Is Your Below-Slab Vapor Barrier Really Doing Its Job?

Matt Risinger on How Builders Can Go Wrong with Poor Foundation Vapor Protection

A number of threats in the soil beneath new homes move upward through concrete slabs — often unseen until flooring fails, mold appears, human health is impacted, or structural issues arise.  

Stego and trusted builder Matt Risinger of The Build Show share how a well-designed vapor barrier under the slab or in a crawl space safeguards a home’s structure and indoor air for the life of the home — helping to ensure long-term durability, healthier living spaces, and peace of mind for builders and homeowners alike. 

Key Takeaways: 

• Concrete is porous and vapor constantly moves upward from the soil.

• 6-mil generic poly doesn’t meet ASTM E1745 performance standards required by recent building codes, ACI guidelines, or flooring requirements.

• Stego barriers maintain extremely low water vapor permeance for decades. Upgrading adds only pennies per sq ft but prevents flooring damage, mold, and lawsuits. 

▶ Watch the full webinar replay: Is Your Below-Slab Vapor Barrier Really Doing Its Job?

Why a Vapor Barrier Matters [1:12-3:45] 

When most people think about a home’s envelope, they picture the roof and walls, but as building-science expert Matt Risinger reminds us, “The sixth wall — the one beneath your feet — is just as critical.” 

A below-slab vapor barrier does far more than meet code. It’s the layer that separates a home from the invisible threats beneath it, including: 

• Water vapor and moisture from capillary rise,

• Soil gases, such as radon, which moves via both concentration and pressure differentials,

• Volatile organic compounds (VOCs) when homes are built on contaminated sites, and

• Even subterranean termites in certain regions. 

As Risinger explains, “Whether it’s a slab-on-grade, basement, or crawl space, there’s always moisture in the ground — and it’s always moving in one direction: up.” 

Without a properly engineered barrier, vapor travels through concrete and has the potential to condense beneath flooring, potentially leading to adhesive failures, mold growth, and long-term structural issues. 

How Moisture Moves Through Concrete [3:46 – 10:12] 

Concrete may look solid, but it’s surprisingly porous. Imagine placing a dry concrete paver into a shallow pan of water — within seconds, the water begins to wick upward. This illustration of water migration is also true for moisture vapor when you think about a home’s foundation – if liquid water wicks through a concrete floor slab, then imagine how easily the tiny water vapor molecules flow through!  

This invisible vapor migration works continuously, carrying moisture toward the conditioned space. Over time, it can elevate humidity beneath flooring, damage finishes like vinyl or engineered wood, and create perfect conditions for mold and indoor-air-quality problems. 

The Real Cost of Cutting Corners [10:13 – 18:49] 

One of the most common mistakes on residential job sites is cutting out the vapor barrier beneath grade beams or using the thinnest material allowed by code. While generic, 6-mil polyethylene may “check a box,” it rarely delivers lasting protection in this application. 

Commodity plastics often contain a high percentage of recycled material which results in inconsistent long-term vapor permeance and in poor strength. Field testing shows these sheets can lose vapor resistance within weeks when exposed to soil, temperature swings, or construction activity. 

High-performance membranes such as StegoHome^® Wrap and StegoCrawl^® Wrap are engineered for the job — maintaining low permeance while resisting punctures and degradation for the life of the structure. The difference between commodity plastic and an engineered foundation vapor barrier can determine whether a home stays dry or becomes a future warranty claim. 

Going Beyond Code: Why ASTM E1745 Matters [27:50 – 32:49] 

The ASTM E1745 standard defines what an effective below-slab vapor barrier must achieve: 

• ≤ 0.1 perms of vapor permeance

• Verified tensile strength and puncture resistance to ensure jobsite durability 

• Proven long-term performance through in-service conditioning testing 

“Code minimum” isn’t the same as best practice. Even where local codes still permit 6-mil poly, organizations like the American Concrete Institute (ACI 302 and ACI 360) and the EPA recommend engineered membranes tested to this standard. 

Modern water-based flooring adhesives are particularly sensitive to moisture and flooring manufacturers often reference engineered, ASTM E1745 compliant vapor barriers beneath slabs in installation and warranty requirements. Builders who use generic poly sheeting risk void flooring warranties before the homeowner even moves in

Protecting Health and Reputation Plus Avoiding Lawsuits [18:50-27:49] 

Moisture intrusion beneath the slab isn’t just a cosmetic issue — it’s a liability. According to industry data, moisture-related flooring failures in concrete slabs cost billions annually. Moisture-related defects make up a large majority of construction-defect claims, and when vapor infiltrates a slab, it can lead to flooring failure, mold, and indoor-air-quality concerns that aren’t always covered by standard insurance policies. (Sources: CTLGroup article; DuPont/ASHRAE data) 

As Risinger often emphasizes, “What’s under your slab is something you get one chance to do right. You can remodel your kitchen in 20 years — but you’ll never rip out your vapor barrier.” 

Treat the ground beneath the slab as an extension of the home’s building envelope. Stego barrier solutions — including StegoHome^® Wrap, StegoCrawl^® Wrap, and Pango^® Wrap — are engineered to control moisture movement, help reduce the entry of soil gases, and provide a durable layer of protection against long-term environmental threats. The result: a healthier, more resilient home from the ground up. 

As Matt Risinger shows in the webinar, cutting corners beneath the slab has real consequences. In one Austin, Texas subdivision now facing a floor failure lawsuit, darkened hardwood floors traced back to beam areas indicated where contractors had cut out the vapor barrier. “You’re not remodeling your vapor barrier,” Risinger warns. “Do it right the first time, and you set that house up for a lifetime of success.” The cost difference? Just pennies per square foot — a small investment that protects against thousands in repairs, warranty claims, and reputational damage. 

Simple Choices, Long-Term Protection [32:50-35:00]  

Upgrading from generic poly to StegoHome Wrap 10 or 15-mil typically adds about $300 to a 1,500-sq-ft home — a pennies-per-square-foot investment that prevents thousands in future repairs and protects your reputation as a builder. 

As Stego puts it, “It’s not just a vapor barrier. It’s the first layer of defense for the entire home.” 

Choosing a high-performance membrane helps ensure a durable, healthy, and resilient foundation — one that stands up to moisture, radon, and time itself. 

 ▶  Watch the Full Webinar Replay 

Want to see the science of below-slab moisture control in action? The live demonstration shows how quickly water moves through concrete — a powerful visual reminder of why an effective vapor barrier is critical beneath every slab. You’ll also hear Matt Risinger and the Stego team break down common misconceptions, installation best practices, and what separates engineered, ASTM E1745 compliant vapor barriers from generic plastic sheeting. 

👉 Watch the full Build Show Webinar Replay: Is Your Below-Slab Vapor Barrier Really Doing Its Job?