tape, no sealing, nothing. Now, building code says you need to overlap the seams by at least six inches in a vented crawl space. But here's what they don't tell you just overlapping the seams doesn't actually seal them. Moisture vapor can still get through those overlaps. Water can still get through those overlaps. And when you look at it with a thermal imaging camera or use a moisture meter, you can see moisture coming right through those unsealed seams. It's like having a raincoat with gaps in it. Sure, you're covered, but you're still getting wet. Proper vapor barrier installation requires you to tape every single seam with a vapor barrier seam tape that's designed for this application. You need to seal around the piers, you need to seal around the penetrations, you need to attach it to the foundation walls with mechanical fasteners and butyl tape. And when you do that, you can see a massive difference in your moisture readings. We're

Moisture, Floating Plastic, Mold

SPEAKER_00 8:28

talking about going from 70 to 80% relative humidity down to 50 to 55% relative humidity just by properly sealing the vapor barrier, and of course, don't forget that dehumidifier. But here's the problem 6mm plastic doesn't come as part of a complete system. Contractors usually buy it in bulk from big box stores. They don't buy the matching tape, they don't buy the proper fasteners, they don't buy the sealants. They just roll it out, overlap it, and call it a day. And that's because they're trying to meet a minimum code requirement as cheaply as possible. They're not trying to give you the best solution. They're trying to give you the cheapest solution that still passes inspection. Versus when you get a proper encapsulation system like what we install here at Crawlspace Ninja, it comes with everything. The 12 mil vapor barrier, the vapor barrier tape, the mechanical fasteners, the sealants. Everything is designed to work together as a complete system. So even if your 6mm plastic somehow stays intact and doesn't get torn or damaged by pests, if it's not properly sealed, it's not doing its job. And most 6mm installations are not properly sealed. Now, all of these problems I've talked about so far, the tearing, the pest damage, the poor sealing, they all lead to one major issue that's going to cost you thousands of dollars if you don't address it properly. And this is probably the most important reason to avoid six mil plastic. Okay, so let's talk about what happens when your six mil vapor barrier fails to do its primary job, which is control moisture. First of all, six mil plastic is designed to stop moisture, vapor, not standing water. So if you've got any kind of groundwater intrusion, any kind of surface water coming into your crawl space through the foundation, that six mil plastic is just going to float. I see this all the time after

The True Cost Breakdown

SPEAKER_00 10:21

heavy rains. The homeowner will go into their crawl space and the vapor barrier is just floating around, completely displaced from where it was originally installed. Now you've got water sitting on top of the plastic in some areas, and in other areas the plastic is floated away and dirt is completely exposed. And when that happens, all the moisture is evaporating up into the floor joists, into your subfloor, into your fiberglass insulation, and that's when you start getting mold growth. That's when you start getting wood rot. That's when you start getting that musty smell coming through your HVAC system. I've been in crawl spaces where the homeowner said, but we just had vapor barrier installed two years ago. How do we have mold? And when I get down there, that six mil plastic is torn in 15 different places. It's floating around and there's mold growing everywhere. Now, even when six mil plastic is working perfectly, no tears, no damage, properly sealed, which is rare, it's still not as effective at blocking moisture vapor as thicker materials. The moisture transmission rate of a 6mm plastic is higher than a 12 mil. And the difference might not seem like a lot on paper, but when you're talking about long-term health of your home and your family, every little bit matters. Especially if you don't have a dehumidifier installed in the crawl space. And of course, why wouldn't you? You see, plastic alone, whether six or twelve mil is not enough to prevent mold. You still need a dehumidifier because vented crawl spaces allow outside humidity to enter. Your crawl space is under attack from ground moisture and outside humidity. Mold starts to grow at around 60% relative humidity. So if your crawl space is sitting at a 70% relative humidity, you're creating the perfect environment for mold growth. Not to mention dust mites, bacteria, viruses, and all the other things that thrive in the high humidity environments. And

What Proper Encapsulation Requires

SPEAKER_00 12:19

remember, 50% of the air you breathe in your home comes from your crawl space. So if you've got high humidity down there, you're breathing that air along with everything that comes up from it. Now at this point, you might be thinking, okay, Michael, I get it. Six mil plastic has a lot of problems. But it's so much cheaper than that thick material that you're recommending. Isn't it worth it to save the money up front? And that's exactly what I want to talk about next. Because this is where contractors really aren't being honest with you about the true cost. All right, so let's talk about money. Because this is usually the number one reason people choose six mil plastic over a properly encapsulated crawl space. A contractor might charge you $1,500 to $3,000 to install a 6mm plastic in your crawl space versus a proper encapsulation system with 12mm material might cost you $4,000 to $8,000 or even more, depending on the size of your crawl space and what else needs to be done. So on the surface, it looks like you're saving $2,500 to $5,000 by going with the 6mm plastic. That sounds pretty good, right? Here's what that calculation doesn't include. And this is based on real numbers from real homeowners I've worked with over the past 20 years. Number one, the cost of replacing that six mil plastic in two to three years when it fails, that's another $1,500 to $3,000. This isn't hypothetical. This is what actually happens, and that's if you even notice the 6 mil has failed. What if you don't notice it's been failing for 10 years? Number two, the cost of increased energy bills because your HVAC system is working harder to deal with the excess moisture and humidity. That could run anywhere between $30 to $75 a month or $360 to $900 a year over a 10-year period that's $3,600 to $9,000. Number three, the cost of a potential mold remediation if moisture gets out of control. And trust me, it gets out of control. That could be an additional $3,000 to $15,000 or more, depending on how bad it gets. Number four, the cost of pest control and cleanup if rodents get underneath your vapor barrier and start living down there. That could be anywhere between $500 to $3,000, plus the cost of replacing the damaged vapor barrier. Number five, the cost of repairing structural damage caused by long-term moisture exposure, wood rot, floor joist damage, subfloor damage. That could be $5,000 to $25,000 or more. Number six, the cost of replacing insulation if you have fiberglass insulation that gets wet and moldy. That could be another $2,000 to $5,000. So when you add it all up over a 10-year period, that cheap 6mm plastic insulation could actually cost you between $15,000 to $50,000 or

When 6 Mil Is Temporarily Useful

SPEAKER_00 15:22

more versus a proper 12mm encapsulation system that costs $4,000 to $8,000 up front and lasts 20 to 25 years with minimal maintenance. Let's break that down even further. If you spend $6,000 on a proper 12 mil encapsulation system and it lasts 20 years, that's $300 a year. If you spend $2,000 on a 6 mil plastic and have to replace it every three years, that's $667 a year, plus all the other costs I just mentioned. And here's the other thing. When you go to sell your home, a proper encapsulation system is a selling point. It's something you can show to potential buyers and say, hey, we took care of this. You don't have to worry about it. We've got a transferable warranty from Crawspace Ninja. But if you've got torn up 6mm plastic in your crawlspace, that's going to show up in the home inspection report. And now you're either going to have to replace it before you can sell, or you're going to have to negotiate on the price of the house. Either way, you're losing money. I've seen home sales where the buyer demanded $5,000 to $10,000 credit because the crawlspace had a failed vapor barrier and moisture problems. So that cheap 6 mil plastic just costs you thousands of dollars at closing. The cheap option really isn't cheap at all when you look at the total cost of ownership. Now I know I've spent a lot of time telling you what not to do, but you're probably wondering, okay, Michael, so what should I do instead? What's the right way to encapsulate a crawlspace? Alright, so here's what a proper crawl space encapsulation system should look like. And this is based on 20 years of experience, thousands of installations, and what actually works in the real world. Number

Health, DIY Options, Next Steps

SPEAKER_00 17:12

one, you want a minimum 12mm vapor barrier thickness. At Crawlspace Ninja, we use either a 12mm solid vapor barrier or a 12mm woven vapor barrier. Both are significantly more durable than the 6mm plastic. The 12mm solid is a single piece of high-density polyethylene. It's not glued together, it's not laminated. It's one solid piece of plastic that's twice as thick as the 6mm. It's white, which causes the crawl space to be bright and easy to inspect, and it's strong enough to hang 12 to 15 feet up the foundation walls without tearing. The 12mm woven has a woven fabric layer in the middle that makes it even more tear resistant. It's great for high traffic areas or crawl spaces where you know contractors are going to be going in and out regularly. Now you might be wondering, what about 20 mil reinforced vapor barriers? And here's something most contractors won't tell you. A lot of reinforced vapor barriers are actually made by gluing two layers of plastic together with a cord or a fabric in the middle. And over time, those layers can start to delaminate or separate. And when that happens, if the cord or fabric gets wet, it can create a smell that's very similar to cat urine. I'm not kidding. We've been in crawl spaces where the homeowner thought they had a cat infestation, but it was actually the vapor barrier breaking down. So we don't recommend reinforced vapor barriers for that reason. Our 12mm solid and 12mm woven don't have that problem because they're not glued together. They're single-piece construction. Number two, you want it to be part of a complete system. That means vapor barrier tape that's designed to seal the seams. That means mechanical fasteners like Christmas tree fasteners or Hilty nails to attach it to the foundation walls. That means sealants to seal around the penetrations. Number three, you want it professionally installed. Or if you're doing it yourself, you want to follow the proper installation process. And here's what that looks like. Step one, site preparation. You need to remove any debris, rocks, roots, or sharp objects from the crawl space floor. If you've got a really rocky crawl space, you might want to consider installing a drainage mat or underlayment first to protect the vapor barrier. A drainage mat is basically a dimpled plastic sheet that goes down first. It protects the vapor barrier from punctures and it also allows water to flow underneath the vapor barrier to your drainage system if you have one. We use these in crawl spaces with gravel or bedrock where we can't remove the sharp objects. Step two, attach the vapor barrier to the foundation walls. Install the two-sided butyl tape from our DIY store three inches below the seal plate and leave a termite gap. But you may want to install foam board insulation prior to the vapor barrier for a more energy efficient hole. You want to bring the plastic up the wall and adhere it to the two-sided butyl tape, then mechanically fasten the vapor barrier to the wall every 12 to 16 inches. We use Hilty Masonry nails, but Christmas tree fasteners work well for the DI wire. You drill a quarter inch hole in the concrete block or poured concrete, and then you push the fastener through the vapor barrier, the butyl tape, and into the hole. It expands and locks in place. Very secure. Step three, install plastic on the pillars just like you did on the walls. Step four, install the ground plastic. You want to roll it out, overlap the seams by at least six inches, but we always recommend 12 because it's better. And then tape every single seam with vapor barrier seam tape. Not duct tape, not masking tape. Vapor barrier seam tape that's designed for this application and is also available on our DIY store. And when you tape the seams, you want to make sure that you're getting good adhesion. Press it down firmly, make sure there are no wrinkles or air bubbles. You want a complete seal. We bring the floor plastic over the wall and pillar plastic for the best moisture control possible. Step five, and this is critical, install a dehumidifier. A vapor barrier alone is not a complete solution. The vapor barrier stops moisture from coming up through the ground, but you still need to control humidity in the air. The Ninja Dry Pro 70 is one of the most efficient and quietest dehumidifiers on the market and is available on our DIY store. And when you do all of that, you end up with a crawl space that's dry, clean, bright, and worry-free. Word of caution if you've got water intrusion issues, you need to address those with proper drainage and a sump pump before installing the vapor barrier. A vapor barrier is not a substitute for proper water management. Now, I promised you at the beginning of this video that I would tell you the only time you should ever use 6mm plastic in your crawlspace. And here it is. The only time that we use six mil plastic is as a temporary Measure during mold remediation. And here's why. If you've got a crawl space with no vapor barrier at all and you've got mold growth, you just can't start remediating the mold without addressing the moisture source. The mold will just come right back. So what we'll do is install a temporary 6mm vapor barrier to dry out the crawl space before we start the mold remediation process. This gives us a baseline level of moisture control so we can actually get the space dry enough to remediate the mold. And once the mold remediation is complete, we remove the 6mm plastic and install a proper 12mm encapsulation system. The 6mm is just a temporary band-aid to get the space dry enough to work in. That's it. That's the only time we use 6mm plastic as a temporary measure during mold remediation, never as a permanent solution. Look, I get it. Nobody wants to spend more money than they have to on their crawl space. It's not the most exciting part of your home. You don't see it every day, you don't show it off to your friends. But here's the thing 50% of the air you breathe comes from your crawl space. So if you've got moisture problems down there, if you've got mold down there, if you've got pests down there, that's affecting your indoor air quality. That's affecting your family's health. And the goal here is to create a worry-free crawl space, something that's protecting your home, protecting your investment, and protecting your family's health. And you're not going to get that with six mil plastic. I'm sorry, but you're just not. Also, if you want to do this yourself, no problem. We've got everything you need on our DIY store at supply.crawlspaceNinja.com. We've got the 12mm solid vapor barrier, we've got the 12 mil woven vapor barrier, we've got the vapor barrier seam tape, we've got the Christmas tree fasteners, the butyl tape, the dehumidifiers, we've got the drainage matting for those rocky crawl spaces, the sump pumps, and of course the exclusive hydroway waterproofing system. Everything you need to do it right. So make sure you check out the links in the description. And I'm Michael Church with Crawl Space Ninja, and I hope you make it a happy and blessed day, and we'll see you later.