Underslab VaporBlock and VaporBlock Plus Buyer’s Guide

Use a continuous underslab vapor or gas barrier to control moisture migration and soil-gas intrusion before concrete placement.

Commercial slabs-on-grade require more than concrete strength and reinforcement. Specify the correct moisture vapor barrier and install it as a continuous system. Soil moisture, water vapor and soil gases can migrate through concrete and affect flooring, adhesives, indoor air quality and long-term building performance.

This buyer’s guide compares Viaflex VaporBlock 10 and VaporBlock 15 and VaporBlock Plus 20. Use it to evaluate underslab vapor barrier installation requirements for commercial buildings, industrial facilities, schools, hospitals, laboratories and other slab-on-grade projects.

Why Underslab Moisture and Soil-Gas Control Matters

Specify an underslab barrier when the building design requires control of moisture vapor or soil gases. Do not treat the barrier as an optional accessory when the project includes moisture-sensitive flooring, occupied spaces or known subsurface contamination.

Control moisture and soil gases to:

• Protect flooring, coatings, adhesives and other moisture-sensitive finishes.
• Reduce the risk of moisture-related mold and mildew growth.
• Support indoor air quality by limiting migration from the soil.
• Reduce vapor-related delays during flooring installation.
• Help control moisture conditions during concrete curing and drying.
• Protect structural and finish systems from long-term moisture exposure.
• Address documented radon, methane, VOC or vapor-intrusion concerns.
• Improve continuity between the slab, foundation wall and perimeter details.

A moisture vapor barrier does not replace drainage, waterproofing, radon mitigation design or an environmental remediation plan. Coordinate the barrier with the complete below-grade system.

Protect the barrier after placement and before concrete placement. Rebar, chairs, utilities and construction traffic can damage even high-strength membranes.

Product Comparison: VaporBlock 10, VaporBlock 15 and VaporBlock Plus 20

Use the following information to distinguish between standard moisture control and enhanced gas-barrier requirements.

VaporBlock 10: Choose VaporBlock 10 when moisture control is the primary performance requirement and the substrate can be prepared to limit puncture exposure. The 10 mil polyethylene barrier provides low moisture vapor permeance, puncture resistance and resistance to decay and degradation.

Use VaporBlock 10 for:

• Standard commercial slabs-on-grade.
• Projects with smooth compacted subgrades.
• Moderate rebar and construction traffic.
• Applications without a documented soil-gas or vapor-intrusion requirement.
• Projects where roll coverage and material cost affect procurement decisions.

Do not specify VaporBlock 10 solely because it is the lowest nominal thickness. Confirm substrate conditions, construction traffic and finish sensitivity before selection.

VaporBlock 15: Choose VaporBlock 15 when the project requires more thickness and durability than VB10 while maintaining a standard polyethylene moisture barrier. The 15 mil configuration increases the material margin against handling damage and construction exposure.

Use VaporBlock 15 for:

• Commercial and industrial slabs with moderate to elevated puncture exposure.
• Projects with extensive rebar, chairs or embedded utilities.
• Rougher subgrades that can be prepared but may still present localized risks.
• Moisture-sensitive flooring where a robust polyethylene barrier is appropriate.
• Projects with longer intervals between barrier placement and concrete placement.

VaporBlock 15 does not provide the same EVOH-based gas-barrier construction as VaporBlock Plus 20. Select VaporBlock Plus 20 when soil-gas transmission is a design concern.

VaporBlock Plus 20: Choose VaporBlock Plus 20 when the project requires moisture control and enhanced resistance to soil gases. VaporBlock Plus uses a seven-layer co-extruded construction made with virgin high-density polyethylene and an EVOH core.

EVOH, or ethylene vinyl alcohol, functions as a highly impermeable barrier layer. It limits transmission of gases and organic vapors more effectively than typical high-performance polyethylene barriers.

Viaflex product documentation reports that VaporBlock Plus 20 is more than 100 times less permeable than typical high-performance polyethylene vapor retarders against methane, radon and other harmful VOCs. The documentation also identifies testing and verification related to BTEX, hydrogen sulfide, trichloroethylene, perchloroethylene, methane, radon and other chemicals and odors.

Use VaporBlock Plus 20 for:

• Southwest and Western projects with radon concerns.
• Brownfield redevelopment and known vapor-intrusion conditions.
• Industrial facilities with subsurface hydrocarbons or chemical exposure concerns.
• Schools, hospitals and laboratories where occupancy sensitivity increases the consequences of soil-gas migration.
• Projects that require a combined moisture and gas barrier.
• Sites where the environmental professional or project specification identifies methane, VOC or other soil-gas risks.

Do not assume VaporBlock Plus 20 alone completes a radon or vapor-intrusion mitigation system. Coordinate the membrane with ventilation, collection systems, sumps, foundation-wall details and environmental design requirements.

Seal seams and transitions to preserve continuity. Treat every penetration and perimeter connection as a system detail.

Regional and Project-Specific Selection

Apply site data instead of selecting a product by region alone.

• Humid Gulf Coast and Southeast: Evaluate high humidity, seasonal rainfall, shallow groundwater and wet subgrades. Select VaporBlock 15 when puncture exposure increases. Select VaporBlock Plus 20 when moisture concerns combine with soil-gas or contamination risks.
• Cold climates: Account for freeze-thaw cycles, seasonal groundwater movement and construction delays. Prepare and stabilize the substrate before placing the barrier. Select thickness based on puncture exposure and project duration.
• Southwest and Western states: Review radon maps, soil-gas data and local mitigation requirements. Use VaporBlock Plus 20 when the design requires enhanced radon or VOC resistance.
• Brownfield redevelopment: Require an environmental professional to identify contaminants and define the vapor-intrusion control system. Evaluate VaporBlock Plus 20 and compatible detailing materials.
• Industrial sites: Account for heavy equipment, extensive reinforcement, penetrations and longer construction sequences. Select VaporBlock Plus 20 based on gas and puncture risk.
• Schools, hospitals and laboratories: Treat indoor air quality and flooring sensitivity as high-priority criteria. Require documented product performance and controlled installation procedures.

Protect the membrane while placing reinforcement and concrete. Repair punctures before the pour begins.

ASTM E1745 and ASTM E1643

ASTM E1745 defines performance requirements for plastic water vapor retarders used in contact with soil or granular fill under concrete slabs. The standard addresses properties such as:

• Water vapor permeance.
• Tensile strength.
• Puncture or impact resistance.
• Conditioning and performance requirements.

ASTM E1745 includes Class A, B and C requirements. Class A has the highest tensile and puncture thresholds. Viaflex product documentation lists VaporBlock 10 and VaporBlock 15 along with VaporBlock Plus 20 as Class A, B and C products.

This classification does not eliminate the need for correct installation. It also does not establish that every installation will meet a project’s code, environmental or design requirements.

Use ASTM E1643 as an installation guidance reference. Review the project drawings, specifications and the current VaporBlock installation instructions or VaporBlock Plus installation guidelines.

Underslab Vapor Barrier Installation Requirements

Require the installer to:

• Prepare a clean, level and compacted substrate.
• Remove sharp debris and materials that can puncture the membrane.
• Place the barrier according to the project drawings and manufacturer instructions.
• Run sheets in a layout that supports efficient concrete placement.
• Overlap and seal seams using the product-specific guidance.
• Seal the perimeter transition to footings and foundation walls.
• Seal plumbing, conduit, columns and other penetrations.
• Repair punctures, cuts and openings before placing concrete.
• Use compatible tapes, butyl sealants, boots and repair materials.
• Protect the membrane during rebar placement and utility installation.
• Use protective plywood and geotextile fabric in high-traffic areas when required.
• Use non-penetrating screed supports where the design permits.
• Inspect the complete barrier before concrete placement.
• Do not leave damaged or unsealed areas beneath the slab.

Viaflex installation guidance specifies a minimum 6 inch overlap for VaporBlock seam sealing. VaporBlock Plus guidance specifies a minimum 6 inch overlap for vapor-retarder applications and a minimum 12 inch overlap with additional sealing measures for gas-barrier applications. Treat these dimensions as product installation guidance and confirm them against the current instructions and project specification.

Buyer Decision Framework

Select the product by answering these questions:

1. What is the primary risk? Choose standard VaporBlock for moisture control. Choose VaporBlock Plus 20 when soil gases or vapor intrusion are documented concerns.
2. How severe is puncture exposure? Increase thickness when the substrate, rebar layout, equipment or traffic increases damage risk.
3. How sensitive is the occupancy? Apply stricter review to schools, healthcare facilities, laboratories and occupied industrial buildings.
4. How long will the barrier remain exposed? Account for weather, scheduling delays and repeated traffic.
5. What substrate conditions exist? Require additional preparation or protective layers where sharp aggregate or rough fill creates puncture exposure.
6. What roll logistics apply? Compare roll width, length, handling equipment and seam quantity against the slab layout.
7. What detailing is required? Plan perimeter transitions, penetrations, repairs, gas-rated tapes and ventilation interfaces before installation.

Frequently Asked Questions

When should a project use VaporBlock instead of VaporBlock Plus?: Use VaporBlock when moisture control is the primary requirement and soil-gas risk is low. Use VaporBlock Plus 20 when the project requires enhanced resistance to radon, methane, VOCs or other soil gases in addition to moisture control.

Does VaporBlock meet ASTM E1745?: Viaflex product documentation lists VaporBlock 10 and VaporBlock 15 as ASTM E1745 Class A, B and C products. The documentation also lists VaporBlock Plus 20 as Class A, B and C.

What does EVOH do in VaporBlock Plus?: EVOH creates a low-permeability barrier layer within the seven-layer co-extruded film. It provides enhanced resistance to transmission of methane, radon and many VOCs compared with typical high-performance polyethylene barriers.

What causes underslab vapor barrier failure?: Common causes include punctures, unsealed seams, incomplete perimeter transitions, unsealed penetrations, poor substrate preparation and damage during rebar or concrete placement. Correct every defect before the concrete pour.

Can VaporBlock Plus replace a complete radon or vapor-intrusion system?: No. Use VaporBlock Plus 20 as part of a project-specific control system when required. Coordinate the barrier with ventilation, collection systems, foundation details, environmental requirements and qualified design professionals.

Select the Correct Barrier Before Procurement

Control the risk before it reaches the slab. Compare VaporBlock 10 and VaporBlock 15  for standard high-performance polyethylene moisture control. Specify VaporBlock Plus 20 when the project requires enhanced resistance to radon, methane, VOCs or other soil gases. Then protect the selected material with disciplined substrate preparation, detailing, inspection and installation.

Confirm final material selection and installation against current Viaflex product documentation, project specifications, applicable codes and qualified design or environmental professionals. For product guidance, project evaluation or a quote, contact Viaflex.