Currently Reading

Concrete Foundations: Under-Slab Vapor Barriers Done Right

Advertisement

Concrete Foundations: Under-Slab Vapor Barriers Done Right

Properly specify and install a layer of protection under concrete slabs to mitigate moisture issues  


By Graham Davis September 23, 2019
Correct way to install the vapor barrier under a concrete slab
For an under-slab vapor barrier to be most effective, it should be in direct contact with the underside of the slab. (Illustration: courtesy IBACOS)
This article first appeared in the October 2019 issue of Pro Builder.

Concrete is hygroscopic, which means a concrete slab foundation will absorb moisture from the earth and transfer it to the basement, crawlspace, or living area where it can cause an array of problems, including finished flooring failures, mold growth, and potential health problems for your homeowners. 

A properly selected and installed polyethylene vapor barrier between the slab and the soil effectively keeps ground moisture away from the concrete to mitigate these issues. Anything less (or nothing at all) is a recipe for myriad problems down the road.

Here are some best practices to follow, as well as some errors and omissions I see on jobsites all over the country.

 

Vapor Barrier Basics

The most obvious best practice is to install a sub-slab vapor barrier in the first place. I recommend a minimum 10-mil-thick polyethylene material, even if your local building code allows a 6-mil, as it lasts longer and is less prone to punctures.

Some builders in desert climates ignore this advice, figuring that they don’t have to worry about under-slab moisture. But whether moisture will be a problem depends in part on the finished floor coverings. For instance, if you apply vinyl flooring directly to the slab, even occasional moisture in markets such as Phoenix can be enough to accelerate the potential for failure between the vinyl and the concrete.

RELATED

 

A poly vapor barrier also helps keep soil gas (such as radon, among others) out of the house. As building scientist Joe Lstiburek noted in 2002, “We are building our houses on more sites that were previously treated with chemicals of all sorts, from pesticides to petroleum to PCBs.” Things have only gotten worse in that regard since.

To further mitigate soil gas intrusion, install the vapor barrier over a 4-inch bed of level, compacted gravel before you pour the slab. If you also install a vent stack for soil gas, the gravel will provide a base layer through which air can move laterally toward the stack. In areas where soil gas isn’t a problem, the gravel will still serve as a capillary break to keep moisture at bay.

 

Curing Issues

For an under-slab vapor barrier to be most effective, it should be in direct contact with the underside of the slab. In this instance, however, the freshly poured concrete can only dry upward; in hot/dry climates, the upper few inches of the slab tend to dry and shrink more quickly than the lower few inches, which can cause the edges of the slab to curl and lose connection with the vapor barrier.

Some contractors put a layer of sand between the poly and the concrete to give the slab a chance to dry in both directions; however, moisture inevitably gets trapped in the sand, and once the concrete dries, that moisture will begin transferring hygroscopically to the slab. Fully drying a sub-slab layer of sand can take months or even years.

Rather than sand, I recommend using a curing agent for the concrete. It will prevent edge curl by slowing the curing rate of the slab enough that all the moisture has time to evaporate through the upper surface.

Other best practices include extending the poly under the footings where permitted by code, as well as sealing all penetrations, as detailed in the illustration on the opposite page. 

 

 

under-slab vapor barrier installation illustration courtesy IBACOS
Illustration: courtesy IBACOS

 

 

Below are some examples of proper underslab vapor barrier installations (all photos courtesy IBACOS).

 

example of underslab vapor barrier correctly installed

 

 

example of underslab vapor barrier correctly installed

 

 

An example of underslab vapor barrier correctly installed

 

 

example of underslab vapor barrier correctly installed

 

 

example of underslab vapor barrier correctly installed

 

 

Graham Davis drives quality and performance in home building as a building performance specialist of the PERFORM Builder Solutions team at IBACOS.

 

RELATED

From Pro Builder's sister site ProTradeCraft.com:

Related Stories

Indoor Air Quality

How to Achieve Affordable Balanced Ventilation

Makeup air is a cost-effective option that yields long-term benefits

Quality Matters

Mitigate Cracks With the Right Detailing

Creating adequate separation between materials is critical to mitigating cracks (and worse) along cementitious surfaces

Moisture Management

Waterproofing Showers, Part 2: Site-Built Pans

Pay attention to these details to properly waterproof site-fabricated shower pans

Advertisement

More in Category




Advertisement

Create an account

By creating an account, you agree to Pro Builder's terms of service and privacy policy.


Daily Feed Newsletter

Get Pro Builder in your inbox

Each day, Pro Builder's editors assemble the latest breaking industry news, hottest trends, and most relevant research, delivered to your inbox.

Save the stories you care about

Lorem ipsum dolor sit amet lorem ipsum dolor sit amet lorem ipsum dolor sit amet.

The bookmark icon allows you to save any story to your account to read it later
Tap it once to save, and tap it again to unsave

It looks like you’re using an ad-blocker!

Pro Builder is an advertisting supported site and we noticed you have ad-blocking enabled in your browser. There are two ways you can keep reading:

Disable your ad-blocker
Disable now
Subscribe to Pro Builder
Subscribe
Already a member? Sign in
Become a Member

Subscribe to Pro Builder for unlimited access

Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.