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The Essentials of Plastic Under a Concrete Slab

Posted by Global Plastic Sheeting on Fri, Dec 22, 2023 @ 10:22

Beneath the Surface: Unveiling the Essentials of Plastic Under a Concrete Slab

Concrete slabs serve as the foundation for countless structures, and the decision to incorporate plastic underneath is a critical consideration. In this comprehensive guide, we'll delve into the why, when, where, types, installation tips, and potential pitfalls to help contractors and homeowners make informed decisions.

Why Use Plastic Under a Concrete Slab?

  1. Moisture Mitigation: Plastic acts as a moisture barrier, preventing ground moisture from seeping into the concrete and causing issues such as mold growth and structural damage.

  2. Radon Protection: In regions prone to radon gas, plastic barriers under slabs serve as a protective layer, minimizing the potential for radon infiltration into indoor spaces.

  3. Termite Prevention: Plastic acts as a deterrent against termites, safeguarding the structural integrity of the concrete.

When and Where is it Necessary?

  1. Cold and Humid Climates: Regions with cold or humid climates benefit significantly from plastic under slabs to prevent moisture-related problems.

  2. Below-Grade Structures: Basements and crawlspaces are prime locations for plastic barriers to counteract moisture infiltration.

  3. High Water Tables: Areas with high water tables increase the risk of moisture intrusion, making plastic a valuable protective measure.

Types of Plastic for Slab Underlayment:

  1. Polyethylene (PE): A common choice due to its durability and resistance to moisture.

  2. High-Density Polyethylene (HDPE): Offers enhanced strength and durability, suitable for more demanding applications.

  3. Vapor Retarder Barriers: Include reinforced options like Griffolyn® for added strength and protection against moisture and radon.

Installation Tips:

  1. Proper Sealing: Ensure seams and edges are adequately sealed to prevent moisture penetration.

  2. Subgrade Preparation: Level and compact the subgrade before laying the plastic to minimize the risk of tears or punctures.

  3. Install on the Warm Side: Place the plastic on the warmer side of the insulation to optimize its moisture mitigation properties.

Mistakes to Avoid:

  1. Inadequate Sealing: Improper sealing can compromise the effectiveness of the plastic barrier.

  2. Incorrect Placement: Placing the plastic on the wrong side of the insulation can diminish its moisture-blocking capabilities.

  3. Ignoring Site Conditions: Failing to consider local soil and climate conditions can lead to suboptimal outcomes.

Is Plastic Under a Concrete Slab Always Necessary?

While highly beneficial in many scenarios, there are instances where plastic may not be necessary:

  1. Dry Climates: In arid regions with low moisture levels, the necessity for plastic may be reduced.

  2. Well-Ventilated Areas: Properly ventilated spaces may naturally mitigate moisture concerns without the need for additional barriers.

Frequently Asked Questions about Plastic Under A Concrete Slab

Do you need it?

It depends.

    • Climate: In dry climates with low water tables, a vapor barrier might not be necessary.
    • Soil moisture: High water table or frequent flooding increase the need for a barrier.
    • Building type: Basements and structures with conditioned spaces below the slab benefit significantly from a vapor barrier.

Types of plastics:

    • Polyethylene (Poly): Most common, affordable, available in various thicknesses (10-15 mil recommended).
    • Cross-Laminated Polyethylene (XPE): More durable and puncture-resistant than poly, ideal for high-traffic areas.
    • High-Density Polyethylene (HDPE): Very strong and puncture-resistant, but less flexible, suited for demanding projects.


    • Prevents moisture damage and related issues like mold and slab cracking.
    • Protects flooring materials from moisture-induced warping and delamination.
    • Extends the lifespan of your concrete slab.
    • Improves energy efficiency by minimizing moisture infiltration and enhancing insulation effectiveness.


    • Adds to the construction cost.
    • Must be installed properly to avoid leaks and moisture traps.
    • Can trap moisture under the slab in poorly ventilated areas.

Material Specifics:

    • Polyethylene vs. XPE vs. HDPE: Choose XPE for durability or HDPE for extreme strength, prioritize poly for affordability.
    • Class A, B, and C: Class A provides the best protection, B is a good balance, C is suitable for low-risk applications.
    • Thickness: 10-15 mil is standard, thicker options offer extra protection in high-moisture environments.
    • Cost: Poly is cheapest, XPE is slightly more expensive, HDPE is the most expensive.

Application and Installation:

    • Install plastic directly on the prepared subgrade before gravel or base materials.
    • Overlap rolls at least 6 inches and seal all seams and penetrations with specialized tape.
    • Ensure proper attic ventilation even with a vapor barrier.

Alternative Options:

    • Liquid-applied vapor barriers: Offer continuous protection but require professional application.
    • Breathable membranes: Allow some moisture vapor to pass through, potentially reducing the risk of moisture buildup under the barrier.

Regional Considerations:

    • Check local building codes for vapor barrier requirements in your area.
    • Consult a professional for specific recommendations based on your climate and soil conditions.

Important Note: This information is general and not a substitute for professional advice. Always consult a qualified building professional for guidance on your specific project.

Conclusion: Making Informed Choices for Durable Foundations

Incorporating plastic under a concrete slab is a proactive measure to enhance the longevity and performance of structures. Consider local conditions, climate, and intended use to determine the necessity. Selecting the right type of plastic and ensuring proper installation are crucial steps in fortifying foundations against moisture-related challenges. By understanding the why, when, and how, contractors and homeowners can make informed decisions that contribute to the lasting resilience of their concrete structures.

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