The Ultimate Guide to Foundation Vapor Barrier

Vapor Barrier on new construction job-site

Its common for soil moisture to soak in through the concrete foundation slab and cause humidity and moisture issues in homes, or new construction projects. This problem is preventable in new construction homes and some existing homes with a crawl space. We want to share the simple solution of using a foundation vapor barrier.   

The foundation vapor barrier is a plastic sheeting placed over compacted dirt during construction. This physical barrier prevents ground moisture from intruding the concrete slab and creating humidity and moisture issues in a home, or building. Vapor barriers also mitigate radon gas from entering the home.  

• What a vapor barrier is and how it works
• Why a vapor barrier is essential
• Common applications

What is a Vapor Barrier?

A vapor barrier is physical membrane typically made of plastic resin sheeting designed to prevent moisture and gas from moving up from the ground into the concrete slab or home. Most barriers of this kind have a very low permeability (water flow through rate).

Foundation Vapor Barrier is different than in wall or house wrap barriers in that this is placed before construction of the foundation slab, or in a dirt floor crawl space.  They are designed to stop moisture and reduce damage to floors, and prevent mold. 

Plastic Vapor Barrier installed before slab pour

Plastic Vapor Barrier installed before slab pour

What is Vapor Barrier Made of?

The highest quality vapor barriers are constructed of 100% virgin resin and produced through multi-layer co-extrusion.  This is noteworthy as not all barriers are created equal.

In years past both clear and black polyethylene sheeting were commonly used in basements and as a vapor retarder. This product commonly known as Visqueen does not provide sufficient protection against moisture or gas infiltration.  Additionally cheaper plastics are less puncture and deterioration resistant, reducing their effectiveness over time. 

How do Vapor Barriers Work and where are they used?

Vapor Barrier is defined as a physical barrier that has moisture and air permeance (pass through) rating 0.01 perms or less. Other plastics often referred to as vapor retarders are not sufficient for preventing water intrusion into a building.  A vapor retarder typically made of thin Visqueen plastic does not have the same low permeability to prevent moisture from transferring from the soil into the concrete foundation slab. 

Without intervention a concrete foundation will naturally absorb moisture. By adding a resistive layer barrier also known as the *"Capillary Break" under the foundation slab we can drastically reduce this water transfer.

Additionally high quality vapor barriers will prevent the transfer of not only water vapor/moisture, but Radon gas. Radon can be a significant issue in various regions of the US, so there is additional value in using a barrier that can prevent multiple issues. 

* See the diagram below, yellow colored Stego Wrap acting as Capillary Break

Diagram of Vapor Barrier preventing migration of moisture and Radon

Stego Wrap Brand Diagram of Moisture and Gas Entering a home

Additionally high quality vapor barriers will prevent the transfer of not only water vapor/moisture, but Radon gas. Radon can be a significant issue in various regions of the US, so there is additional value in using a barrier that can prevent multiple issues. 

Vapor Barriers are meant for new construction projects and encapsulation 

Foundation barriers are unfortunately unable to be retrofitted under existing concrete slabs, so it is important to install them on new construction projects. Another popular use case is in crawlspaces with dirt flooring. 

Encapsulation is a process of sealing a crawl space in a high quality barrier to prevent moisture from entering into the subfloor of a home. 

3 Reasons Vapor Barriers are Essential for Your Home:

1. Moisture And Flooring Damage

There are several issues that can affect the flooring above the crawl space whether your home is new, or not-so-new. 

• Visible Flooring Damage
  - Wood floors may appear scalloped, buckled, cupped, warped, or sagging
  - Vinyl or linoleum flooring may bubble, wrinkle, make popping noises when you step down, or generally delaminate
  
• Visible mold growth on the baseboards and flooring
• Musty, mildewy odors emanating from the crawl space

wood flooring buckling due to moisture

Wood floors can be a big problem for homeowners due to their hygroscopic qualities. That means floors will search out moisture from the environment then hold on to it.  This will cause a number of issues both physically for the flooring, and the health of the people living in the home. 

2. Mold

In order to survive, mold needs three things – moisture, sustained temperatures somewhere between 50° and 122° F, and food sources such as dust, drywall, etc. 

If moisture is coming into a building, mold will grow. According to the Institute of Medicine, 1993, toxic molds such as Stachybotrys can actually be fatal for 5% of people and in others, cause any number of serious health issues. Several studies have shown toxic mold has led to many million-dollar insurance settlements.

Many of the top insurance companies have limited or removed coverage for mold claims believing these claims will ruin and bankrupt them. More than ever before, it's critical that careful attention is paid to prevent moisture vapor from entering buildings from the below-slab environment.

Mold in basement in failed vapor barrier. Picture courtesy of Environix

Vapor Barrier is an excellent choice to protect against these elements. Many architects and engineers are leaning toward this product as inexpensive insurance against these costly problems. Stego Wrap Vapor Barrier is an excellent product for incredible strength and durability. It effectively protects the concrete slab from moisture entering. Stego Wrap Vapor Barrier will help remove the risk of water vapor accumulation and the transmission of various soil gases. Visit Stego Wrap Barrier 15-Mil

3. Radon Gas

Radon gas usually comes from the soil under a home's foundation. Sometimes it's found in water but not that often.  Radon is an odorless, colorless radioactive gas that can enter a home through entry points in the concrete floor slab both in basements and slabs-on-grade. Also, radon can enter your home anywhere that is exposed to outdoor elements. A vented crawl space with no vapor barrier is an easy entry point.

Radon Gas can pass from soil through the home with use of barrier and vent pipe

Many homes throughout the Midwest and Northeast have Radon Vent pipes that run from the gravel beneath the foundation through to the roof of the house, allowing the gas to be directed through the home.

These pipes along with proper installation of vapor barrier help ensure proper air quality within the home/building. 

It's important to know that the federal action limit for radon is 4 pCi/L, anything above that number is considered a very high risk by the EPA and requires immediate removal. Even lower concentrations of radon can still be very harmful and any concentration of it in your home is something you should be concerned about.

Radon is the number one cause of lung cancer among people who do not smoke and if you are a smoker, the risk is even higher. According to the World Health Organization, people exposed to radon are more vulnerable to the effects of ionizing radiation.

What zones in the US are prone to subsoil Radon Gas?

The level of radon in residential construction depends on location and geography. As an example, a family lived in a home for seven years that was contaminated by radon and had no idea their home in northeastern Pennsylvania was in a critical spot on the EPA's radon zone map. According to their map, the United States is split into 3 zones of different risks for radon:

US Map of Radon Zones from EPA

• Zone 1 (Red) has the highest risk of radon levels above the US federal action limit of 4 pCi/L These homes are mostly in the northern and eastern United States.
• Zone 2 (Orange) has a moderate risk of 2-4 pCi/L for homes in the western United States.
• Zone 3 (Yellow) has a low risk 2 pCi/L for homes in the southern United States.

Foundation Vapor Barrier Options:

Resources:

Jake Goodwin

Customer Support

Jake is member of the Overlake product team and as a former landscape contractor lends his expertise to the Overlake blog. You can reach one of our support representatives by emailing Support@overlakesupply.com 

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